development of a lead free piezoelectric (k,na)nbo … will reduce the piezoelectric thin film...
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Development of a Lead−free Piezoelectric (K,Na)NbO3
Thin Film Deposited on Nickel−based Electrodes
by
Alaeddin Bani Milhim
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Mechanical and Industrial Engineering
University of Toronto
© Copyright by Alaeddin Bani Milhim 2016
ii
Development of a Lead−free Piezoelectric (K,Na)NbO3 Thin Film
Deposited on Nickel−based Electrodes
Alaeddin Bani Milhim
Doctor of Philosophy
Mechanical and Industrial Engineering
University of Toronto
2016
Abstract
It is desirable to replace noble metals used as electrode materials for piezoelectric thin film
with base metals. This will reduce the piezoelectric thin film fabrication cost. A nickel−based
layer in conjunction with other protective layers is proposed as a bottom electrode for lead−free
piezoelectric KNN thin film. The obtained results do not indicate the oxidation of the
nickel−based bottom electrode after the deposition of KNN at 600 °C for 10 hours in the
presence of oxygen and/or after annealing the sample at 400 °C for an hour in air. The fabricated
KNN thin film was fully characterized in this work. The effective piezoelectric coefficients d33
and d31 were estimated to be 37 pm/V and 17.2 pm/V, respectively, at 100 kV/cm. The
piezoelectric properties of the fabricated KNN/Ni/Ti/SiO2/Si are affected by the crystal
orientation of the KNN layer, which was preferentially oriented in the (110) direction.
Optimization of the deposition parameters of the fabricated KNN/Ni/Ti/SiO2/Si film is expected
to further enhance the piezoelectric properties.
Two novel systems utilizing the developed KNN piezoelectric thin film are proposed and
their performance simulated based on the achieved KNN thin film parameters. The first is a
precision automated nanomanipulation system using an AFM as a sensor and piezo−actuated
iii
manipulators. Real−time feedback of the particle being manipulated can be achieved
using the proposed system. The length of the manipulators needs to be at least 2 mm to be
incorporated with a commercial AFM system. To fabricate the required manipulators, a
three−step electrochemical etching technique was developed. Tungsten tips combining
well−defined conical shape, a length as large as 2 mm, and sharpness with a radius of curvature
of around 20 nm were fabricated using the proposed technique. By depositing the KNN thin film
on the fabricated manipulator, nanomanipulators with out−of−plane actuation can be produced.
Ultrasonic piezoelectric fan array, the second system, is proposed for GPU cooling applications.
The developed KNN thin film is proposed as the piezo layer in the piezo fan structure. The novel
solution can offer large air flow rate and low power consumption. Since the operating frequency
is beyond the human audible frequencies, non−audible noise fans are expected by using the
proposed ultrasonic piezo fan system. Moreover, fabrication of these ultrasonic piezo fans can be
part of the GPU fabrication process itself.
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Acknowledgments
First of all, I would like to express my sincere gratitude to my supervisor, Prof. Ridha Ben
Mrad, for his continuous abundant support of my PHD study, his patience, his motivation, and
his valued suggestions. This thesis would not have been possible without his guidance and
support. I could not have imagined having a better supervisor for my PHD study. Professor
Ridha Ben Mrad is the one professor who truly made a difference in my life.
I would like to thank the other members of my committee, Prof. Hani Naguib and Prof.
Kamran Behdinan, for their valuable comments and encouragement. I would like to thank Dr.
Edward Huaping Xu, George Kretschmann, Harlan Kuntz, Dr. Henry Lee, Dr. Lindsey Fiddes,
and Dr. Rana Sodhi for their assistance in conducting the experimental work. I was very
fortunate to meet some fantastic colleagues and friends, Mike, Paul, Khalil, Sadegh, Hirmand,
Jacky, James, Imran, Tae, Eu−Jin, Steffen, Chakameh, Ali, Donn, Faez, Ahmed, and Amro for
their advice and for all the fun we have had in the lab in the last four years.
Last but not least, I would like to thank my parents and my brothers and my sister for their
affection and support. I dedicate this thesis to my parents who unremittingly supported me during
my years of study. They made this work possible.
v
Table of Contents
Acknowledgments .......................................................................................................................... iv
Table of Contents ............................................................................................................................ v
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
List of Appendices ........................................................................................................................ xv
Nomenclature ............................................................................................................................... xvi
1 Introduction ................................................................................................................................ 1
1.1 Objectives ........................................................................................................................... 2
1.2 Thesis Outline ..................................................................................................................... 3
2 Lead−free Piezoelectric Thin Film............................................................................................. 5
2.1 Lead−free Piezoelectric Material: KNN ............................................................................. 8
2.2 Piezoelectric Thin Film ..................................................................................................... 11
2.3 Sputtering of KNN Thin Film ........................................................................................... 12
2.4 The State of Art for KNN Thin Film Fabrication ............................................................. 14
2.5 Characterization Methods for Piezoelectric Thin Film ..................................................... 23
2.5.1 Crystal Orientation ................................................................................................ 24
2.5.2 Chemical Compositions ........................................................................................ 24
2.5.3 Polarization Hysteresis Loop ................................................................................ 25
2.5.4 Dielectric Constant ................................................................................................ 27
2.5.5 Leakage Current Density ...................................................................................... 29
2.5.6 Piezoelectric Coefficients (d33 and d31) ................................................................. 30
2.5.7 Electrical Conductivity of the Bottom Electrodes ................................................ 31
vi
2.6 Modeling of Piezoelectric Thin Film Actuators ............................................................... 33
2.7 Summary ........................................................................................................................... 36
3 Fabrication of KNN Thin Film on Nickel−based Electrodes................................................... 37
3.1 Fabrication of KNN on Nickel Silicide Bottom Electrode ............................................... 38
3.1.1 Fabrication Process ............................................................................................... 38
3.1.2 Characterization of the Bottom Electrode ............................................................. 41
3.1.3 Crystal Structure and Chemical Compositions ..................................................... 43
3.1.4 Electric and Piezoelectric Properties .................................................................... 47
3.2 Fabrication of KNN on Nickel−based Bottom Electrode ................................................. 50
3.2.1 KNN Thin Film Structure and Fabrication Process .............................................. 51
3.2.2 Characterization of the Uncovered Bottom Electrode (Nickel Silicide) .............. 53
3.2.3 Crystal Orientation and Chemical Composition of the Fabricated Film .............. 55
3.2.4 Electric and Piezoelectric Properties of KNN/Ni/Ti/SiO2/Si ................................ 59
3.3 Summary ........................................................................................................................... 64
4 A Precision Nanomanipulation System Using an AFM and Piezo−actuated Manipulators .... 66
4.1 Proposed Nanomanipulation System ................................................................................ 66
4.2 Fabrication of Tungsten Tips for Nanomanipulation ........................................................ 72
4.2.1 Tungsten Tips ........................................................................................................ 73
4.2.2 Electrochemical Etching: Static and Dynamic ..................................................... 76
4.2.3 Experimental Setup ............................................................................................... 79
4.2.4 Optimization of the Process Parameters ............................................................... 81
4.2.5 Proposed Three−step Electrochemical Etching Technique .................................. 89
4.3 Assessment of the Manipulation Based on the Developed KNN Thin Film .................... 93
4.4 Summary ........................................................................................................................... 94
5 Development of Ultrasonic Piezo Fans Based on the Developed KNN Thin Film ................. 96
vii
5.1 Piezo Fans for GPU Cooling Systems .............................................................................. 97
5.2 Micro Piezo Fan Operating at 20 kHz ............................................................................ 101
5.3 Summary ......................................................................................................................... 105
6 Concluding Remarks .............................................................................................................. 106
6.1 Conclusions ..................................................................................................................... 106
6.1.1 Lead−free Piezoelectric Thin Film ..................................................................... 106
6.1.2 Fabrication of KNN Thin Film ........................................................................... 107
6.1.3 Proposed Nanomanipulation System .................................................................. 108
6.1.4 Proposed Micro Piezo Fan Array ........................................................................ 109
6.2 Major Contributions ........................................................................................................ 110
6.3 Future Work .................................................................................................................... 111
References ................................................................................................................................... 113
Appendix A: Characteristics of the Macro and Micro Piezo Fans ............................................. 129
viii
List of Tables
Table A. Characteristics of the fabricated tips while varying the etching parameters ................. 89
Table B. Characteristics of the piezo fan designs ....................................................................... 130
ix
List of Figures
Figure 2.1. Perovskite structure. ..................................................................................................... 6
Figure 2.2. Phase diagram for PZT [7]. .......................................................................................... 7
Figure 2.3. Phase diagram for KNbO3−NaNbO3 (KNN) system [17]. ......................................... 10
Figure 2.4. Schematic of an RF magnetron sputtering machine. .................................................. 13
Figure 2.5. A schematic diagram of the classic Sawyer−Tower circuit. ...................................... 26
Figure 2.6. Electrical circuit model of a piezoelectric sample. ..................................................... 28
Figure 2.7. A schematic diagram showing the four point resistivity measurement setup. ........... 32
Figure 2.8. Schematic diagram of a piezoelectric unimorph cantilever........................................ 34
Figure 3.1. XRD patterns of annealed KNN thin film in air, annealed KNN thin film in vacuum,
and as−deposited KNN thin film. ..................................................................................... 41
Figure 3.2. Pictures of fabricated KNN samples. (a) As−deposited Nickel bottom electrode. (b)
As−deposited KNN sample. (c) Post−annealed KNN sample. ......................................... 42
Figure 3.3. XPS depth profiles for nickel silicide bottom electrode. (a) After the KNN deposition.
(b) After the annealing process. ........................................................................................ 43
Figure 3.4. XRD pattern of the annealed KNN thin film. ............................................................. 44
x
Figure 3.5. SEM images of fabricated KNN thin films. (a) As−deposited KNN/Ni/Ti/Si
sample. (b) As−deposited KNN/Ni/Ti/SiO2/Si sample. (c) Annealed KNN/Ni/Ti/Si
sample. (d) Annealed KNN/Ni/Ti/SiO2/Si sample. .......................................................... 45
Figure 3.6. SEM images with EDX line scan elemental profiles for the fabricated KNN thin film.
(a) KNN/Ni/Ti/Si sample. (b) KNN/Ni/Ti/SiO2/Si sample. ............................................. 46
Figure 3.7. Dielectric constant and loss tangent as a function of frequency for the fabricated
KNN thin film. .................................................................................................................. 47
Figure 3.8. Polarization electric field hysteresis loop of the fabricated KNN thin film. .............. 48
Figure 3.9. Leakage current density as a function of the electric field for KNN thin film with
nickel electrodes. ............................................................................................................... 49
Figure 3.10. Schematic diagram of the proposed KNN thin film deposited on nickel−based
electrodes. ......................................................................................................................... 52
Figure 3.11. SEM image and XPS analysis of the fabricated nickel silicide layer. (a)
Cross−sectional SEM image of the nickel silicide. (b) Compositional distribution of the
nickel silicide along the thickness direction. .................................................................... 54
Figure 3.12. XRD patterns of the fabricated KNN/Ni/Ti/SiO2/Si samples. ................................. 56
Figure 3.13. SEM images of the fabricated KNN thin film. (a) SEM image of the cross section of
the sample. (b) SEM image of the fabricated KNN surface. ............................................ 57
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Figure 3.14. Elemental depth profiles for the fabricated KNN film. (a) SEM images with
EDX line scan elemental profiles for the KNN thin film. (b) XPS depth profiles for the
KNN/Ni/Ti/SiO2/Si thin film. ........................................................................................... 58
Figure 3.15. Dielectric constant and loss tangent as a function of frequency for the fabricated
KNN thin film samples. .................................................................................................... 60
Figure 3.16. Polarization electric field hysteresis loop of the fabricated KNN thin film. ............ 61
Figure 3.17. Leakage current density as a function of applied electric field for the fabricated
KNN thin film. .................................................................................................................. 62
Figure 4.1. Schematic diagram of the proposed system. .............................................................. 69
Figure 4.2. Schematic diagrams showing the lateral and vertical views of the manipulators with
the AFM cantilever assembly. (a) Lateral view. (b) Vertical view. ................................. 70
Figure 4.3. Schematic diagram showing the tip−substrate−object model. ................................... 71
Figure 4.4. The relation between the force and the von Mises stress at the manipulator tip. ....... 72
Figure 4.5. Multiple tips are used for multi−point contact measurements. (a) The length of the
tips is 500 µm. (b) The length of the tips is 2000 µm. ...................................................... 74
Figure 4.6. Schematic diagram of a conventional electrochemical etching.................................. 77
Figure 4.7. Electrochemical etching stages. (a) First stage of etching where the voltage is not
applied yet. (b) Formation of the meniscus and the chemical interaction at the anode. (c)
Final stage etching where drop−off happens. ................................................................... 78
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Figure 4.8. Schematics of the etching current during the dynamic electrochemical etching
and the corresponding tip shape. (a) The electrical current during dynamic etching. (b)
The electrical current during one oscillation cycle of dynamic etching and the correspond.
........................................................................................................................................... 79
Figure 4.9. Schematic diagram of the experimental setup including the connections of the
National Instruments (NI) cards that were used in controlling the etching process. ........ 80
Figure 4.10. Investigation the effect of the different positions of the cathode. (a) Measured
electrical current across the tip during the whole process for three different positions of
the immersed wire. (b) SEM image of the tip when the cathode was at the same level as
the air/solution interface. (c) SEM image of the tip when the cathode was 1 mm below the
interface. (d) SEM image of the tip when the cathode was 2 mm below the interface. ... 82
Figure 4.11. Measured currents and SEM images corresponding to different immersed wire
lengths. (a) Measured electrical current across the tip when the immersed length of the
wire was 1, 2, and 3 mm. (b) SEM image of the tip for the 1 mm immersed length of the
wire. The image of the tip corresponding to the immersed wire being 2 mm is shown in
(c) while the 3 mm immersed wire length case is shown in (d). ...................................... 84
Figure 4.12. Measured currents corresponding to different applied voltages and SEM images of
obtained tips. (a) Measured electrical currents corresponding to different applied
voltages: 4, 3, and 5 V. (b) SEM image of the tip when the applied voltage was 4 V. (c)
Image of the tip with 3 V applied. (d) Image of the tip with 5 V applied. ....................... 86
Figure 4.13. Blunt tip produced under applied voltage of 5 V, immersed wire of 1 mm, and the
cathode at the interface. (a) The tip apex is shown in (b). The apex of the produced tip
xiii
under the applied voltage of 4 V, the immersed wire of 1 mm, and the cathode
depth at the air/solution interface is shown in (c). ............................................................ 88
Figure 4.14. Experimental result of the proposed electrochemical etching technique. (a)
Measured electrical current across the external resistor for the complete three−step
electrochemical etching process (b−f) Optical images of the immersed wire during the
process. .............................................................................................................................. 91
Figure 4.15. Scanning electron micrograph of the whole produced tip (a) and the zoom−in image
of the tip apex (b). ............................................................................................................. 92
Figure 4.16. SEM image of a produced tip (a) and the zoom−in image of the tip apex (b). ........ 92
Figure 4.17. Schematic diagram of the proposed nanomanipulator with out−of−plane actuation.
........................................................................................................................................... 93
Figure 5.1. A piezoelectric fan. (a) Schematic of a piezo fan. (b) Schematic diagram showing the
working principle of a piezo fan. ...................................................................................... 96
Figure 5.2. 3D schematic of a typical GPU cooling system. ........................................................ 98
Figure 5.3. Piezo fans incorporated into a cooling system. .......................................................... 99
Figure 5.4. Schematic of a piezo fan showing the estimated air velocity locations. X represents
the length of the considered area, w represents the width of piezo fan, δ represents the
maximum tip deflection, Vi represents the location of the point where the air velocity was
estimated, and α represents the distance between the maximum tip deflection and the Vi.
......................................................................................................................................... 100
xiv
Figure 5.5. Schematic diagrams of the micro piezo fan array configuration. (a) Micro
piezo fans assembly within a GPU cooling system. (b) A large array of micro piezo fans.
(c) Zoom−in schematic of two neighbouring micro piezo fans. ..................................... 102
Figure 5.6. Microscope image of fabricated silicon cantilever. .................................................. 103
xv
List of Appendices
Appendix A: Characteristics of the Macro and Micro Piezo Fans……………………………..129
xvi
Nomenclature
AC alternating current
AFM atomic force microscopy
d31 lateral piezoelectric coefficient
d33 transverse piezoelectric coefficient
DC direct current
GPU graphics processing unit
E applied electric field
EDX energy dispersive X−ray
FSI fluid−structure interaction
FWHM full width at half maximum intensity
HF hydrofluoric acid
I electrical current
ICDD international centre for diffraction data
KBT potassium bismuth titanate (K0.5Bi0.5TiO3)
Kp planar coupling factor
KNN potassium sodium niobate ((K,Na)NbO3)
KOH potassium hydroxide
LNO lanthanum nickel oxide (LaNiO3)
NBT sodium bismuth titanate (Na0.5Bi0.5TiO3)
NI national instrument
P polarization
P−E loop polarization versus applied electric field hysteresis loop
PFM piezoresponse force microscopy
PIMM parallel imaging/manipulation force microscopy
xvii
PLD pulsed laser deposition
PVD physical vapor deposition
PZT lead zirconate titanate (Pb(Zr,Ti)O3)
R resistance
RF radio frequency
SEM scanning electron microscope
SET single electron transistor
SOI silicon on insulator
V voltage
XPS X−ray photoelectron spectrometer
XRD X−ray diffraction
1
1 Introduction
Piezoelectric materials can develop a mechanical strain when they are subject to an applied
electric field; this phenomenon is known as the converse piezoelectric effect. The direct
piezoelectric effect occurs when the electric field is generated as a result of an applied
mechanical stress. The converse piezoelectric phenomenon is normally used for actuation
applications, while the direct effect is used for developing sensing technology. The most widely
used piezoelectric material is lead zirconate titanate (Pb(Zr,Ti)O3) (PZT) due to its excellent
piezoelectric properties and due to the fact that it is widely commercially available [1]. PZT
suffers from containing lead which makes it dangerous for both the environment and health of
the users. Sodium potassium niobate (K,Na)NbO3 (KNN) is considered as a potentially attractive
alternative to PZT due to its interesting piezoelectric properties which are comparable to those of
PZT [2]. Piezoelectric material can be fabricated as a thin film, at which the thickness of the
piezoelectric material is less than 10 µm. The piezoelectric film is often deposited on an elastic
beam to function as a sensor or an actuator, e. g. to generate an out−of−plane actuation.
Piezoelectric thin film has been recently used in industrial applications such as in inkjet printers
and accelerometers [3,4]. Piezoelectric thin film typically consists of a piezoelectric layer
sandwiched between two electrodes. Noble metals are usually used as electrode materials due to
their high−temperature oxidation resistance. However, using noble metals in piezoelectric thin
film fabrication increases the cost of these films. Therefore, it is desirable to replace these
materials with base metals to reduce the fabrication cost. In this work, nickel is proposed to be
used as an electrode material for KNN thin film. In this way, low cost lead−free piezoelectric
thin film can be produced.
2
Two novel systems that use KNN piezoelectric thin film as an actuator are proposed. The
systems share the need for miniature actuators. The first is a precision automated
nanomanipulation system using an atomic force microscopy (AFM) and piezo−actuated
manipulators. The proposed system can provide real−time feedback of the particle being
manipulated. The system is expected to enable the practical applications of AFM−based
nanomanipulation.
Ultrasonic piezo fan array, the second system, is proposed to replace current rotary fans in
a commercial graphics processing unit (GPU) cooling systems. The ultrasonic piezo fan (micro
piezo fan) array can potentially provide large air flow rate and low power consumption. This
enhances the thermal management methods for the GPU which has a direct influence on the
development of the GPU itself.
This thesis discusses several systems. Therefore, a detailed introduction is presented in
each chapter along with the state of the art of that system. It is worth mentioning again that these
systems share the use of the developed KNN thin film.
1.1 Objectives
The main objective of this thesis is to develop lead−free piezoelectric thin film on base
metal electrodes. The developed thin film is to be implemented on two proposed systems.
More specifically, the objectives of this work can be listed as follows:
Fabrication and characterization of KNN piezoelectric thin film on nickel−based
electrodes.
3
Design of an automated nanomanipulation system incorporating a commercial AFM and
piezo−actuated manipulators and simulated assessment of its performance based on the
KNN thin film achieved properties.
Development of nanomanipulators that fit within the AFM working area.
Design of an ultrasonic piezo fan array for GPU cooling applications and assessment of
its performance based on the KNN thin film properties achieved.
1.2 Thesis Outline
KNN piezoelectric thin film is the focus of Chapter 2. This chapter starts with a discussion
on the well−known PZT piezoelectric material and then discusses the need for lead−free
piezoelectric material. KNN piezoelectric thin film is then presented followed with an extensive
literature review of the KNN thin film fabrication processes. Characterization methods of
piezoelectric thin film are also discussed in Chapter 2. KNN thin film deposition on
nickel−based bottom electrodes is discussed in Chapter 3. A discussion of the motivation for
replacing the current noble materials used as a bottom electrode with a base metal is included.
Two fabrication runs that were conducted are also discussed in Chapter 3. In the first run, the
KNN thin film was deposited on a nickel silicide bottom electrode. In the second run, the KNN
film was deposited on a hybrid bottom electrode including both pure nickel and nickel silicide
portions. The fabricated KNN thin films in both runs were fully characterized. In Chapter 4, a
novel automated nanomanipulation system using an AFM and piezo−actuated manipulators is
proposed. A three−step electrochemical etching technique is developed to fabricate the
manipulators that fit within the AFM working area. Then, the developed KNN thin film is
proposed to be deposited on the fabricated manipulators to act as an out−of−plane actuator. The
4
out−of−plane displacement of the fabricated manipulator is evaluated based on the properties of
the developed KNN thin film. The manipulation process is analyzed and the development of an
XY nano−positioning stage is suggested. The second application of the developed piezoelectric
thin film is introduced in Chapter 5. In this chapter, a novel ultrasonic piezo fan array is
discussed and proposed to replace the current rotary fan in commercial GPU cooling systems.
The developed KNN thin film is proposed as the piezo layer in the piezo fan structure. Then, the
performance of the proposed system is assessed based on the piezoelectric properties of the
developed KNN thin film. In Chapter 6, conclusions and a summary of the thesis work are
presented.
5
2 Lead−free Piezoelectric Thin Film
Piezoelectric materials play a large role in the development of a large array of precision
and/or high speed of response micro actuators and sensors [5,6]. Piezoelectricity stems from the
crystal structure of the material. A crystal consists of atoms that form a periodically repeated
pattern in the three spatial dimensions. Some of these crystals exhibit piezoelectricity such as
when a mechanical stress is applied on a piezoelectric material, the atomic structure of the crystal
changes. This leads to changing the distance between the positive and negative ions in a
crystallographic unit cell and thus a dipole moment is formed. This is referred to as an internal
dipole moment formation which leads to a spontaneous polarization. However, the dipole
moment must not be cancelled out by other formed dipoles in order for the crystal to develop a
net polarization. So, the atomic structure of the material must not have a center of symmetry in
order to produce a piezoelectric effect. There are 32 crystalline classes, within which 21 classes
do not have centers of symmetry [1]. Only 20 classes are piezoelectric while the other class does
not display any piezoelectric effect because the piezoelectric charges along the polarization
directions cancel each other. Of these 20 piezoelectric classes, 10 classes are pyroelectric, and
the remaining are non−pyroelectric classes. Pyroelectricity indicates that the spontaneous electric
polarization of the material varies when the temperature is changed. Some of these pyroelectric
materials are ferroelectric. Ferroelectricity refers to the ability of a material to inverse its
spontaneous polarization under the application of an electric field. It can be stated that
ferroelectrics are a special subset of piezoelectrics. Ferroelectric materials include perovskite and
ilmenite families. In this work, the perovskite structure is focused on. This perovskite structure
includes that of calcium titanium oxide (CaTiO3) (ABO3) and is shown in Figure 2.1 [1,6] .
6
Perovskite is the most well−known ferroelectric structure. It has a simple cubic phase, as
shown in Figure 2.1, which appears above its Curie temperature [1]. The cubic phase refers to
the cubical shape of the unit cell in a crystal structure. The Curie temperature is the temperature
at which the ferroelectric material starts to lose its spontaneous polarization. Since the cubic
phase in perovskite structure materials appears above the Curie temperature, the cubic phase of a
perovskite structure is not ferroelectric. The most widely used perovskite material is lead
zirconate titanate (Pb(Zr,Ti)O3) (PZT) due to its excellent piezoelectric properties and due to the
fact that it is widely available commercially [1]. The PZT structure is such that Pb is placed on
the A sites in the perovskite structure, Zr and Ti ions are randomly placed on the B sites, and O is
placed on the O sites in the perovskite structure.
Figure 2.1. Perovskite structure.
7
The chemical concentrations of Zr and Ti in the overall mix as well as the surrounding
temperature control the phase of the PZT as shown in Figure 2.2 [7]. Accordingly, the
piezoelectric properties are based on the PZT phase. The tetragonal phase, which appears when
the concentration of PbTiO3 is greater than 48% as shown in Figure 2.2, can be represented as
stretching the cubic phase along one of its axes (X, Y, or Z). This stretching leads to developing
a distance between the positive and negative ions in the unit cell along the stretching axis.
Therefore, the spontaneous polarization is generated along that axis. Since that axis is x, y, or z,
the Miller index of that axis is (001) [8]. Stretching the cubic phase along the (111) direction
leads to the rhombohedral phase, at which the spontaneous polarization is generated along the
(111) direction. The (111) direction is perpendicular to a plane that intercepts on all axes (x, y,
and z). The maximum piezoelectric properties of Pb(ZrxTi1-x)O3 are found to occur at the
boundary between the tetragonal and the rhombohedral phases which is referred to as the
morphotropic phase boundary (and at x= 0.52 where x represents the weight percentage of Zr)
[9]. This might be because the tetragonal and rhombohedral phases have equal energy states,
Figure 2.2. Phase diagram for PZT [7].
8
which optimizes the domain rotation during the poling process. The poling process is applied to
align the polarization of the unit cells in a piezoelectric element so that they all point in the same
direction and thus enhance the piezoelectric behavior of the intended piezoelectric element. This
enhances the polarization and thus maximizes the piezoelectric response of PZT. However, the
exact reason for the optimized properties at the values mentioned above is still a topic of intense
interest [2,7].
PZT ceramics have a polycrystalline structure, which consists of many crystallites with
varying sizes and orientations. If a structure has only one crystal orientation, it is referred to as a
single crystal material. Improved electromechanical properties can be generally achieved in a
single crystal structure. This is due to the absence of interfaces between the crystallites, which
are referred to as grain boundaries. These grain boundaries act as mechanical constrains. This
explains the superior piezoelectric properties of relaxer−based single crystals (PZNT) [10].
However, single crystal PZT has a lower Curie temperature in comparison with that of
polycrystalline PZT [11]. It is worth pointing out that the crystal structure is not perfect within a
single crystal, which may contain defects such as vacancies.
PZT contains lead which makes it dangerous for both health and environment. Europe, The
US and China have all enacted laws to restrict and regulate the use of hazardous substances such
as lead in the future in electrical and electronic equipment [12,13]. Therefore, there is a strong
interest to find a lead−free piezoelectric material with comparable properties to those of PZT.
2.1 Lead−free Piezoelectric Material: KNN
Recent research led to the development of lead−free piezoelectric materials including
Barium Titanate (BaTiO3), Sodium Bismuth Titanate (Na0.5Bi0.5TiO3) (NBT), Potassium
9
Bismuth Titanate (K0.5Bi0.5TiO3) (KBT), and Potassium Sodium Niobate (K0.5Na0.5NbO3) (KNN)
[7,14,15]. Among these lead−free piezoelectric materials, KNN is considered the most attractive
alternative to PZT due to its interesting piezoelectric properties which are comparable to those of
PZT [2]. Ferroelectric behavior in potassium niobate was discovered by Matthias et al. [16]. It is
worth pointing out that the pure potassium niobate (KNbO3) is a ferroelectric material; however,
the pure sodium niobate (NaNbO3) is an anti−ferroelectric material. An anti−ferroelectric is a
crystal material with adjacent dipoles oriented in opposite direction and thus the macroscopic
spontaneous polarization is zero. KNN, which is KNbO3 and NaNbO3 together, is a ferroelectric
material.
The phase diagram of KNbO3 −NaNbO3 (KNN) is shown in Figure 2.3 [17]. The symbol F
in the diagram represents ferroelectric, P represents paraelectric, and the subscript represents the
phase (FO: ferroelectric orthohobmic phase, FT: ferroelectric tetragonal phase, PC: paraelectric
cubic phase). Both vertical axes on the phase diagram of the KNN system represent the
temperature. KNN ceramics have an orthohombic structure at around room temperature. The
orthohombic structure refers to a cube structure stretching along two of its orthogonal axes by
different factors while keeping all angles 90°. The interesting ratio of KNbO3 and NaNbO3 is
1:1, at which the best piezoelectric properties of KNN can be obtained (d33=80 pC/N, kp=0.36)
[17]. An accepted explanation of the improved piezoelectric properties of KNN at the mentioned
ratio is the polymorphism phase transition theory which claims that the good properties of KNN
is due to the phase transition shifting from tetragonal to orthohombic downwards from around
200 °C to room temperature [2]. The tetragonal and orthorhombic phases are shown in Figure
2.3.
10
It can be seen that the phase diagram of KNN is more complicated than that of PZT. In
addition, the development of KNN as a commercial piezoelectric material is a challenge due to
the processing difficulties, especially densification. It is difficult to maintain stoichiometry due to
the volatility of the alkali metals (K and Na) used. Processing conditions can be optimized by
doping the KNN with suitable materials to improve the piezoelectric properties of KNN (d33=
120 pC/N and kp=0.4) [18,19,20]. The enhanced piezoelectric properties of KNN ceramics are
considered relatively low in comparison with that of PZT (for polycrystalline PZT: d33 400~6000
pC/N, kp ~ 0.7, for Pt−PZT single crystal: d33 > 1500 pC/N, kp > 0.9 [10]).
Saito et al. [21] significantly improved the piezoelectric properties of KNN ceramics
through the discovery of the morphotropic phase boundary and the highly (001) oriented
Figure 2.3. Phase diagram for KNbO3−NaNbO3 (KNN) system [17].
11
polycrystalline. The peak d33 value was reported to be 416 pC/N. The strain to electric field ratio
in this material was independent of the temperature between room temperature and 160 °C. The
morphotropic phase boundary was obtained between the orthorhombic perovskite structure
K0.5Na0.5NbO3 and the hexagonal pseudo−ilmenite structure LiTaO3. These new developments
showed interesting properties of KNN piezoelectric material and thus motivated further
development of this lead−free material.
In various applications, reducing the size of the piezoelectric element, while enhancing the
performance, is highly demanded. However, when the thickness of a piezoelectric element
becomes less than 10 µm, it approaches the grain size of the element. This leads to a degradation
of the piezoelectric material. Therefore, the production methods of piezoelectric material with
thickness of less than 10 µm are different than that of the thicker piezoelectric film (bulk). Thin
film technology studies the development of such thin piezoelectric elements. This is discussed in
the next section.
2.2 Piezoelectric Thin Film
Piezoelectric thin film is different from piezoelectric bulk material in a number of ways
including in the implementation mechanism. Piezoelectric bulk material can be used as an
actuator by itself such as a piezoelectric stack to provide a linear motion. However, thin film
needs to be used in conjunction with a support structure [22]. Piezoelectric material can be
fabricated as a thin film by depositing it on an elastic beam to function as a sensor or an actuator,
e. g. to generate out−of−plane actuation. In addition, the piezoelectric properties of the material
depend on the fabrication process of the material itself. In bulk form, the piezoelectric is
developed using a sintering process. However in the thin film case, the piezoelectric is fabricated
12
based on depositing the material on an elastic structure. Therefore, the properties of the thin film
including the structural, electrical, and piezoelectric properties are different than those of the
bulk ones for the same piezoelectric material.
This work focuses on the fabrication of KNN piezoelectric thin film. KNN can be
deposited as a thin film through different techniques such as pulsed laser deposition (PLD)
[23,24], sol−gel process [25,26], and RF magnetron sputtering [27,28]. RF magnetron sputtering
is the technique that leads to the highest−quality KNN thin film [29,30]. RF magnetron
sputtering is discussed next.
2.3 Sputtering of KNN Thin Film
Sputtering is a physical vapor deposition (PVD) process to deposit material on a substrate.
Sputtering involves removal of a material from a target through bombarding the target by
energetic atoms such as Argon. The collision of these atoms into the target ejects the atoms from
the target into the space. These ejected atoms reach the substrate after travelling a short distance
(a few cm). Then, they condensate to form a film. When more and more atoms condensate on the
substrate, they bind to each other at the molecular level to form an atomic layer. One or more of
these atomic layers can be formed depending on the duration of the sputtering process.
A schematic diagram describing the sputtering process is shown in Figure 2.4. Argon
atoms are introduced into a vacuum chamber at a low pressure (1 to 10 mTorr). An AC voltage is
applied between the target (Cathode) and the substrate (Anode). This electric field ionizes the
Argon atoms and creates plasma. Since these ions (Ar+) are now charged, they accelerate
towards the target and thus ejecting target atoms. The ejected atoms travel to the substrate and
settle there. Electrons released during the Argon ionization are accelerated to the substrate,
13
colliding with additional Argon atoms, creating more ions and free electrons in the process,
continuing the cycle.
As mentioned earlier, the properties of KNN thin film depend on the fabrication process.
Using the RF magnetron sputtering technique, the elastic modulus of the fabricated KNN thin
film was estimated to be 115 GPa [31]. In another study, the elastic modulus for KNN thin film
fabricated using the same technique was evaluated to be 92 GPa [32]. The Curie temperature and
the thermal expansion coefficient of the KNN thin film fabricated through the RF magnetron
sputtering technique were evaluated to be 360 ºC and 8×10-6
(1/°C), respectively [32]. A review
of the literature for KNN thin film fabricated through RF magnetron sputtering is presented next.
Figure 2.4. Schematic of an RF magnetron sputtering machine.
14
2.4 The State of Art for KNN Thin Film Fabrication
KNN thin film fabrication has been studied by different research groups [27−60]. The
objective of these studies has been to produce KNN thin film with good electric and high
piezoelectric properties to enable the practical applications of these films. This includes
optimization of the deposition parameters (e. g. deposition temperature, Ar/O2 gas concentration,
and post−annealing treatment). The influence of various material parameters such as the grain
size, crystal orientation, and chemical compositions of the KNN thin film were discussed in these
studies. The effects of the bottom electrode and the substrate were also investigated. In addition,
piezoelectric energy harvesters using KNN and PZT thin film were fabricated and characterized
to compare the performance of the KNN thin film with that of PZT film. A number of these
studies also investigated the effect of argon gas, carbon, and hydrogen concentrations within the
KNN thin film. These elements are inherent in the film due to the nature of sputtering technique
which requires argon gas and/or a target material which contains carbon and hydrogen elements.
Therefore, understanding the effect of all these elements and process parameters is essential to
fabricate high−quality KNN thin film.
Lee et al. [27] prepared KNN on a Pt/Ti/SiO2/Si substrate using RF magnetron sputtering.
The substrate temperature was heated up to 600 ºC and then the deposited thin film was annealed
at 700 ºC for an hour in an oxygen atmosphere. The effective piezoelectric coefficient (d33) was
estimated to be 45 pm/V using piezoresponse force microscopy (PFM). Patanapreechachi et al.
[33] optimized the KNN thin film composition by the deposition of KNN with a composition
gradient on a Pt/Ti/SiO2/Si substrate by using multi−target magnetron sputtering. KNbO3 and
NaNbO3 target materials were used. The obtained K/(K+Na) ratios range from 0.05 to 0.95,
when the substrate temperature was 600 °C. The chemical composition measured through energy
15
dispersive X−ray spectroscopy (EDX) confirmed the composition gradient of K/(K+Na) ratio of
the KNN thin film along the line between the two target materials.
Nili et al. [34] studied the alkali loss in the KNN thin film. The deposition parameters
including the oxygen partial pressure and the substrate temperature, as well as the
post−annealing conditions were examined against the volatility of the alkali metals in the film. It
was shown that these parameters have high impact on controlling the alkali concentration in the
KNN thin film.
Wakasa et al. [35] fabricated Si micro−cantilevers using silicon on insulator (SOI) wafers
and then deposited KNN on the cantilevers through RF magnetron sputtering. The transverse
piezoelectric coefficient d31 was determined from tip deflection measurements of the cantilevers
subject to KNN thin film actuation. The d31 was estimated to be 53.5 pm/V.
Shibata et al. [28] deposited KNN on Pt/MgO and Pt/Ti/SiO2/Si substrates through RF
magnetron sputtering. Tip deflections of Pt/MgO and Pt/Ti/SiO2/Si unimorph cantilevers subject
to KNN thin film actuation were measured to determine the effective piezoelectric transverse
coefficients (e31*=d31/S11
E, where d31 is the piezoelectric transverse coefficient and S11
E is the
elastic compliance), which were reported to be −3.6 and −5.5 C/m2, respectively. The deposition
was conducted in Ar/O2 mixed gas and the substrate was heated to 550 ºC. Shibata et al. [36]
also investigated the effect of the annealing process after sputtering and the effect of the sodium
potassium ratio on the piezoelectricity and performance of the deposited KNN. It was found that
annealing at 750 °C in air and a ratio of Na/(K+Na) of 0.55 result in the best piezoelectric
properties. The piezoelectric transverse coefficient (e31*) was estimated to be in a range of −10.0
to −14.4 C/m2. The d31 coefficient was estimated to be in the range of −96.3 to −138.2 pm/V.
16
Kim et al. [37] studied the effect of the annealing treatment on the quality of the KNN thin film
deposited by RF magnetron sputtering. KNN thin film was amorphously developed at a low
deposition temperature (300 °C) and then it was annealed at 800 °C under Na2O, K2O, and KNN
atmospheres. It was found that annealing amorphous KNN thin film in KNN atmosphere leads to
the best electric and piezoelectric properties among the cases studied. KNN piezoelectric thin
film with a low leakage current of 2.6×10-9
A/cm2 at an electric field of 200 kV/cm, dielectric
constant of 620, remnant polarization of 11.7 µC/cm2 and coercive electric field of 133.8 kV/cm,
and an effective d33 coefficient of 74 pm/V at 50 kV/cm was obtained.
KNN thin film was deposited on SrRuO3/Pt/MgO substrates by RF magnetron sputtering in
[38]. The SrRuO3 layer was used as a buffer layer to improve the epitaxial growth of the KNN
thin film. Epitaxial growth is the process of growing crystal of one substance on the crystal face
of another substance such that both substances have the same crystal orientation. The effective
piezoelectric transverse coefficient e31* was calculated to be −2.4 C/m
2 when the concentration of
potassium with respect to sodium and potassium was 0.16. The values were determined based on
tip deflection measurements. The substrate temperature was 580~650 ºC during the sputtering.
The thin film was deposited in an Ar/O2 mixed gas without a post−annealing process. It was
found that the concentration of potassium in KNN being at 0.16 resulted in larger piezoelectric
properties in comparison with that of KNN without potassium.
Li et al. [39,40] investigated the effect of implementing lanthanum nickel oxide (LaNiO3)
(LNO) as electrodes for KNN thin film. LNO bottom electrodes were deposited on silicon at 450
ºC and then annealed at 600 ºC. KNN was deposited in an Ar/O2 mixed gas while the substrate
temperature was 550 ºC. The deposition process was also done by RF magnetron sputtering. The
deposited KNN thin film was subsequently annealed at 750 ºC. It was found that using an LNO
17
top electrode improves the dielectric permittivity and piezoelectric coefficient in comparison to
those using platinum as a top electrode. However, the piezoelectric properties were not stable
while it was used and the temperature was increased. The leakage current density of the
fabricated Pt/KNN/LNO film was about 6×10-8
A/cm2 at an electric field of 50 kV/cm. Li et al.
[41] deposited KNN thin film on SrRuO3/SrTiO3(001) single crystal substrates by RF magnetron
sputtering. The remnant polarization was estimated to be 8 µC/cm2 and coercive field of 40
kV/cm. The leakage current density was estimated as 3.48×10-6
A/cm2 at an electric field of 250
kV/cm. Also, the fabricated film exhibited low−fatigue behavior. The piezoelectric coefficient
(d33) was evaluated to be 36 pm/V.
Kanno et al. [42] compared the power generation performance of KNN thin film with that
of PZT thin film. KNN and PZT piezoelectric thin film were developed on Pt/Ti/Si through RF
magnetron sputtering. The performance of the fabricated KNN and PZT thin films was evaluated
based on simple unimorph cantilevers of KNN/Si and PZT/Si, respectively. The dielectric
constant of the fabricated KNN and PZT thin film was evaluated to be 744 and 872, respectively.
The effective piezoelectric coefficient e31* (e31
*=d31/(S11
E+ S12
E) where S11
E and S12
E are the
elastic compliances of the piezoelectric thin film was calculated to be around −11 C/m2 for both
the fabricated KNN and PZT thin films. The averaged output power of KNN and PZT was
estimated to be 1.1 µW and 1.0 µW, respectively. This indicates that the performance of KNN
thin film as an energy harvester is comparable to that of PZT films. Minah et al. [43] fabricated a
KNN thin film based piezoelectric energy harvester by using a bulk micromachining technique.
In this technique, KNN was deposited on a Pt/Ti/SiO2/Si substrate through RF magnetron
sputtering. Then, the roof mass of the harvester was patterned through dry etching. The obtained
results showed that the normalized power density of the piezoelectric energy harvester fabricated
18
through bulk micromachining was improved in comparison with that of a non−micromachined
one. KNN thin film was deposited through RF magnetron sputtering technique in both
micromachined and non−micromachined energy harvesters. Bulk micromachining processing
refers to the fabrication process of the proof mass, and non−micromachined one refers to
fabrication of the unimorph cantilever of KNN/Si [42].
Minh et al. [44] fabricated cantilevers based on KNN piezoelectric thin film by dry etching
and wet etching. Dry etching was applied through a fast atomic beam technique and the wet
etchant was 25% HF solution. Both etching techniques were successful to fabricate KNN
cantilevers in dimensions of 1000×120×2 µm3. It was shown that it is possible to fabricate
KNN/Si micro−cantilevers by either dry etching or wet etching. Kurokawa et al. [45] fabricated
KNN thin film on silicon micro−cantilevers with superior piezoelectric properties by dry etching.
The piezoelectric coefficient d31 was evaluated by measuring the tip deflection of the KNN
unimorph cantilever to be 99−219 pm/V. The high piezoelectric value was attributed to the
release of the internal stress of the KNN thin film by the etching of the silicon substrate.
Patents
Shibata et al. [46] developed (K1-xNax)(NbO3) (0.4<x<0.7) on Pt/Ti/SiO2/Si substrate. High
piezoelectric properties were obtained when the average crystal grain diameter was between 0.1
and 1 µm in the plane direction of the substrate. The specified range of the grain size is because
the thickness of the piezoelectric thin film is normally between 2 to 5 µm. Therefore, if the grain
size is greater than 1 µm, pin holes will be formed and thus leakage current flows through the
film. If the thickness of the film is less than 1 µm, high piezoelectric film cannot be obtained.
The desired grain size was achieved through optimization of the KNN sputtering conditions
19
including sputtering power, chamber pressure, and atmosphere gas concentration. The effective
piezoelectric coefficient (d31) was estimated to be 130 pm/V based on the tip deflection measured
through the use of a vibrometer. The sputtering conditions for KNN were set at 600 °C, 100 W,
Ar gas, 0.05 Pa, and 2 hours and 30 minutes.
Shibata et al. [47] investigated the effects of the Na and K compositions in (K,Na)NbO3.
The outcome of the research lead to that in (Kl_xNax)yNbO3, the composition ratios x (x is the
weight percentage of Na), y (y is the weight percentage of K+Na) should be in a range of
0.4≤x≤0.7 and 0.7≤y≤0.94 in order to obtain a film with high piezoelectric properties with the
coefficient d31 greater than 90 pm/V. Also, the leakage current is remarkably increased when the
(K+Na)/Nb ratio is smaller than 0.7. To fabricate a KNN thin film with an (K+Na)/Nb ratio of
less than 1, KNN target material with a smaller (K+Na)/Nb ratio is used or KNN is deposited at a
higher temperature (e.g. 800 °C). The leakage current density was less than 1×10-7
A/cm2 with
an applied electric field of 50 kV/cm.
Shibata et al. [48] investigated the effect of the difference in the thermal coefficients of the
substrate and the KNN piezoelectric thin film. A mismatch in thermal coefficients leads to
generating a compressive or tensile stress in the piezoelectric thin film. This stress leads to a
warping and thus degrades the piezoelectric performance of the thin film. The degradation is
more pronounced when the piezoelectric thin film is operated for a long time. The degradation
was reduced by reducing the KNN sputtering temperature from 680 °C down to 540 °C. As a
result, the decreasing rate of the coefficient d31 after 1,000,000,000 times of bending the
fabricated KNN/Pt/Ti/SiO2/Si cantilever was decreased from 7.4% to 3.3% (The decreasing rate
of the d31 = (the initial d31 – the post drive d31)/the initial d31 ×100%). Also, the decreasing rate of
the d31 can be reduced by having a substrate with a thermal expansion coefficient close to that of
20
the KNN thin film. It is worth pointing out that when the piezoelectric thin film thermal
expansion coefficient is less than that of the substrate, a tensile stress is generated in the
piezoelectric layer and thus the shape of the piezoelectric thin film deposited on the substrate is
convex downwards and vice versa.
Shibata et al. [49,50,51,52] developed KNN/Pt/Ti/SiO2/Si piezoelectric thin film on silicon
substrate with high piezoelectric coefficient d31 under low applied electric field (e. g. the absolute
value of [d31 at electric field of 70 kV/cm – d31 at electric field of 7 kV/cm] / d31 at electric field
of 7 kV/cm is less than or equal to 0.2). The piezoelectric thin film contains a pseudocubic or
tetragonal polycrystalline thin film. High−quality piezoelectric thin film was achieved by
forming KNN thin film preferentially oriented in the (001) direction with an occupation ratio of
80% in the X−ray diffraction (XRD) measurements to the surface of the film (Occupation ratio
of (001) direction = the intensity of (001) in XRD pattern / (the intensity of (001) direction in
XRD pattern + intensity of (110) direction in XRD pattern) × 100%). KNN with a stronger (001)
orientation preference can be obtained by using a Pt bottom electrode which is highly
preferentially oriented in the (111) direction. It should be mentioned that this is not an epitaxial
growth. In other words, the crystal orientation of the KNN layer does not have to follow the
crystal orientation of the layer beneath it (Pt bottom electrode). For Pt, the (111) crystal plane is
the thermodynamically stable configuration because Pt has the largest packing density in the
(111) crystal orientation, which leads to the smallest surface energy [53]. Highly (001) oriented
KNN can be also achieved by interposing an orientation control layer between the KNN thin film
and the Pt bottom electrode, such as an LaNiO3, NaNbO3, or (Kl-xNax)NbO3 (0<x<l) layer having
a composition ratio x,.
21
Shibata et al. [54] investigated the effect of lattice constants on the piezoelectric properties
of KNN thin film deposited on silicon substrate. It was found that high piezoelectric properties
can be obtained when the ratio of an out−of−plane directional lattice constant to an in−plane
directional lattice constant of the thin film is in a range of 0.98 to 1.01. This ratio leads to
minimize the stress generated in the piezoelectric thin film. The stress is mainly added by the
difference of the thermal expansion coefficients between the KNN film and the silicon substrate.
The magnitude of the added stress can be controlled by changing the orientation state of the
KNN film, the Na/(Na+K) composition, the deposition temperature, and by conducting a
post−annealing treatment.
Sakuma et al. [55] developed KNN piezoelectric thin film with a high coercive electric
field. The coercive electric field is the minimum electric field applied on a piezoelectric material
that leads to reverse the polarization direction of the material. This was achieved when the
piezoelectric thin film contains a rare gas element and has a content gradient of the rare gas
element in the thickness direction of the piezoelectric thin film. High piezoelectric properties of
KNN thin film can be realized when the rare gas element content has a minimum value of about
5 atomic % or less in one of the electrode layer side of the film and a maximum value of about
10−15 %. This can be achieved by changing the deposition conditions during the KNN
sputtering. Maejima et al. [56] reduced the leakage current of the KNN thin film by having an
average crystal grain diameter between 60 and 90 nm. KNN thin film with a low leakage current
density as 1×10-6
A/cm2 or less and a d31 of 70 pm/V or more was obtained by doping the KNN
film with Mn in a range of 0.1 to 3.0 atomic %.
Suenaga et al. [57] developed KNN piezoelectric thin film doped with Li according to a
the formula (NaxKyLiz)NbO3 (0≤x≤1,0≤y≤1,0≤z≤0.2, x+y+z=1), where x, y, and z are the weigh
22
percentages for Na, K, and Li, respectively. The developed piezoelectric thin film has a crystal
structure of a pseudocubic crystal. It is preferentially oriented in the (001) direction with a
volume fraction of the component (001) with respect to components (001) and (111) falling
within a range of 60−100 %. Piezoelectric constants depend on the (111) and (001) volume
fractions. As the (111) volume faction is increased, the piezoelectric constant is increased.
However, when the (111) volume fraction exceeds 20%, it is found that the piezoelectric
constant is reduced. The piezoelectric constant is increased with the increase of the (001)
orientation component. However, when the (001) volume fraction is 80% or more, there is a
tendency that the piezoelectric constant is reduced. The total of the (001) and (111) volume
fractions is assumed to be 100%. Suenaga et al. [58] discussed the effect of the inert gas content
contained in the KNN piezoelectric thin film. The piezoelectricity performance of KNN thin film
can be enhanced by containing the inert gas (Ar) between 30 ppm and 70 ppm. Therefore, high
piezoelectric thin film can be realized by controlling the inert gas element content in the film.
Shibata et al. [59] studied the influence of carbon and hydrogen concentrations in the KNN
thin film on the dielectric loss of the film. It was shown that when the carbon concentration of
the piezoelectric thin film is 2×1019
/cm3 or less, or when the hydrogen concentration of the
piezoelectric thin film is 4×1019
/cm3 or less, a dielectric loss of 0.1 or less occurs. This low
dielectric loss is required to use the KNN thin film in an inkjet printer. The source of the carbon
is the carbon contained in a KNN sintered target. The KNN sintered target is usually formed
through a process of a mixture using K2CO3, N2CO3, and Nb2O5 powder as raw materials. Most
of the carbon in the raw materials is removed in a sintering step due to the high temperature; but,
a small part of the carbon remains in the KNN sintered compact. The concentration of the carbon
can be controlled by reducing the carbon concentration in the sintered target, increasing the ratio
23
of O2 in atmosphere gas during film formation, or applying heat treatment in oxygen atmosphere
after formation of the KNN thin film.
Shibata et al. [60] studied the quality of the fabricated KNN thin film through rocking
curves by X−ray diffraction measurements. The full width at half maximum intensity (FWHM),
which can be revealed from the rocking curves, is related to the dislocation density in the film.
The leakage current was found to be large when the half width of the rocking curve of the KNN
(001) plane was smaller than 0.5°. The deterioration rate of the piezoelectric constant was also
found to be small when the half width was larger than 2.5°.
It can be concluded that the fabrication of high−quality KNN thin film is a challenge as
there are many factors that need to be considered. The quality of the fabricated KNN thin film
needs to be determined through a variety of characterization tools. These are discussed next.
2.5 Characterization Methods for Piezoelectric Thin Film
In this work, KNN was deposited on silicon substrates through RF magnetron sputtering
and then the samples were post−annealed under different conditions. The fabricated samples
were subsequently characterized. Based on the characterization results, the next round of the
fabrication process was conducted. This was repeated many times to improve the quality of the
produced film. The accuracy of the characterization methods is essential to enable the
development of a useful and reliable fabrication process. The characterization methods of the
fabricated KNN thin film include measurement leading to characterization of crystal orientation,
chemical compositions, polarization hysteresis loop, dielectric constant, leakage current density,
effective piezoelectric coefficients d31 and d33, and four−point resistivity measurements.
24
2.5.1 Crystal Orientation
Crystal orientation of the fabricated KNN thin film needs to be examined. XRD is used to
identify the atomic and molecular structure of a crystal. When an X−ray is penetrated through a
specimen surface, the crystalline atoms cause a beam of incident X−ray to diffract into different
specific directions. By measuring the angles and intensities of these diffracted beams, the crystal
structure of a material can be identified.
XRD pattern is generated on an XY plot. The X−axis of the plot contains angles (2θ) and
the intensity of the diffracted beam (counts) is plotted on the Y−axis. Each crystal material has a
unique XRD pattern (International Centre for Diffraction Data (ICDD)) [61]. By comparing the
generated XRD pattern of a material with that of the corresponding ICDD standard, the crystal
orientation of the material can be identified. The tool used in this work is the Philips XRD
system whose basic components are a PW 1830 HT generator, a PW 1050 goniometer, PW3710
control electronics, and an X−Pert system software.
2.5.2 Chemical Compositions
The piezoelectricity of KNN thin film is a function of K, Na, Nb, and O chemical
compositions. The chemical concentration of the film is analyzed through energy dispersive
X−ray spectroscopy (EDX) and X−ray photoelectron spectrometer (XPS).
The function of an EDX is based on stimulating the sample with uniform energy through
an electron beam. In this way, each element in the sample reflects X−ray of specific energies.
This reflected X−ray provides information about the element composition of the sample. The
electron beam in a scanning electron microscope (SEM) is used. Therefore, EDX is combined
25
with an SEM. SEM is an imaging tool based on scanning the sample by a focused beam of
electrons. These electrons carry significant amounts of kinetic energy. When the electrons
interact with the atoms in the sample, they produce different signals that can be used to generate
an image of the sample surface. The SEM used in this work is an JEOL JSM6610−Lv,
complemented by an Oxford/SDD EDS detector (ultra−thin window) allowing for X−ray
microanalysis and digital imaging via SE, BSE, and X−ray signals.
Matrix effects, which come from components in the sample other than the ones of interest,
are pronounced in EDX especially for thinner layers (e. g. electrode layer). Therefore, XPS in
this work is dedicated to study the interface between the piezoelectric layer and electrode layers
as well as the chemical compositions of the electrode layer. The XPS results are used to verify
the results obtained by EDX. The XPS technique is based on analyzing the photoemission of
electrons from the sample surface due to the matter of an X−ray. This information is used to
obtain the binding energy of the emitted electrons and thus to identify the chemical elements.
XPS analysis is performed using a Thermo Scientific K−Alpha [62].
2.5.3 Polarization Hysteresis Loop
The polarization versus applied electric field hysteresis (P−E) loop is used to examine the
ferroelectric behavior of the fabricated KNN thin film. As mentioned earlier, ferroelectric
materials are a special class of piezoelectrics, and KNN piezoelectric is a ferroelectric material.
The dielectric polarization is defined as the dipole moment per unit volume, which describes the
behavior of a material under an applied electric field.
In this work, the P−E loop of the fabricated KNN thin film is measured using the classis
Sawyer−Tower circuit [38]. The classic Sawyer−Tower circuit includes placing a reference
26
capacitor (Cr) in series with the piezoelectric sample then an alternating current (AC) signal is
applied to the circuit as shown in Figure 2.5. The principle of this technique is based on the fact
that the charge across the reference capacitor is the same as the charge across the piezoelectric
sample as both capacitors (Cr and the piezoelectric capacitor (CPiezo)) are connected in series. The
charge (Q) on the reference capacitor can be calculated by multiply the measured voltage (V)
across the reference capacitor with the capacitance (C) of the same capacitor (Q=V×C). The
polarization (P) of the piezoelectric sample is then calculated by dividing the charge by the area
of the electrode (P=Q/A).
Selecting the value of Cr is a challenge. If Cr is selected to be small, then the equivalent
impedance is high (the impedance Cr is not negligible). This results in large voltage drop across
Cr, which eventually leads to more accurate readings. However, if the impedance is much larger
than that of the piezoelectric sample, the impedance of the piezoelectric sample is negligible. If
Cr is set to be large, then the equivalent impedance is low (impedance of Cr is negligible). This
results in low voltage drop across Cr, which eventually leads to less accurate readings. As a rule
for measurement, the value of Cr should be larger than 10 times that of CPiezo.
AC
Vx
Vy
Piezo
Cr
Figure 2.5. A schematic diagram of the classic Sawyer−Tower circuit.
27
2.5.4 Dielectric Constant
The dielectric constant, which is also called relative permittivity (εr), indicates the ability
of a material to store energy. Permittivity is composed of real and imaginary parts. The real part
represents how much energy from an external applied field is stored in a material while the
imaginary part represents the dissipation of energy in the material. Piezoelectrics can be modeled
as a resistor and capacitor (Cp) in parallel as shown in Figure 2.6 [1], where G represents the
conductivity (G=1/R). Therefore, the equivalent impedance can be represented as G+(1/jωCp),
where ω represents the operating frequency. The current passing through the piezoelectric
sample (I) is then defined as follows:
)( GCjVIII PRC (2.1)
where IC and IR represent the currents passing through the capacitance and resistance,
respectively.
The constitutive equation for a capacitor is:
rr C
t
AC
0
0 (2.2)
where ε0 represents the permittivity of free space (8.85×10-12
F/m), A is the surface area of the
top electrode, t is the piezoelectric thickness, and C0 represents the capacitance of an equivalent
size of free space.
Let’s define Y as Y=G+jωCp, so:
28
00
0C
Gj
C
CCjY P
(2.3)
Then, the complex relative permittivity is defined as:
"'
00
rrP
r jC
Gj
C
C
(2.4)
So, the real part of the permittivity is:
A
tC
tA
C
C
C P
r
PPr
00
'
(2.5)
The real part is referred to as the relative dielectric constant. And the imaginary part is:
AR
t
RCC
Gr
000
" 1
(2.6)
Piezo
Cp
G
Figure 2.6. Electrical circuit model of a piezoelectric sample.
29
The loss tangent can be defined as the tangent of the phase angle between the real and
imaginary parts of the permittivity, as follows:
'
"
)tan(r
r
(2.7)
Therefore, permittivity and the loss tangent can be extracted by measuring the equivalent
capacitor and resistor of the piezoelectric sample using an impedance analyzer. Permittivity and
loss tangent are usually determined to indicate the ability of a piezoelectric sample to store
electrical energy in an electric field. In this work, two impedance analyzers were used: the
Agilent 4294A and the Keysight E4990A.
2.5.5 Leakage Current Density
The leakage current density represents the electrical current passing through a unit area of
a cross section of the sample. The top electrode area is usually considered for the calculation.
The leakage current density of the thin films as a function of applied electric field was measured
using a semiconductor parameter analyzer (HP 4155A Semiconductor). A staircase−shaped
direct current (DC) bias voltage, with a 0.5 V step and 2 seconds span, was applied to the
fabricated thin films. The current across the piezoelectric samples was also measured by placing
a low noise current preamplifier instrument (SR570 current preamplifier) in series with the piezo
sample. Then, using the oscilloscope (Tekronix DPO 3014) to monitor and record the data from
the current instrument.
30
2.5.6 Piezoelectric Coefficients (d33 and d31)
The inverse piezoelectric effect is used to measure the longitudinal piezoelectric coefficient
d33. The coefficient d33 is related to the change in the strain in the third direction when an electric
field is applied across the thin film (also third direction), according to the following:
TE
Sd
3
333
(2.8)
where E is the applied electric field and S is the corresponding strain. The subscript 3 indicates
the direction of the piezoelectric polarization.
Since the piezoelectric thin film is clamped on a substrate and the effect of the substrate
cannot be excluded, the ratio S3/E3 represents the effective piezoelectric coefficient instead of the
real one just limited to the piezoelectric thin film. The real coefficient limited to the piezo thin
film is related to the effective coefficient according to the following relation [63]:
EE
E
SS
Sddd
effec
1211
13313333
2. (2.9)
where S13, S12, and S11 are the mechanical compliances of the piezoelectric film and d31 is the
transverse piezoelectric coefficient. S12, S13, and d31 are usually negative and S11 is positive and
larger than S12. Therefore, the value of the effective d33 is underestimated in comparison to that
of the real coefficient [63,64]. Since the elongation in the third direction is very small (picometer
range), PFM is usually used to detect the elongation. In PFM, the potential is applied on the top
electrode through the tip of the PFM probe while the bottom electrode is grounded. PFM consists
of an AFM system and a lock−in amplifier. The system used in this thesis is the Bruker Bioscope
31
catalyst AFM with a lock−in amplifier. Prior to the measurement of the fabricated thin films, the
PFM cantilever was calibrated using a standard sample (a piezoelectric periodically−poled
lithium niobate sample).
The transverse piezoelectric coefficient d31 for piezoelectric thin film was determined as
outlined in the section entitled: Modeling of Piezoelectric Thin Film Actuator (next section). The
tip deflections due to the piezoelectric thin film subject to a potential are experimentally
measured using a vibrometer. Then, the tip deflection piezoelectric heterogeneous unimorph
equation is used to determine the d31 [66]. Similar to the d33 measurements, the estimated d31 is
referred as the effective d31 as the effect of the substrate on the measurements cannot be
removed.
2.5.7 Electrical Conductivity of the Bottom Electrodes
The conductivity of the electrode layer must be tested after the fabrication of the
piezoelectric thin film. A convenient electrical resistivity measurement method uses two probes
to measure the resistivity between any two points on a sample. However, this method leads to
incorrect results in the case of small resistivity values such as the case when the resistivity of a
metal film (bottom electrode layer) is measured. In this case, the influence of the contact
resistance between the probe and the sample is significant. Therefore, the resistivity for a metal
film is measured through four−point resistivity measurement [65]. The schematic diagram of the
measurement setup is shown in Figure 2.7. The measurement is executed by passing an electrical
current through the outer probes and measuring the voltage through the inner probes. By
measuring the voltage and current, the sheet resistance (the thin film layer in Figure 2.7) can be
estimated as follows:
32
I
V
I
VS 5234.4
2ln
(2.10)
where V and I are the measured voltage and current, respectively. The voltage is measured by a
voltmeter placed in the inner loop and the current is measured by an ammeter placed in the outer
loop as shown in Figure 2.7. Strictly speaking, the unit of the thin film resistance is Ω. However,
the unit Ω/square is used to distinguish between thin film (sheet) resistance and bulk resistance.
Square represents a square sheet at which width equals to length. The resistivity (ρ) can be
estimated by multiplying the sheet resistance by the thickness of the tested film.
tI
V
2ln
(2.11)
where t is the thickness of the tested film. The unit of the resistivity is Ω.m.
It can be seen that the thickness of the tested film needs to be accurately measured to
estimate the resistivity. In order to estimate the resistivity of a metal film, the metal film should
be deposited on an insulating layer such as silicon oxide. However, the metal film was directly
Substrate
Thin film
DC
A
V
sssd d
t
Figure 2.7. A schematic diagram showing the four point resistivity measurement setup.
33
deposited on a silicon substrate in some cases during this work. Therefore, the sheet resistance is
a better quantity to represent the resistivity of the film.
The following assumptions need to be met in order to use the four−point measurement
method [65]:
1) The spacing between the probes (s) should be at least 4 times thickness (t) of the film
2) The distance between the probe and the edge of the sample (d) should be at least 4 times
of the spacing (s)
This work focuses on developing and using KNN thin film as an actuator, which leads to
developing KNN/Si unimorph cantilevers. So, modeling of piezoelectric unimorph cantilevers is
discussed next and is pursued prior to any fabrication, prototyping and characterization attempts.
2.6 Modeling of Piezoelectric Thin Film Actuators
Piezoelectric thin film actuators typically consist of a bottom electrode, piezoelectric
material, and a top electrode. The most commonly used example of a piezoelectric thin film
actuator is the unimorph cantilever, which is basically a piezoelectric thin film deposited on an
elastic cantilever such as one made of silicon as shown in Figure 2.8.
When a voltage is applied across the electrodes, an electric field is generated parallel to the
polarizations of the piezoelectric material but in the opposite direction. Due to the properties of
piezoelectric material, the piezoelectric material will expands in the in−plane direction and
contracts in the out−of−plane direction. Since the piezoelectric element is attached to the upper
surface of the cantilever, reaction forces will be generated at the interface between the
piezoelectric thin film and the cantilever opposing the expansion of the piezoelectric element. As
34
a result, the structure including the cantilever and the piezo element bends down and an
out−of−plane displacement is generated. The direction of the displacement can be changed by
switching the polarity of the applied voltage.
In this structure, the piezoelectric thin film is deposited on an elastic layer and thus the
structure has only one active layer (i.e. the piezoelectric layer). Therefore, the structure can be
modeled as a piezoelectric heterogeneous bimorph. The analytical model of the tip deflection of
piezoelectric heterogeneous bimorphs is given in [66] as follows:
VK
LhhhSSd pss
ps 2
111131 )(3 (2.12)
where δ is the tip deflection under the actuation of the piezoelectric layer, d31 is the transverse
piezoelectric strain coefficient,sS11 and
pS11 are the compliance under constant mechanical stress of
the substrate and the thin film, respectively, hs and hp are the thicknesses of the substrate and the
thin film, respectively, V is the applied voltage between the bottom and top electrodes of the thin
film, L is the length of the cantilever that been actuated, and K is defined as follows:
Figure 2.8. Schematic diagram of a piezoelectric unimorph cantilever.
35
(2.13)
It can be observed that the analytical solution depends mainly on the thickness of the
layers, the elastic coefficient of the layers, and d31. The analytical model was developed based on
the following assumptions:
1. The beam is free to expand vertically, thus no strain is developed in the thickness
direction (Z−axis). So the stress in the Z−axis is considered to be zero.
2. The beam is considered to be long and slender, thus no strain is developed along the
Y−axis.
3. Shear effects are negligible (T4,5,6 = 0).
4. The cross section of the beam remains plane and perpendicular to the X−axis.
5. A constant curvature is generated throughout the beam.
6. The Poisson’s ratio is isotropic for the two layers.
In order to calculate the tip deflection under the piezoelectric thin film actuation, the piezo
strain coefficient (d31) has to be known. As mentioned earlier, the coefficient d31 for piezoelectric
thin film is different than that of piezoelectric bulk ceramics. The coefficient d31 for piezoelectric
thin film is determined as follows. The tip deflection under the actuation of piezoelectric thin
film is experimentally measured using an optical system such as a laser Doppler vibrometer.
Then, the tip deflection piezoelectric heterogeneous unimorph equation is applied to determine
the d31. The elastic properties of the piezo layer as well as the elastic layer are assumed to be
similar to those of the bulk forms. Moreover, the measured tip deflection depends on the internal
stress in the cantilever. Therefore, the estimated d31 is referred as the effective d31. Hereafter, the
d31 refers to the effective d31 unless it is specified otherwise.
2
11
4
11
4
11
2
11
22
1111
3
1111
3
1111 )()()()()()(6)(4)(4 spsp
ps
sp
ps
sp
ps
sp SSSShhSShhSShhSSK
36
2.7 Summary
In this chapter, KNN piezoelectric thin film was introduced. Starting from the well−known
PZT piezoelectric material, the piezoelectric behavior of KNN thin film was presented. State of
the art for KNN thin film fabrication and properties were listed. In order to accurately determine
the electric and piezoelectric properties of the fabricated KNN thin film, the characterization
methods need to be well−established. These methods were also presented in this chapter.
37
3 Fabrication of KNN Thin Film on Nickel−based Electrodes
The continued trend towards miniature actuators and sensors has motivated the
development of piezoelectric thin film. As mentioned earlier, a piezoelectric thin film is a
piezoelectric layer sandwiched between two electrodes. Nobel metals are used as electrode
materials for piezoelectric thin film due to their good electrical conductivity and
high−temperature oxidation resistance. High temperature treatment in the presence of oxygen gas
is done while fabricating piezoelectric thin film during the deposition and annealing processes
[27,28]. Pt is the most commonly used electrode material for KNN thin film. Using other noble
metals, such as gold, or an oxide conductive layer was reported and lead to developing
high−quality KNN thin film too [54,67]. Using noble metals in piezoelectric thin film fabrication
increases the cost of these films. Replacing these materials with base metals is of interest to
reduce the fabrication cost and thus enable their use in many new applications.
Using a base metal as an electrode material for KNN thin film has never been reported. For
lead−based piezoelectric thin film, PZT was successfully deposited on copper electrodes [68].
The concentration of oxygen in the processing environment was optimized to avoid the oxidation
of the copper while achieving high−quality PZT piezoelectric thin film.
Nickel (base metal) was successfully implemented as inner electrode material for KNN
multilayer ceramics [69]. However, nickel has not been used as an electrode material for KNN
thin film. Dawley et al. [70] fabricated (Ba,Sr)TiO3 (BST) capacitors on nickel tapes using a
chemical solution deposition technique. The pressure of oxygen during the annealing treatment
was optimized to prevent the oxidation of the nickel bottom electrode while crystallizing the
BST films. Valladares et al. [71] investigated the oxidation of nickel thin film on SiO2/Si
38
substrates in air. The deposited nickel thin film samples were annealed at different temperatures
up to 700 °C for 3 hours in air. It was found that the nickel layer was oxidized when the
annealing temperature was higher than 350 °C.
In this work, nickel is used as an electrode material for KNN thin film. Two types of
nickel−based bottom electrodes were investigated. The first was nickel silicide bottom electrode.
The second was a hybrid layer consisting of both nickel silicide and pure nickel. The electric and
piezoelectric properties of the KNN thin film in both cases were measured and compared with
those previously reported for KNN thin film. The KNN thin film on a nickel silicide electrode is
presented next.
3.1 Fabrication of KNN on Nickel Silicide Bottom Electrode
The KNN thin film presented in this section includes depositing KNN on nickel silicide
bottom electrodes. KNN thin film was deposited on Ni‒coated silicon wafers at an elevated
temperature, followed by annealing of the sample. The chemical compositions and the electrical
resistivity of the bottom electrode were analyzed. The fabricated KNN thin film was fully
characterized. The characterization includes the crystal orientation, the chemical compositions,
the dielectric constant and loss tangent, the ferroelectric polarization versus the electric field
hysteresis loop, the leakage current density of the thin films as a function of the applied electric
field, and the effective piezoelectric coefficient d33 of the sample.
3.1.1 Fabrication Process
K0.35Na0.65NbO3 target material (99.9% purity, 3″ disk) was loaded into the sputtering
machine (AJA International System). A Si wafer substrate was also loaded in the sputtering
machine. A thin titanium layer (2 nm thickness) was deposited on the Si wafer as an adhesive
39
layer followed by a deposition of a nickel layer (200 nm thickness). This was all done at room
temperature in the presence of pure argon gas.
The next step was to deposit the KNN by RF magnetron sputtering. The deposition
parameters (i. e. substrate temperature, gas concentration, and sputtering power) play an
important role in determining the piezoelectricity of the deposited KNN thin film. The effects of
some of these parameters were investigated by other researchers [28,34]. The deposition
parameters were further optimized in this work because of the use of different electrode material
as well as other fabrication variability inherent to the use of a different sputtering system. It was
first confirmed that the high temperature during the deposition is required to grow the
piezoelectric KNN on the substrate. Initially, the KNN was deposited at room temperature;
however, the XRD pattern of the corresponding deposited KNN indicates the formation of an
amorphous structure of the deposited thin film instead of the crystal one. Therefore, the
temperature of the substrate was increased up to 600 °C during the KNN deposition. The
concentration of the oxygen into the chamber during the deposition process was investigated and
it was confirmed that the oxygen has to be introduced during the process to replace the oxygen
loss from the target material during sputtering. The concentration of the oxygen in the chamber
was set to be 17%. Also, the sputtering power was examined; an initial power level of 50 W was
applied. However, the deposition rate was very slow. It was found that as the power level
increases, the deposition rate increases. Therefore, the power was increased up to 200 W and the
corresponding deposition rate was approximately 100 nm/hr.
As mentioned earlier, the critical issue in using nickel as an electrode material for KNN
thin film is the oxidation of the nickel layer used as a bottom electrode. The nickel layer might
be oxidized due to the deposition of KNN at high temperature in the presence of oxygen gas
40
and/or during the annealing process which also occurs at high temperature. To avoid any
unexpected oxidation of the nickel bottom electrode while the substrate temperature was being
raised to 600 ºC, the introduction of oxygen into the chamber was delayed until the temperature
reached the target value. The required time to heat the substrate up to 600 ºC was approximately
200 seconds.
The annealing process is required to improve the crystallization as well as improve the
electrical properties of the deposited KNN thin film. The annealing time and the annealing
environment were experimentally investigated. Along with the conductivity of the nickel−based
layer used as a bottom layer, the crystal orientation of the KNN thin film was examined through
XRD patterns (Philips XRD, PW1830). The deposited KNN samples were annealed in vacuum
and air. Then, the crystal orientation of these samples along with as−deposited KNN sample was
characterized through XRD as shown in Figure 3.1. It is worth mentioning that the peaks at 2θ =
22.5° and 32° in the XRD pattern of the KNN thin film correspond to the (100) and (110) KNN
crystal orientations, respectively. It can be seen in Figure 3.1 that the KNN sample being
annealed in air resulted in the best improvement of the crystal orientation of the sample among
the three investigated conditions as sharp peaks represent crystal structure. This can be explained
that an oxygen environment is required to improve the oxidation of niobium toward a state that is
required for formation of KNN [34]. When the annealing time was increased from an hour to two
hours, the electrical conductivity of the annealed nickel layer deteriorated. The annealing
temperature was selected to be 750 ºC during the annealing process. Consequently, the selected
annealing treatment includes annealing the deposited KNN thin film in air by rapid thermal
annealing for an hour. It is worth pointing out that the broad peak at 2θ = 27° is contributed by
the silicon wafer. This was verified by generating the XRD pattern for the annealed Si wafer.
41
After examining the crystallographic of the fabricated KNN thin film, the bottom electrode was
investigated as presented next.
3.1.2 Characterization of the Bottom Electrode
Pictures of the fabricated KNN thin film at different stages are shown in Figure 3.2. The
color of the nickel bottom electrode was changed after the KNN deposition. Also, it can be
observed that the color of the nickel bottom layer further changed after the annealing process. To
gain insight into the modified nickel bottom electrode, the chemical depth profile was generated
as shown in Figure 3.3. The depth profile was generated through XPS. It can be seen that the
investigated layer contains nickel and silicon with similar concentration, and thus it indicates the
formation of a nickel silicide layer.
Figure 3.1. XRD patterns of annealed KNN thin film in air, annealed KNN thin film in vacuum,
and as−deposited KNN thin film.
20 25 30 35 40 45 50 55
2 (deg)
Inte
nsity (
cp
s)
KNN annealed in vacuum
KNN annealed in air
As-deposited KNN
42
The most important property of the fabricated nickel silicide layer used as a bottom
electrode for KNN piezoelectric thin film is the electrical conductivity. The electrical resistivity
of the nickel silicide layer samples was measured through the four−point resistance measurement
method [72]. The resistivity of the post−annealed nickel silicide layer (12.9×10−8
Ω.m) was
increased 1.3 times of that of the as−deposited nickel silicide layer (5.7×10−8
Ω.m). The
resistivity value was estimated based on the average of 4 measurements. The resistivity of an
annealed platinum thin film (27.0×10−8
Ω.m) was reported to be increased 0.4 times of that of the
as−deposited platinum (18.7×10−8
Ω.m) [72]. Next, the crystal orientation and chemical
characterizations of the fabricated KNN layer are investigated.
(a) (b) (c)
Figure 3.2. Pictures of fabricated KNN samples. (a) As−deposited Nickel bottom electrode. (b)
As−deposited KNN sample. (c) Post−annealed KNN sample.
(a) (b) (c)
Figure 3.
43
3.1.3 Crystal Structure and Chemical Compositions
The XRD pattern of the KNN sample is further analyzed as shown in Figure 3.4. It can be
seen that the fabricated KNN thin film consists of multiple crystallographic orientations. The
strongest peak at 2θ=22.5° indicates the (001)−oriented KNN thin film. Also, the XRD pattern
indicates the formation of the (110) crystal orientation in the sample. The thickness of the sample
was approximately 0.4 µm and thus the XRD pattern includes the peak contributed by the Si
substrate (peak position is at 2θ=33°). The peaks corresponding to the annealed nickel were
verified through the XRD pattern of an annealed Ni−coated silicon sample. The peak positions in
the XRD pattern corresponding to KNN are in agreement with those reported XRD patterns of
KNN thin film fabricated by RF magnetron sputtering [36,39].
(a) (b)
Figure 3.3. XPS depth profiles for nickel silicide bottom electrode. (a) After the KNN
deposition. (b) After the annealing process.
0 500 1000 1500 2000 2500 3000 3500 40000
10
20
30
40
50
60
70
80
90
100
Etch time (seconds)
Ato
mic
perc
enatg
e (
%)
Nickel
Silicon
0 500 1000 1500 2000 2500 3000 3500 40000
20
40
60
80
100
Etch time (seconds)
Ato
mic
pe
rcen
atg
e (
%)
Nickel
Silicon
44
Cross−section SEM images of the fabricated KNN/Ni/Ti/Si and KNN/Ni/Ti/SiO2/Si
samples are shown in Figure 3.5. The irregular thickness of the nickel layer in the as−deposited
KNN/Ni/Ti/Si sample (see Figure 3.5(a)) indicates that the nickel has diffused down into the
silicon. Also, a thin layer between the nickel electrode and the KNN thin film was formed
because the silicon might be diffused up to the nickel layer. This interdiffusion of the nickel and
silicon layers was due to the lack of a buffer layer between the nickel layer and the silicon
substrate. The buffer layer is usually grown between the piezoelectric thin film and the substrate
to prevent the interdiffusion of the metal electrode and the substrate. In order to confirm the
effectiveness of the buffer layer, silicon oxide as a buffer layer was thermally grown on the
silicon wafer. Then, the nickel layer was deposited on the Ti/SiO2/Si substrate followed by
deposition of the KNN layer. It can be seen in Figure 3.5(b) that the thin layer between the nickel
and KNN layers was not formed. The interdiffusion of the metal electrode and silicon substrate
due to the lack of the buffer layer was discussed in [73]. It is worth pointing out that titanium did
not appear in the SEM images as a very thin titanium layer (2 nm thickness) was deposited.
Figure 3.4. XRD pattern of the annealed KNN thin film.
20 25 30 35 40 45 50 55
2 (deg)
Inte
nsi
ty (
cps)
SiKNN
(001)
Annealed
Ni
Annealed
Ni
KNN
(002)KNN
(021)
KNN
(110)
45
The thin layer between the nickel and KNN layers became thicker when the KNN/Ni/Ti/Si
sample was annealed at 750 ºC for an hour in air as shown in Figure 3.5(c). Thus, the silicon has
further diffused into the nickel layer during the annealing treatment. Also, the SiO2 buffer layer
prevented the interdiffusion of the nickel electrode and KNN thin film during the annealing
treatment as can be seen in Figure 3.5(d).
The chemical composition of the fabricated KNN thin film was identified through EDX
(JOEL JSM 6610/LV SEM complemented by Oxford SDD detector). Cross−section SEM
images of the KNN/Ni/Ti/Si and KNN/Ni/Ti/SiO2/Si samples with EDX line scan elemental
mapping indicating Si, O, Ni, Nb, K, and Na elemental profiles are shown in Figure 3.6. The
trend of the silicon elemental profile in the KNN/Ni/Ti/SiO2/Si sample, which decreases as it
goes from the silicon layer to the KNN layer, does indicate that no interdiffusion layer has
formed. However, when the KNN/Ni/Ti/SiO2/Si sample was annealed, the nickel layer was
totally oxidized and thus the conductivity of the nickel layer deteriorated.
(a) (b) (c) (d)
Figure 3.5. SEM images of fabricated KNN thin films. (a) As−deposited KNN/Ni/Ti/Si
sample. (b) As−deposited KNN/Ni/Ti/SiO2/Si sample. (c) Annealed KNN/Ni/Ti/Si sample.
(d) Annealed KNN/Ni/Ti/SiO2/Si sample.
46
The ratio of Na/(Na+K) was estimated based on the EDX analysis to be 0.79 while the
ratio of the target material was reported from the manufacturer to be 0.65. The increase in the
ratio was due to the irregular loss of alkali metals during the sputtering process. It was reported
in [36] that a ratio of 0.65 in the target material leads to KNN with a ratio of 0.55, at which the
best piezoelectric properties can be obtained. Unfortunately, the loss of the alkali metals could
not be controlled [34]. Also, the ratio of alkali to niobium concentration ((Na+K)/Nb), which is
expected to be 1, was estimated to be 0.81. The loss of the alkali metals during the sputtering
process leads to a lower concentration of the alkali metals in comparison to that of niobium.
However, the concentration of the alkali metals in the fabricated KNN indicates that the loss of
potassium was much higher than that of sodium during the sputtering process.
In conclusion, the concentration of potassium in the fabricated KNN thin film was low and
a buffer layer was not used in the sample. Therefore, the fabricated KNN thin film is expected to
exhibit low piezoelectric behavior. This is further discussed in the next section.
(a) (b)
Figure 3.6. SEM images with EDX line scan elemental profiles for the fabricated KNN thin
film. (a) KNN/Ni/Ti/Si sample. (b) KNN/Ni/Ti/SiO2/Si sample.
47
3.1.4 Electric and Piezoelectric Properties
To measure the electric and piezoelectric properties of the fabricated KNN thin film, a
rectangular 150−nm−thick nickel electrode was deposited on the sample at room temperature to
serve as top electrode. Also, the thickness of the KNN thin film was set to 1 µm to avoid
unexpected shorts between the electrodes due to the pin holes. The properties measured include
dielectric constant and loss tangent, ferroelectric polarization and electric field hysteresis loop,
leakage current density, and the effective d33 coefficient.
The dielectric constant and loss tangent were measured as a function of frequency using an
impedance analyzer (Agilent 4294A). These are shown in Figure 3.7. It can be seen that the
dielectric constant for the deposited KNN thin film with nickel electrodes is 58.71 at 1 kHz,
which is lower than those reported by other research groups that fabricated KNN thin film
[32,39]. This lower value can be attributed to the lower concentration of potassium element in
Figure 3.7. Dielectric constant and loss tangent as a function of frequency for the fabricated
KNN thin film.
102
103
104
105
106
0
10
20
30
40
50
60
70
Frequency (Hz)
Die
lectr
ic c
on
stan
t
102
103
104
105
1060
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Lo
ss t
an
gen
t
48
the deposited KNN layer as well as the lack of the buffer layer between the nickel bottom
electrode and the silicon substrate. When the frequency increases above 20 kHz, the permittivity
is sharply reduced and the loss tangent is increased.
Ferroelectric polarization and electric field hysteresis loop of the deposited KNN was
measured using the classis Sawyer−Tower circuit [38]. The reference capacitor used in the
circuit was 68 µF. The polarization hysteresis loop is shown in Figure 3.8. The polarization
hysteresis loop was generated at an applied frequency of 22 kHz. While the figure is a typical
polarization hysteresis loop, the hysteresis loop was relatively round in shape indicating that the
dielectric loss of the film was high. This is consistent with the dielectric constant measurement
result which indicated a low dielectric constant value. The remnant polarization was 4.2 µC/cm2
and the maximum polarization was 5.8 µC/cm2. The coercive field was 150 kV/cm. These values
are comparable with those of KNN thin film fabricated by the sol−gel technique (remnant
polarization was 3.45 µC/cm2 and coercive field was 160 kV/cm) [74]. However, these values
are lower than the values of the KNN fabricated using RF magnetron sputtering [38,45]. This is
Figure 3.8. Polarization electric field hysteresis loop of the fabricated KNN thin film.
-200 -150 -100 -50 0 50 100 150 200-6
-4
-2
0
2
4
6
Electric field (kV/cm)
Po
lari
zati
on
(C
/cm
2)
49
also likely due to the low concentration of the potassium element in the KNN as well as the lack
of the buffer layer.
The leakage current density of the thin films as a function of applied electric field was also
measured using a current preamplifier (SR570 current preamplifier). The measurements are
shown in Figure 3.9. It can be seen that the current density characteristics were nearly symmetric
with respect to the voltage polarity. The current density was around 47 µA/cm2 at a positive
electric field of 100 kV/cm, which is higher than those reported by other research groups [27,39].
As the applied electric field was increased, the current density was less dependent on the applied
voltage, which is in agreement with the published data for fabricated KNN thin films [36].
The effective d33 coefficient of the deposited KNN thin film was measured using PFM
(Bruker Bioscope catalyst AFM). The applied frequency was 0.5 kHz. The effective d33 was
estimated to be 28.7 pm/V at 100 kV/cm and 25.6 pm/V at 50 kV/cm. The d33 value was
estimated based on the average of 4 measurements. These values are comparable with those of
Figure 3.9. Leakage current density as a function of the electric field for KNN thin film with
nickel electrodes.
-200 -150 -100 -50 0 50 100 150 20010
-7
10-6
10-5
10-4
10-3
Electric field (kV/cm)
Cu
rren
t d
ensi
ty (
A/c
m2)
50
Pt/KNN/LNO sample (26 pm/V) [39]. However, this value is less than other reported values for
KNN thin film. For instance, the effective d33 was reported to be 58 pm/V for LNO/KNN/LNO
samples [39] and 45 pm/V for Pt/KNN/Pt sample [27].
It can be observed that the characteristics of the fabricated KNN thin film on nickel silicide
bottom electrode were lower in comparison to those reported for KNN thin film obtained through
sputtering. It was found that the nickel silicide leads to form a buffer layer between the KNN
layer and the nickel silicide bottom electrode layer. This layer acts a barrier between the KNN
layer and the bottom electrode layer, which reduces the properties of the KNN film. Therefore, it
can be stated that the nickel silicide layer should not be used as a bottom electrode for KNN
piezoelectric thin film. However, it can be used as an uncovered bottom electrode while a pure
nickel layer is used as a bottom electrode under the KNN layer. The uncovered bottom is used to
access the pure nickel used as a bottom electrode under the KNN layer. As a result, a buffer layer
is needed to prevent the interdiffusion of the metal electrode and the silicon substrate and thus
prevent the formation of nickel silicide under the KNN layer. This is achieved in the next
section. Moreover, the chemical concentration (K/(Na+K)) of the target material was measured
to be close to that of the developed KNN film, which was lower than that of the desired value.
Therefore, the low quality of the developed KNN thin film is also attributed to the low quality of
the target material. A new target material was obtained from another supplier for the second run.
3.2 Fabrication of KNN on Nickel−based Bottom Electrode
KNN was initially deposited on a Ni/Ti/SiO2/Si substrate. During the KNN deposition at
high temperature in the presence of oxygen, the uncovered bottom electrode was protected by a
mask. The uncovered bottom electrode refers to the portion of the bottom electrode exposed to
51
the atmosphere that is used to gain access to the bottom electrode under the piezoelectric thin
film. When the deposited KNN sample was annealed in air atmosphere, the uncovered nickel
bottom electrode was oxidized. To overcome this problem, a hybrid layer is proposed as a
bottom electrode. This electrode layer consists of pure nickel deposited under the KNN layer and
nickel silicide for the uncovered bottom electrode. The crystal orientation of the sample was then
analyzed through XRD. The chemical composition was identified using EDX spectroscopy and
XPS. Subsequently, nickel was deposited on the sample to serve as top electrode. The dielectric
constant and loss tangent were determined through an impedance analyzer. The ferroelectric
polarization versus electric field hysteresis loop of the sample was measured using the classis
Sawyer−Tower circuit. The leakage current density of the thin films as a function of the applied
electric field was also measured using a parameter analyzer. The effective piezoelectric
coefficient d33 was estimated using PFM. The effective coefficient d31 was determined from the
tip deflection of the fabricated KNN unimorph cantilever.
3.2.1 KNN Thin Film Structure and Fabrication Process
The proposed KNN thin film structure uses two forms of nickel−based materials used as a
bottom electrode. Pure nickel is used as bottom electrode under the KNN film and nickel silicide
is used as an uncovered bottom electrode to gain access to the bottom electrode under the KNN
layer. Figure 3.10 shows a schematic diagram of the proposed structure. It can be observed that
the pure nickel used as bottom electrode under the KNN thin film is connected to the nickel
silicide used as an uncovered bottom electrode.
52
The KNN thin film was fabricated as follows. A silicon wafer was subjected to an RCA
standard cleaning process. Then, silicon oxide was thermally grown by wet oxidation. The wafer
was then patterned with photoresist and silicon oxide was etched in a buffered hydrofluoric acid
(HF) solution for the uncovered bottom electrode. A thin titanium (Ti) layer (2 nm thickness)
was sputtered to serve as an adhesive layer between the nickel bottom electrode and the silicon
oxide. A 200−nm−thick nickel was deposited on the top of the silicon wafer including the silicon
oxide and silicon portions. Ti and Ni were deposited using a sputtering machine (AJA
International System) at room temperature in the presence of pure argon gas.
The KNN sputtering target used was K0.35Na0.65NbO3 target material (99.9% purity, 3″
disk). The sputtering parameters including substrate temperature, gas concentration, and
sputtering power all play an important role in determining the piezoelectricity of the deposited
KNN thin film. In this work, the temperature of the substrate was set at 600 °C during the KNN
deposition. O2/Ar concentration was set at 5%, the discharge power and vacuum pressure were
set at 140 W and 3 mTorr, respectively. The deposition time was 10 hours and the corresponding
Figure 3.10. Schematic diagram of the proposed KNN thin film deposited on nickel−based
electrodes.
53
deposition rate was approximately 100 nm/hr. It is worth pointing out that the deposition
parameters were not optimized in this run.
Nickel silicide was formed by depositing nickel directly on silicon followed by annealing
the sample at high temperature. The high annealing temperature to form the nickel silicide was
applied during the KNN deposition. It is worth to mention that the silicon oxide deposited under
the nickel bottom electrode was used to prevent the interaction between nickel and silicon.
The critical issue in using nickel as an electrode material for KNN thin film is the
oxidation of the nickel layer used as a bottom electrode. The nickel layer might be oxidized due
to the deposition of KNN at high temperature in the presence of oxygen gas. To avoid any
oxidation of the nickel bottom electrode while the substrate temperature was raised to 600 ºC, the
introduction of oxygen into the chamber was delayed until the temperature reached the target
value. The required time to heat the substrate up to 600 ºC was approximately 200 seconds. The
uncovered bottom electrode (nickel silicide) was protected by a mask during KNN sputtering.
The uncovered bottom electrode was characterized and results are presented in the next section.
3.2.2 Characterization of the Uncovered Bottom Electrode (Nickel Silicide)
Nickel silicide has been widely used as contact pads in the fabrication process of integrated
circuits due its low contact electrical resistivity [75]. Nickel silicide is usually formed by
thermally reacting deposited nickel on silicon, at which the silicon is consumed in this process.
Different nickel silicide forms (Ni2Si, NiSi, and NiSi2) can be fabricated based on the annealing
temperature [76]. Different recipes can be used to fabricate nickel silicide. For example, nickel
silicide can be formed by annealing the deposited nickel on silicon at a temperature between 400
to 900 °C for 30 seconds in a nitrogen ambient [77], annealing of sputtered nickel at 600 °C for
54
90 minutes in vacuum, or annealing of evaporated nickel at 350 °C for 30 minutes in vacuum
[78,79].
In this work, 200−nm−thick nickel was sputtered directly on silicon. Then, the nickel
silicide was formed by annealing the deposited nickel at 600 °C for 10 hours during the KNN
sputtering step. It is worth pointing out that formation of nickel silicide was observed when the
KNN was deposited at 600 °C for 4 hours.
A cross−sectional SEM image of the fabricated nickel silicide is shown in Figure 3.11(a).
The irregular contact line between the nickel and silicon indicates the diffusion of nickel into
silicon. XPS analysis results are depicted in Figure 3.11(b) to reveal the composition distribution
of nickel and silicon along the thickness direction. XPS measurements of the fabricated nickel
silicide suggest an approximate composition of 50% nickel and 50% silicon, which indicates
complete reaction between the sputtered nickel and the silicon substrate.
(a) (b)
Figure 3.11. SEM image and XPS analysis of the fabricated nickel silicide layer. (a)
Cross−sectional SEM image of the nickel silicide. (b) Compositional distribution of the nickel
silicide along the thickness direction.
(a) (b)
0 500 1000 1500 2000 2500 30000
20
40
60
80
100
Etch time (seconds)
Ato
mic
perc
enatg
e (
%)
Nickel
Silicon
55
Electrical resistivity of the fabricated nickel silicide (uncovered bottom electrode) was
tested through four−point resistivity measurements. The resistivity of the as−deposited nickel
layer was estimated to be 1.143 Ω/sq while the resistivity of the fabricated nickel silicide was
estimated to be 0.115 Ω/sq. The resistivity value was estimated based on the average of 4
measurements. Those values are comparable to those of reported nickel silicide in the literature
[80,81].
3.2.3 Crystal Orientation and Chemical Composition of the Fabricated Film
As−deposited KNN/Ni/Ti/SiO2/Si sample was annealed at 400 °C using a muffle furnace
(Thermolyne). Then, the crystal orientation of both post−annealed and as−deposited
KNN/Ni/Ti/SiO2/Si samples was examined through XRD patterns (Philips XRD, PW1830) as
shown in Figure 3.12. The peaks at 2θ=22.5° and at 2θ=32° correspond to the crystal orientation
in the (001) and (110) directions, respectively. The other peaks are identified as shown in the
figure. The ratio of component (001) to component (110) for the annealed KNN thin film was
estimated to be 45%. This pattern reveals the formation of a polycrystalline KNN thin film with
a preferential orientation in the (110) direction. High piezoelectric properties of KNN thin film
can be obtained when the KNN thin film is preferentially oriented to (001) with the ratio of
component (001) to component (110) being more than 80% [51]. Therefore, it is expected to
observe lower piezoelectric properties for the KNN thin film in this work. It is worth pointing
out that the broad peak at 2θ=27° corresponds to the silicon oxide layer. Also, the crystal
structure of the nickel layer is shown through the peak at 2θ=44.5°. This was confirmed by
generating the XRD pattern of the as−deposited Ni/Ti/SiO2/Si sample.
56
SEM images of the fabricated KNN/Ni/Ti/SiO2/Si sample are shown in Figure 3.13. Figure
3.13(a) shows the cross−sectional SEM image of the fabricated film. The KNN, Ni, SiO2, and Si
layers are clearly shown in the figure. Also, this image confirms the thickness and uniformity of
each layer. An SEM image of the sample surface is shown in Figure 3.13(b). The average grain
diameter was estimated to be around 90 nm for the fabricated 1.2−µm−thick KNN film. KNN
thin film with an average grain diameter between 0.1 and 1 µm is desired to realize high
piezoelectric properties [56].
Figure 3.12. XRD patterns of the fabricated KNN/Ni/Ti/SiO2/Si samples.
20 25 30 35 40 45 50 55 60
2 (degrees)
Inte
nsi
ty (
co
un
ts)
Post-annealed KNN at 400 C in air
Ni(111)
Si
KNN(110)
KNN(001)
As-deposited KNN
57
The compositional distribution of the fabricated KNN/Ni/Ti/SiO2/Si sample along the
depth direction is shown in Figure 3.14. EDX line scan profiles of the Si, O, Ni, Nb, Na, and K
are shown in Figure 3.14(a). XPS depth profiles are shown in Figure 3.14(b) to confirm the
results obtained by EDX analysis. These scan lines indicate the distribution of the chemical
element along the thickness direction. It can be seen that the interface between KNN, Ni, and
SiO2 is clear, showing no interaction between those layers. It can be seen that the distributions of
the K, Na, Nb, and O are uniform along the thickness direction, indicating uniform atomic ratio
within the KNN layer.
(a) (b)
Figure 3.13. SEM images of the fabricated KNN thin film. (a) SEM image of the cross section
of the sample. (b) SEM image of the fabricated KNN surface.
58
(a)
(b)
Figure 3.14. Elemental depth profiles for the fabricated KNN film. (a) SEM images with EDX
line scan elemental profiles for the KNN thin film. (b) XPS depth profiles for the
KNN/Ni/Ti/SiO2/Si thin film.
0 2000 4000 6000 8000 100000
20
40
60
80
100
Etch time (seconds)
Ato
mic
perc
enta
ge
(%
)
O
Nb
K
Na
Ni
Ti
Si
59
The ratio (Na/(K+Na)) was estimated through EDX analysis to be 0.52 while the ratio of
the target material was reported to be 0.65 from the manufacturer. The ratio (Nb/(Na+K)) was
estimated to be 1.1 while it was reported to be 0.998 from the manufacturer.
It can be stated that the structural and chemical characterizations of KNN/Ni/Ti/SiO2/Si
indicate the growth of crystal KNN on the nickel bottom electrode without the formation of an
oxide layer between the nickel bottom electrode and the KNN layer.
3.2.4 Electric and Piezoelectric Properties of KNN/Ni/Ti/SiO2/Si
To measure the electric and piezoelectric properties of the fabricated KNN thin film, a
rectangular 100−nm−thick nickel electrode was sputtered on a 1−µm−thick KNN film to serve as
a top electrode. The properties measured include dielectric constant and loss tangent,
ferroelectric polarization versus electric field hysteresis loop, leakage current density, and the
effective d33 and d31 coefficients.
The dielectric constant and loss tangent were measured as a function of frequency using an
impedance analyzer (Keysight E4990A). It can be seen from Figure 3.15 that the dielectric
constant of the fabricated KNN thin film is 280 at 1 kHz with a loss tangent of 0.1. Those values
are comparable to those of fabricated KNN/SRO/Pt/MgO [82]. However, the dielectric constant
values for KNN/LNO and KNN/Pt/Ti/SiO2/Si samples were reported to be 584 and 620,
respectively [32,37].
60
Ferroelectric polarization versus electric field hysteresis loop of the deposited KNN was
measured using the classis Sawyer−Tower circuit. The reference capacitor used in the circuit was
selected to be 10 µF. The polarization hysteresis loop is shown in Figure 3.16. The polarization
hysteresis loop was generated at an applied frequency of 20 kHz. The polarization hysteresis
loop reveals typical ferroelectric behavior for the fabricated KNN thin film. The remnant
polarization was 12 µC/cm2 and the maximum polarization was 18 µC/cm
2. The coercive field
was 35 kV/cm. The maximum polarization is comparable to that of the KNN/Pt/Ti/SiO2/Si (16
µC/cm2) [45]. However, it is lower than that of KNN/SRT/Pt/MgO (26 µC/cm
2) [36].
Figure 3.15. Dielectric constant and loss tangent as a function of frequency for the fabricated
KNN thin film samples.
102
103
104
105
0
50
100
150
200
250
300
350
Frequency (Hz)
Die
lectr
ic c
onsta
nt
102
103
104
105
-0.1
1
5
10
15
X: 1000Y: 0.1013
Loss tangent
61
The leakage current density of the thin films as a function of applied electric field was also
measured using a parameter analyzer (HP 4155A Semiconductor). The measurements are shown
in Figure 3.17. The current density was around 1 mA/cm2 at an applied electric field of 100
kV/cm. The fabricated KNN thin film exhibits higher leakage current than that of
KNN/Pt/Ti/SiO2/Si [27,39]. As the applied electric field was increased, the current density was
less dependent on the applied voltage, which is in agreement with the published data for the
fabricated KNN thin film [36].
Figure 3.16. Polarization electric field hysteresis loop of the fabricated KNN thin film.
-100 -80 -60 -40 -20 0 20 40 60 80 100-20
-15
-10
-5
0
5
10
15
20
Electric field (kV/cm)
Po
lariza
tio
n (C
/cm
2)
62
The leakage current was significantly reduced when the deposited KNN/Ni/Ti/SiO2/Si was
annealed at 750 °C for an hour in air. However, the post−annealed sample exhibited low
dielectric constant. This can be attributed to the excessive thermal process during the fabrication
process including deposition of KNN at 600 °C for 10 hours and post−annealing the sample at
750 °C for an hour. Annealing treatment is known to reduce the leakage current density of
piezoelectric thin film by increasing the grain size and therefore reducing the volume of the grain
boundaries [36,37]. In case of KNN thin film, alkali metals (K,Na) are volatilized at high
temperature, at which oxygen will replace the volatilized metals. Moreover, a high annealing
temperature could change the oxidation state of the niobate which changes the properties of the
KNN [82]. The volatility of the alkali metals can be compensated by having a target material
with excess of alkali metals or adding organic compounds such as Diethanolamine [83].
Consequently, annealing treatment conditions are a trade−off between achieving low leakage
Figure 3.17. Leakage current density as a function of applied electric field for the fabricated
KNN thin film.
0 50 100 15010
-9
10-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
Electric field (kV/cm)
Curr
ent density (
A/c
m2)
63
current (less grain boundaries or oxygen vacancies) while maintaining high piezoelectric
properties of KNN thin film.
The effective d33 coefficient of the deposited KNN thin film was measured using PFM
(Bruker Bioscope catalyst AFM with a lock−in amplifier). The applied frequency was 30 kHz.
The PFM cantilever was calibrated using a standard sample (piezoelectric periodically−poled
lithium niobate sample). The effective d33 was estimated to be 36.5 pm/V at 50 kV/cm and 37
pm/V at 100 kV/cm (the d33 values was estimated based on the average of 4 measurements).
These values are comparable with those of Pt/KNN/LNO sample (26 pm/V) [39]. However,
these values are less than other reported values for KNN thin film. For instance, the effective d33
was reported to be 58 pm/V for an LNO/KNN/LNO samples [39] and 45 pm/V for a Pt/KNN/Pt
sample [27].
The effective transverse piezoelectric coefficient (d31) was estimated by measuring the tip
deflection using a laser Doppler vibrometer (Polytec OFV 5000). A rectangular beam of the
fabricated Ni/KNN/Ni/Ti/SiO2/Si with length of 12.5 mm and width of 5 mm was prepared. The
beam was fixed using a small vise grip to form a KNN unimorph cantilever. A negative sine
wave signal at 600 Hz was applied between the top and bottom electrodes, and the maximum tip
deflection was measured. The applied frequency was selected so as to avoid the mechanical
resonant frequencies. Since the thickness of the sample (525 µm) is much thicker than that of the
piezoelectric thin film (1 µm), Equation 2.12 is rewritten here:
(3.1)
The values of the Young’s modulus for the Si and KNN used are 168 and 107 MPa, respectively
[56]. The effective d31 was estimated to be 17.2 pm/V at an applied voltage of 100 kV/cm. The
VLhhhSS
Kd
pss
ps 2
1111
31)(3
.
64
d31 value was estimated based on the average of 4 measurements. This value is comparable to
that of the KNN/SRO/Pt/MgO, which the d31 values were 8.6 and 23.1 pm/V [38]. However, it is
lower than those for KNN/Pt/Ti/SiO2/Si, which the d31 values were 53.5 and 45.1 pm/V [28,35].
3.3 Summary
In the first run, nickel silicide was used as an electrode material for KNN thin film. The
deposition process as well as the annealing treatment was experimentally optimized. The
resistivity of the annealed nickel silicide layer was estimated to be 12.9×10−8
Ω.m. The electric
and piezoelectric characteristics of the fabricated KNN thin film were determined. It was found
that these characteristics were lower in comparison to those reported for KNN thin film obtained
through sputtering. The dielectric constant of the fabricated KNN thin film was low which
indicates a dielectric loss in the film. This resulted in a formation of a relatively round shape of
the polarization electric field hysteresis loop. Also, the effective d33 was estimated to be 28.7
pm/V at 100 kV/cm. The concentration of potassium with respect to sodium (K/(Na+K)) in the
fabricated KNN thin film was estimated to be 0.21. The concentration of the target material used
was reported from the manufacturer to be 0.45. When the chemical concentration of the target
material was measured using EDX, it was found to be close to that of the developed KNN film.
This indicates that the target material was not ideal. Therefore, a new target material was
obtained from another manufacturer for the next run. Also, it was shown that the buffer layer is
needed to prevent the formation of nickel silicide under the KNN layer. This was achieved in the
second run.
In the second run, the bottom electrode consists of pure nickel and nickel silicide portions.
Pure nickel is implemented under the KNN film while the nickel silicide is served as an
65
uncovered bottom electrode to gain access to the electrode under the KNN film. The nickel
silicide has high−temperature oxidation resistance in comparison with that of pure nickel. This
prevents the nickel silicide exposed to the atmosphere to be oxidized when it is annealed at high
temperature. The crystal and chemical composition investigations suggest the possibility of using
the proposed nickel−based layer as bottom electrode for KNN thin film. The resistivity of the
fabricated nickel silicide layer was estimated to be 0.115 Ω/sq. The effective d33 and d31 were
estimated to be 37 pm/V at 100 kV/cm and 17.2 pm/V at 100 kV/cm, respectively. The electric
and piezoelectric characteristics of the fabricated KNN thin film were determined. It was found
that these characteristics were lower in comparison to those reported for KNN deposited on Pt
electrodes. The fabricated KNN thin film is preferentially oriented in the (110) direction and the
average grain diameter was estimated to be less than 0.1 µm. High piezoelectric properties of
KNN thin film can be realized when the film is preferentially oriented in the (001) direction with
an average grain diameter of between 0.1 and 1 µm. The piezoelectric properties of the
fabricated KNN/Ni/Ti/SiO2/Si can be further improved by further optimizing the KNN sputtering
conditions.
66
4 A Precision Nanomanipulation System Using an AFM and
Piezo−actuated Manipulators
In addition to the development of KNN piezoelectric thin film on nickel−based electrodes,
two novel applications utilizing the developed KNN piezoelectric thin film are proposed. The
developed KNN thin film is proposed as an out−of−plane actuator for both systems. The systems
are a precision automated nanomanipulation system using an AFM and piezo−actuated
manipulators and an ultrasonic piezoelectric fan array. The first proposed system is discussed in
this chapter while the piezoelectric fan array system is presented in the next chapter.
In the proposed nanomanipulation system, the developed KNN piezoelectric thin film is
proposed to drive the manipulators in the out−of−plane direction. This provides sub−nanometer
displacement in the intended direction. As a result, high−precision nanomanipulation can be
achieved. This chapter is organized as follows. The proposed system is described in the first
section. Then, the fabrication of the manipulators is presented in Section 4.2. The assessment of
the nanomanipulation based on the developed KNN thin film is discussed in Section 4.3, and
finally the chapter is summarized in Section 4.4.
4.1 Proposed Nanomanipulation System
Nanomanipulation systems are designed to precisely move, arrange, or control the
orientation of nano−scale objects. Such systems are used in a wide range of application areas
such as biotechnology, material sciences, and nano−fabrication. Some of these applications
involve the isolation of a single bio−species from a mixture for further analysis, such as an
67
unknown virus in a mixture, and building new high performance devices such as single electron
transistors (SETs) [84,85].
A nanomanipulation system consists of a set of imaging/position sensors, a control system,
actuators, and manipulators. Due to the small size of the objects, an imaging tool at nano−scale
(microscope) needs to be incorporated in the system as a sensor. AFM is often used as it has the
advantage of imaging conductive and non−conductive samples in ambient conditions without the
need for sample preparation [86,87]. A key component in the nanomanipulation system is the
manipulator, which is the link between the actuator (macro−scale) and the object (nano−scale).
The development of AFM−based nanomanipulation systems is being pursued by different
research groups [88,89]. The typical AFM−based manipulation process consists of a series
combination of imaging the surface using the AFM tip; and then manipulating (pushing) the
object by using the same AFM tip. This process is repeated until the object reaches the desired
position. An interesting research was presented in [89]. They developed an automated parallel
imaging/manipulation force microscopy (PIMM) system that makes use of two AFM cantilevers.
The first cantilever is used for imaging while the second acts as a manipulating tool. Real−time
automated manipulation has not been demonstrated. It can be stated that the typical AFM−based
nanomanipulation process is a blindly executed process involving a push and look approach.
Also, current systems share the idea of using the AFM probe as a manipulating tool. This leads
to some drawbacks such as losing the particle being manipulated as the particle is guided by a
single probe. Also, this approach leads to a large contact area between the AFM probe and the
object as the contact occurs on the side of the probe instead of the tip apex. Current systems lack
the desired level of automation, speed, repeatability, and real−time feedback.
68
To address the shortcomings of the AFM−based nanomanipulation systems, an automated
nanomanipulation system incorporating an AFM with piezo−actuated manipulators is proposed.
The proposed design involves the development of nanomanipulators that fit within the AFM
working area. The fabricated KNN lead−free piezoelectric thin films are proposed to act as
out−of−plane actuators for the nanomanipulation tasks.
The proposed design consists of two manipulators with out−of−plane actuation, two XY
nano−positioning stages, an AFM system, and a control system. To facilitate the system
construction, the manipulators are developed so that they can be integrated with a commercial
AFM system. The AFM system will be used to provide real−time feedback of the particle
position as well as the position of the manipulators. This can be achieved by imaging a small
area (less than 0.4 µm side dimension), that includes the nano−object and a small portion of the
end−effector of the manipulators. Figure 4.1 shows a schematic diagram of the proposed system.
An XY nano−positioning stage is driving the manipulator in the in−plane direction; thus, it will
be developed to provide high accuracy and sub−nanometer resolution over a long displacement
range of hundreds of micrometers (which is a typical operating range of a commercial AFM
system). Precision control algorithms will be developed to provide a high level of automated
control based on fast real−time feedback. This work focuses on two main topics to make this
system feasible. The first is the proposed design to achieve a fully automated nanomanipulation
system using a commercial AFM system and the proposed manipulators. This includes the
manipulation strategy. The second is fabrication of the nanomanipulators that fit within the AFM
working area. Moreover, the performance of the nanomanipulation is assessed based on the
developed KNN thin film.
69
Based on the dimensions of the working area underneath the AFM cantilever tip and to
avoid contact between the manipulators and the AFM cantilever tip, the angle between the
centerline of the manipulator end−effector should make 45° in the lateral plane and 7° in the
vertical plane as shown in Figure 4.2. This results in an applied force on the particle being 1.4
times the force that would need to be transmitted by one single manipulator.
Figure 4.1. Schematic diagram of the proposed system.
Actuator 1
xyz
scanner
AFM
Sample holder
Substrate
Nanomanipulator Nanomanipulator
Actuator 2
Lasersource
Controller
AFM probe
Position-sensitive
photodetector
Actuator 1: x-y motion
Actuator 2: x-y motion
Thin-film 1: z motion
Thin-film 2: z motion
x
yz
Thin-film 1 Thin-film 2
Holder Holder
70
The interaction between the tip, substrate, and object can be modeled (tip−substrate−object
model) as shown in Figure 4.3 [90,91]. The main forces incorporated in the tip−substrate−object
model are adhesive and frictional forces. These forces are due to different phenomena such as
van der Waals force, capillary force, repulsive contact, and surface tension [92]. Based on the
tip−substrate−object model (see Figure 4.3), the following condition needs to be satisfied to
successfully push a nano−object on a surface:
maxsincos)(S
f
t
f
t
a
t
n FFFF (4.1)
where F represents the force, and the subscripts n, a, and f stand for normal, adhesive, and
friction, respectively. The superscripts t and s stand for the tip and the substrate, respectively. For
example, Fat represents the adhesive force applied on the tip.
(a) (b)
Figure 4.2. Schematic diagrams showing the lateral and vertical views of the manipulators
with the AFM cantilever assembly. (a) Lateral view. (b) Vertical view.
(a) (b)
(a) (b)
71
It is impossible to determine simultaneous force values for normal and friction forces;
therefore, the worst case scenario can be considered as follows [91]:
maxcosSf
tn FF (4.2)
where FfSmax
=τ.A, τ is the shear strength, and A is the contact area between the object and
substrate. The contact area is a function of the radius of the particle [91].
FfSmax
required to push a 15−nm−radius gold particle on a mica surface was reported to be
130 nN [93]. If the pushing task is performed on an object with a radius of 50 nm under the same
conditions, FfSmax
can be estimated to be 645 nN.
In order to validate the ability of such a manipulator to transmit the required force, the
stress at the tip apex generated due to the required pushing force was determined through
ANSYS software; the results are shown in Figure 4.4. A stress of 278 MPa was generated due to
an applied force of 1 µN at the tip with an end−effector of 60 nm in diameter. The proposed
manipulator is made of Tungsten and therefore the tensile strength of the manipulator is 1920
MPa. It can be concluded that the proposed manipulator can safely transmit the required force for
Figure 4.3. Schematic diagram showing the tip−substrate−object model.
Tip
Substrate
Particle
t
fF
t
aF
S
nFt
nF
S
fFS
aF
θ
θ
X axis
Z axis
72
the manipulation task. The first natural frequency of the manipulator was investigated using
ANSYS software to be 21.164 kHz. Also, the stiffness of the fabricated tip was estimated to be
1001 N/m.
One of the main challenges of the proposed system is the fabrication of the manipulators.
The manipulator needs to be 2 mm long with nano−sized end−effector and conical shape to fit
within the limited AFM working area. Therefore, prior to assessing the potential performance of
the system based on deposition of KNN thin film on the manipualtors, the manipualtors were
fabricated as presented in the following section.
4.2 Fabrication of Tungsten Tips for Nanomanipulation
The proposed nanomanipulator is a cantilever beam with a nano−sized end−effector (sharp
tip) and is made up of tungsten. This sharp tip offers a small contact area between the
manipulator and the object which results in a high accuracy of the manipulation process. Using
two manipulators has the advantages of avoiding the problem of losing the particle, distributing
Figure 4.4. The relation between the force and the von Mises stress at the manipulator tip.
0
200
400
600
800
1000
1200
1400
1600
1800
2000
0 1 2 3 4
Vo
n M
ises
str
ess
(MP
a)
Force at the tip (µN)
Stress generated at the tip
Yield strength
73
the required forces for manipulation on both manipulators, and performing pick−and−place tasks
with minor modifications. As mentioned earlier, the manipulator was developed to fit within the
limited environment of the AFM system (underneath the AFM cantilever tip).
4.2.1 Tungsten Tips
In addition to the application of sharp tips as manipulators for the proposed
nanomanipulation system, they have a wide range of applications in scanning probe microscopy,
multi−point contact measurements, and nanolithography. The size and the shape of the tip play
an important role in enabling various tasks [94,95,96]. In multi−point contact measurements, the
tips need to be long enough to enable the handling of a number of manipulators/probes in close
proximity as shown in Figure 4.5. In Figure 4.5(a), the tip length and cone angle are 500 µm and
28º, respectively, and the circle shows the contact area between two probes. The cone angle can
be defined as the angle between the lines that define the apex as shown in the inset in Figure
4.5(a). Longer tips, as shown in Figure 4.5(b), enable the use of the probes in closer proximity.
In nanolithography, a tip is used to etch or write at nano−scale. Therefore, sharp tips lead to high
resolution lithography as the thickness of etching or writing is smaller. Consequently, the tip of
the probe needs to be long as well as sufficiently sturdy to perform various tasks.
74
Sharp tips can be produced through different techniques such as cutting [97,98], grinding
[99,100], mechanical pulling [101,102], ion milling [103,104], and electrochemical etching
[94,105,106,107,108]. Electrochemical etching is the most widely used technique because of its
low−cost and ease of implementation [109], while tungsten is the most commonly used material
for sharp tips due to its high strength and due to the success of a number of researchers in
obtaining nano−sized tips [110,111].
Electrochemical etching involves dipping a small portion of a tungsten wire into an acid
solution, such as potassium hydroxide (KOH), and then applying an electrical potential to the
wire. In this way, the etching occurs mainly at the air/solution interface causing a neck−in
phenomenon on the wire at the interface. When the weight of the lower portion exceeds the
tensile strength of the etched wire neck, the lower portion drops in the solution and sharp tips are
formed at the upper and lower portions. This method is called the ―drop−off‖ method [112].
(a) (b)
Figure 4.5. Multiple tips are used for multi−point contact measurements. (a) The length of the
tips is 500 µm. (b) The length of the tips is 2000 µm.
75
Electrochemical etching has been explored by different research groups [110−118]. Guise
et al. [113] developed the technique by reversing the electrical potential after the occurrence of
drop−off. Tips with a radius of curvature, the radius of the circle of curvature at the tip, that is
less than 5 nm were achieved without post−etching. Kim et al. [114] developed a two−step
etching procedure by reducing the etching rate at the final stage of the tip formation; and the
diameter of the produced tip was 10 nm. Hobara et al. [115] implemented dynamic
electrochemical etching in which the tip was continuously lifted up from the solution during the
etching process until drop−off occurs. More recently, Ju et al. [116] optimized the etching
parameters including the lifting up speed to produce long and thin tips. Chang et al. [117]
applied four stages of DC pulses to produce long and sharp tips. The technique did not require
any mechanical setup or electronic cut−off circuit. However, the produced long tips had spindly
shape and as the tip was longer, the sharpness of the tip was reduced. Khan et al. [118] proposed
a two−step electrochemical etching to produce long and sharp tips. Initially, the tip was
electrochemically etched for about 15 minutes in a dynamic manner, where the solution was
moved up and down, in order to achieve a thinner tip, followed by a cleaning step. Then, fine
dynamic etching was applied until the occurrence of drop−off. Finally, another cleaning step was
applied. The length of the tip was reported to be 647 µm. Consequently, significant progress has
been done by a number of researchers in producing either short−and−sharp tips or
long−and−blunt tips. However, long−and−sharp tips exhibit spindly shape. In order to use sharp
tips for nano−applications such as nanomanipulation, the tip needs to be sharp, long, and have a
well−defined conical shape. Such tips are pursued in this work.
A two−step electrochemical etching is well suited as a technique to fabricate long and
sharp tips, as was demonstrated in the literature [118]. When we implemented the technique to
76
produce long tips as long as 2 mm, the wire did not break and thus sharp tips could not be
produced.
Therefore, a three−step electrochemical etching that combines a drop−off mechanism,
dynamic electrochemical etching where the tip is moved up and down, and reversing of the
electrical potential is proposed to produce well−defined conical shape, long, and nano−sized
tungsten tips. It includes static etching as a first step to neck down on the wire, followed by
dynamic etching to form a long conical tip. Finally, static etching is applied again to achieve
drop−off. The best conditions of the etching parameters were also experimentally obtained to
achieve the desired geometry. Physical insight into these parameters is discussed based on the
measured etching current.
4.2.2 Electrochemical Etching: Static and Dynamic
Electrochemical etching involves immersing a tungsten wire, which is used as an anode, in
an aqueous solution such as KOH. An electrical potential is applied between the tungsten
terminal and another metal terminal such as one using a stainless steel wire (see Figure 4.6).
Because of the surface tension of the solution, a meniscus is formed around the tungsten wire as
shown in Figure 4.7(a). The etching occurs at the air/solution interface and at the immersed
portion of the wire. A meniscus is formed at the interface while the lower part becomes thinner
as shown in Figure 4.7(b). A complex electrochemical reaction corresponding to the etching
process occurs according to the following chemical reactions [103]:
77
Oxidative dissolution of tungsten (W) results into tungsten ions ( 24WO ), which are soluble
in water, at the anode. These tungstate anions flow down on the sides of the wire, as shown in
Figure 4.7(b). The concentration of the ions at the interface is less than that at the immersed
portion of the wire; thus, the etching rate at the interface is higher than that at the immersed
portion. Therefore, a neck−in phenomenon is formed at the interface and, thus, a meniscus is
created. The cross section of the etched wire neck is reduced until the weight of the lower part of
the wire exceeds the tensile strength of the etched wire neck. As a result, the lower part drops off
and sharp tips are produced on both lower and upper parts. At the cathode terminal, bubbles of
hydrogen gas are formed.
Figure 4.6. Schematic diagram of a conventional electrochemical etching.
Anode: 6eO2HWO8OHW 2
2
4(s)
Cathode: 6OH2H6eOH6 2(g)2
Overall: 2(g)
2
42(s) 3HWOO2H2OHW
78
The aspect ratio and the shape of the tip are determined by the shape as well as the location
of the meniscus on the wire. When the location of the meniscus is fixed, the electrochemical
etching is referred to as static etching. In such a case, the meniscus shifts over time because of
reduction of the wire cross section. As a result, an irregular tip is formed as shown in Figure
4.7(c). However, regular and continuous shaped tips can be achieved in static etching mode
under low applied voltage where the etching rate is slow [115]. In contrast, the location of the
meniscus on the wire is changed over time in dynamic etching in which the wire is slowly pulled
off from the solution, or it is moved up and down in the solution [115,118]. In such a case, a
smooth and uniform tip can be fabricated. The method of oscillating the wire up and down yields
more uniform and smooth tips in comparison to that of lifting the wire up from the solution. This
is due to the fact that a thin layer of the wire surface is etched in every oscillating cycle. Figure
4.8(a) shows a schematic of the electrical current throughout the process in the dynamic mode in
which the wire is moved up and down in the solution. Each cycle in the graph represents a full
(a) (b) (c)
Figure 4.7. Electrochemical etching stages. (a) First stage of etching where the voltage is not
applied yet. (b) Formation of the meniscus and the chemical interaction at the anode. (c) Final
stage etching where drop−off happens.
79
oscillation of the wire (see Figure 4.8(b)). It can be observed from this graph that the electrical
current is proportional to the cross section of the tip.
In the case of lifting the wire up from the solution, the etching mechanism can be
considered as a large number of static etching steps at different height levels of the immersed
wire depth, where the position of the highest etching rate is continuously moved down.
4.2.3 Experimental Setup
Tungsten wire (99.95 % purity) of 250 µm diameter was obtained from NanoScience
Instruments Inc. The wire was cut into 4−cm length segments and cleaned with isopropyl
alcohol. A KOH solution of concentration 1 M (Sigma−Aldrich) was used as an electrolyte for
the electrochemical etching and a stainless steel wire of 1 mm diameter was used as a cathode. A
(a) (b)
Figure 4.8. Schematics of the etching current during the dynamic electrochemical etching and
the corresponding tip shape. (a) The electrical current during dynamic etching. (b) The electrical
current during one oscillation cycle of dynamic etching and the correspond.
80
ring form with 3 cm diameter of stainless steel was used to form a uniform electrical field around
the tungsten wire [119]. Coarse positioning in the z direction was implemented to place the
tungsten wire into the electrolyte. A micro−motor stage, along with the micropositioning
controller (MC−4B) from National Aperture, Inc., a low−cost solution, was used to precisely
control the position of the wire in static mode. It also drives the wire up and down to achieve
dynamic etching. A schematic diagram of the experimental setup is shown in Figure 4.9.
A cut−off circuit controls the applied voltage as well as the switching of the voltage when
drop−off occurs. The electrical potential was provided by a national instrument (NI) signal
generator (NI PXI−5412). A multifunction I/O DAQ (NI PXI−6070E) was used to measure the
voltage across an external resistor (Re). The external resistor is connected to the negative
electrode terminal (a stainless steel wire), as shown in Figure 4.9. When the voltage across the
external resistance dropped below a threshold value, the applied voltage to the tip electrode was
switched to a specific value (−1 V). The threshold value was selected to be 0.001 V (equivalent
Figure 4.9. Schematic diagram of the experimental setup including the connections of the
National Instruments (NI) cards that were used in controlling the etching process.
81
to 2.3 µA), for an external resistor of 440 Ω. This is automatically executed through a control
algorithm that was implemented using LabVIEW software. The threshold value (0.001 V), which
controls the switching of the applied voltage; and the external electrical resistor (440 Ω) were
chosen based on trial and error. Initially, the experiment was run until the wire breaks without
switching the voltage while measuring the etching current. The threshold value was then selected
to be below the breaking voltage. This process was repeated many times to ensure the selected
value is appropriate. The external resistor was selected to ensure a smooth etching current.
4.2.4 Optimization of the Process Parameters
A number of parameters including the position of the cathode, the length of the immersed
wire, and the applied voltage affect the etching process [112], and thus they were investigated
through a number of experiments. The electrical current throughout the whole process was
measured in each experimental run.
The height level of the cathode plays an important role in determining the aspect ratio and
the sharpness of the tip. Three different cathode positions were examined. The first position of
the cathode was set at the same level as that of the air/solution interface. The second and third
positions were set at 1 and 2 mm below the air/solution interface, respectively.
The electrical currents measured throughout the process for all positions are shown in
Figure 4.10(a). The red line represents the current when the cathode position was at the same
level as the air/solution interface; the blue and green lines represent the currents when the
cathode was at 1 and 2 mm below the interface, respectively. SEM images of the produced tips
in all cases are shown in Figure 4.10.
82
Although the lines in Figure 4.10(a) look similar in value, it can be observed that the red
line is smoother than the other ones especially in the last 200 seconds. This indicates that the
reduction in the wire cross−section in the first case was more uniform than those in the other two
cases. In addition, SEM images show that the most uniform V−shaped tip was fabricated when
the cathode was at the interface level. This can be explained by a strong and uniform electrical
field at the interface where the etching is at the highest rate [116]. It is worth mentioning that the
(a)
(b) (c) (d)
Figure 4.10. Investigation the effect of the different positions of the cathode. (a) Measured
electrical current across the tip during the whole process for three different positions of the
immersed wire. (b) SEM image of the tip when the cathode was at the same level as the
air/solution interface. (c) SEM image of the tip when the cathode was 1 mm below the
interface. (d) SEM image of the tip when the cathode was 2 mm below the interface.
83
measured electrical current across the external resistor, which is the same as the current across
the tip, can be used to predict the shape of the fabricated tip. For example, it can be observed that
the large drop in the current in the third position (green line) at 1375 seconds represents the
formation of the meniscus on the tip as shown inside the black circle in Figure 4.7(d).
The radius of curvature and the cone angle of the tip fabricated in case of the cathode at the
interface level are 50 nm and 25°, respectively. The radius of curvature of the tip produced in the
second case is as large as 220 nm and the length of the tip is 217 µm. The third position of the
cathode (2 mm below the interface) resulted in a sharper tip with a 40 nm radius of curvature and
a cone angle of 20°. However, the shape of the tip is irregular as shown in Figure 4.10(d). Table
4.A summarizes the characteristics of the tips produced in all cases.
The length of the immersed wire is another factor that directly influences the aspect ratio
and the sharpness of the tip [120]. Wires with immersed lengths of 1, 2, and 3 mm were
investigated as shown in Figure 4.8. Figure 4.11(a) shows the measured currents throughout the
process in the three cases. Red, blue, and green lines represent the currents when the lengths of
the immersed wire were 1, 2, and 3 mm, respectively. SEM images of the produced tips in all
cases are also shown in Figure 4.11.
84
The measured current in the 3 mm immersed length case, the green line in Figure 4.11(a),
is higher than the measured current in the other two cases. This is due to the fact that when a
longer portion of the wire is immersed in the solution, the equivalent electrical resistance of the
process is reduced. Therefore, a deeper immersed length yields a higher etching rate.
(a)
(b) (c) (d)
Figure 4.11. Measured currents and SEM images corresponding to different immersed wire
lengths. (a) Measured electrical current across the tip when the immersed length of the wire
was 1, 2, and 3 mm. (b) SEM image of the tip for the 1 mm immersed length of the wire. The
image of the tip corresponding to the immersed wire being 2 mm is shown in (c) while the 3
mm immersed wire length case is shown in (d).
85
The reduction in the measured current started at 900 seconds and 400 seconds in the cases
where the immersed wire was of 3 and 2 mm of length, respectively. However, it took place
immediately at the beginning of the process in the case of 1 mm length of the immersed wire.
This may be due to the fact that the rate of change of the equivalent resistance in the cases of 3
and 2 mm immersed wire depths were smaller than that of the 1 mm immersed wire depth case,
which resulted in a high current (5.7 mA), at the beginning of the etching process. For example,
the equivalent resistance in the 3 mm immersed depth case at 900 seconds was the same as that
of the 2 mm immersed depth case at 400 seconds.
It is worth mentioning that the etching current is constrained by the conductance of the
solution and the dimensions and material purity of the tungsten wire. The maximum current
flowing through this etching experiment is around 5.7 mA which is the value at the beginning
stage of the etching process.
The etching rates in the cases of 2 and 3 mm length immersed wire cases were higher than
the corresponding rates for the 1 mm immersed wire case. This can be observed through a
reduction rate of the current as shown in Figure 4.11(a). The reduction in current in the 1 mm
immersed case results in a uniform and sharp tip as shown through the SEM image in Figure
4.11(b). The radius of curvature and the cone angle of the tip produced in the 1 mm immersed
wire case are 60 nm and 28°, respectively. Although the image of the tip in the 3 mm immersed
wire length case (see Figure 4.11(d)) shows a more uniform tip than the one shown in Figure
4.11(b) (1 mm immersed wire depth case), the surface of the tip in the 1 mm immersed wire
length case is smoother and the tip is sharper than the tip produced based on the 3 mm immersed
length case. This result did not match the results shown in [121]. As mentioned earlier, the
drop−off happens when the weight of the lower part exceeds the tensile strength of the etched
86
wire neck. The weight of the lower part in the 3 mm immersed length case was greater than the
weight of the lower part corresponding to the 1 mm immersed length. Therefore, the drop−off in
the case of the 3 mm immersed length happened faster than that of the 1 mm immersed length
case. It can also be seen that the produced tip in case of 2 mm (see Figure 4.11(c)) is not uniform
or sharp (the radius of curvature and the cone angle are 185 nm and 47°, respectively).
(a)
(b) (c) (d)
Figure 4.12. Measured currents corresponding to different applied voltages and SEM images
of obtained tips. (a) Measured electrical currents corresponding to different applied voltages:
4, 3, and 5 V. (b) SEM image of the tip when the applied voltage was 4 V. (c) Image of the tip
with 3 V applied. (d) Image of the tip with 5 V applied.
87
The etching rate depends on the applied voltage between the anode and the cathode.
Different DC voltages, 3, 4, and 5 V, were applied. Figure 4.12(a) shows the electrical currents
that were generated throughout the process in the three cases. Red, blue, and green lines
represent the measured currents corresponding to the applied voltages of 4, 3, and 5 V,
respectively. SEM images of the produced tips in all three cases are shown in Figure 4.9.
In the case of the lower voltage (3 V), the process was slow producing an unstable tip, as
shown in Figure 4.12(c). Also, the radius of curvature and the cone angle of the tip are 40 nm
and 25°, respectively. It can be observed that the current flow in this case (blue line) was largely
reduced at 1450 seconds and 1750 seconds. A spindly tip, shown in Figure 4.12(d), was
produced when a 5 V potential was applied, which resulted in faster etching in comparison to the
3 V and 4 V cases. However, the produced tip in the 5 V case is sharp as the radius of curvature
is 50 nm and the cone angle is 6°. It can be observed that the etching current dropped at 1000
seconds and during the last 100 seconds starting at around 1400 seconds, as shown through the
green line in Figure 4.12(a). From this observation, the shape of the tip can be predicted in which
a meniscus, as shown inside the circle in Figure 4.12(d), was formed at 1000 seconds and the
cross section of the tip apex was largely reduced.
Blunt tips were observed. For instance, Figure 4.13(a) shows the tip produced when the
applied voltage, the immersed wire, and the cathode depth were 5 V, 1 mm, and 0 mm,
respectively. Figure 4.13(b) shows the tip apex while Figure 4.13(c) shows the tip apex produced
under an applied voltage of 4 V, an immersed wire of 1 mm, and a cathode depth at the
air/solution interface. It can be seen that the sharpness of the tip in Figure 4.13(b) is 20 times
larger than that of the tip in Figure 4.13(c). This tip is referred to as a blunt tip. Blunt tips were
formed due changes in the etching process such as a change of the etching rate at the end of the
88
process. This is noticed in the last 100 seconds of the green line in Figure 4.12(a), where the
current quickly dropped. It may result in removing a large amount of material at the air/solution
interface leading to a break in the wire (tip is formed). A large etching of material can also lead
to an open circuit which results in etching of the tip under the natural potential difference. This
leads to a blunt tip instead of a fine V−shape sharp tip.
(a)
(b) (c)
Figure 4.13. Blunt tip produced under applied voltage of 5 V, immersed wire of 1 mm, and the
cathode at the interface. (a) The tip apex is shown in (b). The apex of the produced tip under
the applied voltage of 4 V, the immersed wire of 1 mm, and the cathode depth at the
air/solution interface is shown in (c).
89
It was found that the application of 4 V potential led to the most stable tip among the three
cases studied. The measured electrical current corresponding to the 4 V case was also the
smoothest of the three signals measured. Table 4.A summarizes the geometrical characteristics of
the produced tip when varying one of the etching parameters during each test. Based on the tests
done, it was determined that the cathode position being at the air/solution interface, the length of
the immersed wire being 1 mm, and the applied voltage being 4 V are the best parameters.
Table A. Characteristics of the fabricated tips while varying the etching parameters
Etching parameters Corresponding
figures
Characteristics
Applied
voltage (V)
Immersed
wire (mm)
Immersed
cathode (mm)
Radius of
curvature
(nm)
Cone angle (
°)
Length (µm)
5 1 0 Figure 4.12(d) 50 6 504
Figure 4.13(a) 560 51 442
4 1 0 Figure 4.10(b) 50 25 525
— 60 33 520
— 125 38 548
3 1 0 Figure 4.12(c) 40 25 700
— 830 28 500
4 2 0 Figure 4.11(c) 185 47 450
— 1800 85 320
4 3 0 Figure 4.11(d) 60 28 500
— 250 33 417
4 1 1 Figure 4.10(c) 220 45 217
4 1 2 Figure 4.10(d) 40 20 460
— 40 33 504
— 1000 70 427
4.2.5 Proposed Three−step Electrochemical Etching Technique
Based on the chosen process parameters, a three−step electrochemical etching is proposed
to produce uniform V−shaped, long, and sharp tips. The first step includes static etching for four
minutes to form a neck. The neck−in phenomenon is detected through a voltage drop across the
resistor, where the voltage across is proportional to the cross section of the etched wire. Dynamic
etching is applied next, with the wire oscillating up and down in the solution. A triangular
90
oscillation with an average speed of 200 µm/s is applied to the wire. This speed leads to a
smooth motion of the wire. The amplitude of the oscillation represents the length of the etched
tip, which is set to be 2 mm in this work. The second step is completed when the voltage across
the resistor becomes 0.4 V, which corresponds to a cross section of 50 µm. In the third step, the
dynamic etching is switched off and static mode is applied again. The wire is etched until the
lower part drops in the solution and the electrical current drops to zero. At this time, the voltage
is inverted to −1 V to prevent any post−etching. The tip is then cleaned in distilled water and
dried with compressed nitrogen gas.
Figure 4.14(a) shows the electrical current across the external resistor throughout the
etching process. Region I represents the first etching step (static mode). A snapshot of the video
taken during this step is shown in Figure 4.14 (b). Oscillation in the measured current due to
oscillation of the wire is seen in Region II. Figure 4.14 (c) shows the wire corresponding to a low
measured current (small cross section), while Figure 4.14 (d) shows a scenario when the wire is
fully immersed in the solution (the current is high). The neck−in phenomenon can be detected in
this picture (inside the circle). This region is responsible for producing a long smooth conical tip
shape. Figure 4.14 (e) shows a snapshot during Region III where the wire becomes thinner. The
picture in Figure 4.14 (f) was taken at the moment of drop−off, where the lower part of the tip is
shown inside the circle. The etching of the wire was watched using a microscope (Leica MZ16
F).
91
.
The proposed technique can successfully produce conical long sharp tips. The length of
the conical tip was 2 mm, which is equivalent to a 7º cone angle since the wire diameter is 250
µm. In addition, tips with radius of curvature of around 20 nm were achieved. An SEM image of
a produced tip and a zoom−in image of the tip apex are shown in Figure 4.15. The produced tip
(a)
(b) (c) (d) (e) (f)
Figure 4.14. Experimental result of the proposed electrochemical etching technique. (a)
Measured electrical current across the external resistor for the complete three−step
electrochemical etching process (b−f) Optical images of the immersed wire during the process.
Region I
Region II
Region III
92
is uniform. This is due to the fact that the position of the highest etching rate on the tip was
continuously moved up and down. Another produced tip and a high magnification at the tip apex
are also shown in Figure 4.16. During the process, the voltage is continuously applied between
the electrodes, and the wire is kept immersed. The process takes approximately 30 minutes to
produce a tip, which is longer than what has been reported by others [94,115,116,119,121]. The
success rate of producing tips similar to the ones in Figure 4.15 and Figure 4.16 under the
selected parameters for the three−step electrochemical etching method was estimated to be
around 85%.
(a) (b)
Figure 4.16. SEM image of a produced tip (a) and the zoom−in image of the tip apex (b).
(a) (b)
Figure 4.15. Scanning electron micrograph of the whole produced tip (a) and the zoom−in
image of the tip apex (b).
93
4.3 Assessment of the Manipulation Based on the Developed KNN
Thin Film
The developed KNN thin film is proposed as an out−of−plane actuator for the fabricated
manipulators, at which the KNN film drives the tip of the manipulator in the out−of−plane
direction. The KNN thin film can be deposited on the manipulator as shown in Figure 4.17.
The relation between the tip displacement of the tungsten manipulator and the applied
voltage across the piezoelectric thin film is governed mainly by the transverse piezoelectric
coefficient (d31). The relation was discussed in details in Section 2.6. The value of the effective
d31 of the developed KNN thin film was estimated to be 17.2 pm/V. This value leads to an
out−of−plane displacement of 35 nm for the fabricated tungsten manipulator at an applied
voltage of 20 V. This value was calculated based on depositing a 2−µm−thick KNN thin film on
2 mm length of a square tungsten wire with a cross section of 0.25×0.25 mm2. Tip deflection of
69 nm can be achieved by increasing the thickness of the KNN thin film to 4 µm while applying
the same electric field of 100 kV/cm. Therefore, sub−nanometer resolution in the out−of−plane
direction can be achieved by using the developed KNN piezoelectric thin film. However, the
estimated out−of−plane displacement is low in comparison with the out−of−plane displacement
Figure 4.17. Schematic diagram of the proposed nanomanipulator with out−of−plane
actuation.
Piezo thin film
Tungsten nanomanipulator
94
(Z direction) of commercial nano−positioning stages [122]. The range of the Z direction of
commercial high−precision nano−positioning stages is typically ones of micrometers [122].
Therefore, the tip deflection of the manipulator should be increased. This can be achieved by
improving the piezoelectric properties of the KNN thin film.
At this point, nanomanipulation with out−of−plane actuation can be achieved. An XY
nano−stage can be obtained and integrated with the developed manipulator. As a result, an XYZ
nano−positioning stage can be produced. This platform is a three−degree−of−freedom system.
The nanomanipulation platform can be incorporated with a commercial AFM system.
4.4 Summary
A novel automated nanomanipulation system was proposed in this chapter. The proposed
system uses an AFM as a sensor and uses manipulators to interact with the target particle. The
system requires specific manipulators to fit within the limited environment of a commercial
AFM system. Uniform V−shaped sharp manipulators (tips) as long as 2 mm are needed. Theses
manipulators can be fabricated through a novel electrochemical etching technique developed in
this work as well.
The electrochemical etching was investigated experimentally to make long sharp tips. The
best operating conditions were experimentally obtained. V−shape tips with radius of curvature of
around 20 nm and length of 2 mm were produced using the proposed technique. The developed
etching technique can be extended for other applications at which the limited working area is a
challenge, e. g. multi−point contact measurements. The length of the tip can be controlled by
varying the amplitude of the dynamic oscillation applied during the dynamic etching step
(second step).
95
The fabricated KNN thin film is proposed as a micro actuator for the developed
manipulators. Based on the current results of the KNN thin film, a tip deflection of 69 nm can be
achieved at an applied electric field of 100 kV/cm. However, the estimated out−of−plane
displacement is low in comparison with that of the commercial nano−positioning stages.
Therefore, the tip deflection of the manipulator should be increased. This can be achieved by
improving the piezoelectric properties of the fabricated KNN thin film. This is further discussed
in the section entitled: Future Work at the end of the document.
96
5 Development of Ultrasonic Piezo Fans Based on the
Developed KNN Thin Film
The second proposed system that uses the developed KNN thin film is an ultrasonic piezo
fan array. An ultrasonic piezo fan operates at a frequency above the upper limit of the human
audible frequencies. The developed KNN thin film is proposed as an out−of−plane actuator in
the ultrasonic piezo fan system. A piezo fan is a vibrating cantilever beam. It can be fabricated
by attaching a piezoelectric patch on an elastic layer. The piezoelectric patch consists of a bottom
electrode, a piezoelectric material, and a top electrode. A schematic diagram of a piezo fan is
shown in Figure 5.1(a). By applying an alternating voltage between the electrodes (across the
piezo layer), the elastic layer bends at the same frequency as the input signal. This bending
motion leads to generating an air flow (see Figure 5.1(b)). When the signal is applied at the
resonant frequency of the fan (including the piezo and elastic layers), the cantilever tip deflection
is maximized.
(a) (b)
Figure 5.1. A piezoelectric fan. (a) Schematic of a piezo fan. (b) Schematic diagram showing
the working principle of a piezo fan.
(a) (b)
Air flow
Elastic layer
Piezo layer
~
97
Current piezo fans use bulk PZT piezoelectric material with an operating frequency of
less than 100 Hz [123,124]. It should be mentioned that the piezo fans, operating at a resonant
frequency below 100 Hz, have been reported to have a low acoustic noise (i.e. below 25 dBA)
[124]. The piezo layer in such piezo fans is tens of mm in length with a thickness of around 0.2
mm [123,124]. These piezo fans are referred to as macro piezo fans. In this work, the developed
KNN thin film on nickel−based electrodes is proposed as a piezo layer in the piezo fan structure.
The proposed ultrasonic piezo fans are submillimeter in size and as such are referred to as micro
piezo fans. Using the developed KNN thin film offers an environmental friendly solution as it
does not contain lead as well as a low cost alternative as it uses nickel as bottom electrodes. The
micro piezo fan system is proposed to be used in GPU cooling applications. In the next section,
current piezo fans are discussed. The novel micro piezo fan solution is proposed and the
performance of the proposed GPU cooling system is assessed based on the developed KNN
piezoelectric thin film.
5.1 Piezo Fans for GPU Cooling Systems
The rapid rate of innovation in GPUs requires improving the conventional methods of
thermal management. Current thermal management methods include a heat sink and rotary fans.
The rotary fans lead to an acoustic noise and a relatively high power consumption. Piezo fans
have been researched as an air flow generator to replace the current rotary fans [125,126]. Piezo
fans have the advantages of low power consumption, low acoustic noise, and simple compact
designs that contain no moving parts.
Many figures characterizing the performance of piezo fans have been presented in the
literature. Yoo et al. [123] developed a piezoelectric cooling fan with dimensions of
98
31.8×25.8×0.1 mm, producing a wind velocity of 3.1 m/s measured 0.1 cm away from the fan tip
when driven by a 220 V, 60 Hz power source. Acikalin et al. [127] investigated using piezo fans
inside portable electronics. One of the fabricated piezo fans was 15 cm in length and 0.13 mm
thick brass, a 0.19 mm thick PZT led to a wind velocity of 30 cm/s when driven by a 40 V and
20 Hz power source [127]. Petroski et al. [124] reported on a fan driven by a 120 V and 60 Hz
signal that led to a wind velocity of 1.5 m/s. Examples of commercial piezo fans can also be
found in [128] and [129].
A schematic diagram of a typical GPU cooling system is shown in Figure 5.2. The heat
sink is presented as a cuboid shape as shown in Figure 5.2.
Piezo fans can be incorporated in such a system by replacing the rotary fans with piezo
fans. Figure 5.3 shows a schematic diagram of a piezo fan configuration incorporated into the
cooling system.
Figure 5.2. 3D schematic of a typical GPU cooling system.
Heat sink
Rotary fan
99
The volumetric air flow rate, the fan size, and power consumption for the fan configuration
addressed are established based on numerical finite element simulations. The volumetric flow
rate is defined as the volume of air passing through a surface per unit of time (air velocity × cross
sectional area). The piezo fan presented in [123] was reported to produce an air velocity of 3.1
m/s. By considering a rectangular area with dimensions equal to the fan width and using the
maximum tip deflection, the un−obstructed air flow rate is estimated to be around 5.75 cfm. The
flow rate estimated in this work does not consider any resistance placed at the front of the piezo
fans.
In order to have a low acoustic signature, the piezo fans to be developed should be
operated at a frequency beyond the human audible frequencies (greater than 20 kHz). The
following procedure was pursued in order to design the piezo fans:
1. Find the dimensions of the piezo fan that leads to the desired resonant frequency (i.e. 20
kHz) and maximum tip deflection at the resonant frequency. Analysis was done using
commercial finite element software (ANSYS, ANSYS Inc.).
Figure 5.3. Piezo fans incorporated into a cooling system.
100
2. Using the results from Step 1 (dimensions of the piezo fan, resonant frequency, and
maximum tip deflection), the generated air velocity was estimated by running a
fluid−structure interaction program (ANSYS FSI, ANSYS Inc.).
3. By estimating the air velocity at different locations using the software package (see
Figure 5.4 where each of the Vis indicates the location of the estimated air velocity), the
volumetric air flow rate was estimated according to the following:
Volumetric flow rate (Q) =air velocity × area,
X
dxyAirVelocitWidthQ
0
)(
(5.1)
QTotal=Q×Number of piezo fans (5.2)
The volumetric air flow rate was discretized over 4 segments; each segment is defined
by any two neighboring locations of the estimated air velocity (Vi), as shown in Figure 5.4.
Figure 5.4. Schematic of a piezo fan showing the estimated air velocity locations. X
represents the length of the considered area, w represents the width of piezo fan, δ represents
the maximum tip deflection, Vi represents the location of the point where the air velocity was
estimated, and α represents the distance between the maximum tip deflection and the Vi.
101
4. The required electrical power was also estimated according to the following:
P=V(t).I(t)=2πfCV2sin(2πft)cos(2πft)= πfCV
2sin(4πft)
Pmax=πfCV2 (5.3)
where V(t)=Vsin(2πft), I(t)=C(dV/dt), C is the capactiance of the piezo layer, and f is the
operating frequency.
To design an ultrasonic piezo fan (operating frequency is greater than 20 kHz), the piezo
fan should be submillimeter in size (micro piezo fan). The above−mentioned procedure was
followed to design the micro piezo fan array operating at 20 kHz, as presented next.
5.2 Micro Piezo Fan Operating at 20 kHz
Piezo fans operating at 20,004 Hz can be obtained by using a silicon layer of
1.7×0.8×0.037 mm and a KNN piezo layer of 0.8×0.3×0.005 mm. Figure 5.5 shows a schematic
of the micro piezo fan assembly. Due to the small size of these fans, up to 4620 fans can be
integrated into a space of a 90−mm−diameter rotary fan. This can be achieved by fabrication of
the 4620 silicon micro−cantilevers (elastic layer of the piezo fan) and then deposition the KNN
thin film on these cantilevers as shown in Figure 5.5(c). The micro−fabrication process for the
micro piezo fan array requires three masks; two masks to fabricate the cantilevers and a third
mask to fabricate the piezo layer. We have fabricated silicon micro−cantilevers. Figure 5.6
shows an image of one of the fabricated cantilevers under a microscope.
102
(a)
(b) (c)
Figure 5.5. Schematic diagrams of the micro piezo fan array configuration. (a) Micro piezo fans
assembly within a GPU cooling system. (b) A large array of micro piezo fans. (c) Zoom−in
schematic of two neighbouring micro piezo fans.
103
To optimize the dimensions of the micro piezo fan using ANSYS software, a complete set
of piezoelectric properties for the thin film is required. The piezoelectric properties including the
transverse piezoelectric coefficient (d31= 17.2 pm/V) and the dielectric constant (ε= 280) were
based on the determined characteristics of the developed KNN piezoelectric thin film. The rest of
the set was assumed to be similar to that of KNN bulk ceramics [130]. The tip deflection was
estimated to be 0.0422 mm at an applied voltage of 17 V. When the applied voltage was
increased up to 31 V, the tip deflection was estimated to be 0.0769 mm, which leads to
producing a volumetric air flow rate of 10.14 cfm at 0.5 mm from the maximum tip deflection.
The effect of the neighboring fans on the flow around any of the micro fans was neglected in the
estimation. Also, no air resistance at the front of the fan was taken into account during the
estimation. The corresponding power consumption was estimated to be 13.19 W.
To realize the values of the air flow rate and the power consumption of the micro piezo fan
array, macro piezo fan array operating at 60 Hz was also studied for comparison. Twelve fans
can be fitted into a similar space, 90 mm in diameter, (see Figure 5.3). The dimensions of the
elastic and piezo layers were 50×10×0.1 mm and 18.5×10×0.3 mm, respectively. The material
for the elastic layer was selected to be phosphor bronze which maximizes the tip deflection
Figure 5.6. Microscope image of fabricated silicon cantilever.
104
[123]. It should be mentioned that the air velocity of a piezo fan depends mainly on the resonant
frequency and the tip deflection of the fan. The material properties of the piezoelectric layer were
based on PZT−5A [131]. The corresponding maximum tip deflection was estimated to be 11.5
mm. The total volumetric air flow rate and the required power for the twelve fans were estimated
to be 15.72 cfm and 0.26 W, respectively.
It can be noted that the micro piezo fans require high power consumption in comparison
with that of the macro piezo fans. The total power consumption is high due to the high operating
frequency as well as the size of the total capacitance of the piezo layers used. In order to reduce
the required power, the capacitance of the piezo layer (capacitance = permittivity × area /
thickness) can be reduced by minimizing the area/thickness ratio of the piezo layer. Yet, this will
affect the maximum tip deflection which obviously controls the air flow rate. In another solution,
the power consumption can be reduced by decreasing the applied voltage. However, this will
reduce the maximum tip deflection and thus affect the air flow rate. Decreasing the distance
between the piezo fan and the heat sink will significantly increase the air flow rate. The flow rate
at different distances between the maximum tip deflection of the macro piezo fan and the heat
sink under low operating voltages is presented in Appendix A. The corresponding total power
consumptions are also shown in the appendix.
The proposed micro piezo fan system is a promising potential for GPU cooling
applications. In addition to the advantages of large air flow rate and low acoustic noise,
fabrication of the micro piezo fan array can be part of the GPU fabrication process itself.
Moreover, the proposed piezoelectric thin film does not contain lead and it uses a low cost
electrode material (nickel). The generated air flow rate of the micro piezo fans can be increased
by increasing the tip deflection of the fan. This can be achieved by improving the transverse
105
piezoelectric coefficient (d31) of the KNN thin film. Therefore, prior to fabrication of the micro
piezo fan array, the piezoelectric properties of the KNN thin film need to be further improved. It
should be mentioned that the solution of using micro piezo fan array for GPU cooling
applications remains novel.
5.3 Summary
Piezo fans have attracted attention to replace rotary fans in GPU cooling systems. This is
due to their low acoustic noise and simple design. In this work, novel micro piezo fans are
proposed to replace the rotary fan in a GPU cooling system. In addition to the ease of fabrication
of micro piezo fans, they can be integrated into any available space in a cooling system as they
are small in size. Based on the piezoelectric properties of the developed KNN thin film, the
generated air flow rate and required power consumption of the micro piezo fan array were
estimated to be 10.14 cfm and 13.19 W, respectively. The power consumption of the micro piezo
fan array can be reduced by optimizing the dimensions of the piezo layer to reduce the equivalent
capacitance. Also, the piezoelectric properties of the fabricated KNN thin film should be further
improved to increase the tip deflection and thus increase the generated air flow rate. This also
will allow low operating voltages and consequently reduce the power consumption.
106
6 Concluding Remarks
Several systems are discussed in this thesis. KNN piezoelectric thin film was developed on
nickel−based electrode. A nanomanipulation system using a commercial AFM and
piezo−actuated manipulators was proposed. A three−step electrochemical etching technique was
developed to fabricate the manipulators (tungsten nano−tips). A novel micro piezo fan system
using the developed KNN thin film was proposed. In this chapter, conclusions, contributions,
and future work are presented.
6.1 Conclusions
The conclusions of this thesis are summarized based on four themes: lead−free
piezoelectric thin film, fabrication of KNN thin film, a proposed nanomanipulation system, and
proposed micro piezo fan array.
6.1.1 Lead−free Piezoelectric Thin Film
KNN, lead−free material, thin film is a promising candidate to replace PZT thin film due to
its interesting piezoelectric properties. To gain better understanding of the KNN piezoelectric
thin film, the well−known PZT piezoelectric material was presented and then the piezoelectric
behavior of KNN thin film was discussed. A review of the literature on KNN thin film indicates
the potential of this material to replace the PZT thin film as well as the challenges with
producing high−quality KNN piezoelectric thin film. This work focuses on the development of
KNN thin film on base metal electrodes. This reduces the fabrication cost of the piezoelectric
thin film. The fabrication of KNN thin film on nickel−based electrodes was discussed in Chapter
3. The KNN piezoelectric thin film is proposed as an out−of−plane actuator in this work.
107
6.1.2 Fabrication of KNN Thin Film
Pt is the most widely used material as a bottom electrode for KNN thin film. In this work, a
nickel−based bottom electrode for KNN thin film is proposed. In particular, two KNN thin film
fabrication runs were conducted. In the first run, nickel silicide was proposed as a bottom
electrode material. Nickel was deposited directly on a silicon substrate. The nickel silicide was
then formed due to the high deposition temperature of KNN layer as well as the lack of the
buffer layer between the nickel layer and the silicon substrate. It was found that the nickel
silicide bottom electrode leads to form a buffer layer between the KNN layer and the nickel
silicide bottom electrode. This buffer layer acts as a barrier reducing the quality of the KNN thin
film. To overcome this problem, a hybrid bottom electrode was proposed in the second
fabrication run. The bottom electrode consists of pure nickel and nickel silicide portions. Pure
nickel is implemented under the KNN film while nickel silicide is used as an uncovered bottom
electrode to gain access to the electrode under the KNN film. The nickel silicide has a
high−temperature oxidation resistance in comparison with that of pure nickel. This prevents the
nickel silicide exposed to the atmosphere from oxidation when it is annealed at high temperature.
The crystal and chemical composition investigations suggest the possibility of using the
proposed nickel−based layer as a bottom electrode to grow KNN piezoelectric thin film. The
resistivity of the fabricated nickel silicide layer was estimated to be 0.115 Ω/sq. The effective d33
and d31 were estimated to be 37 pm/V at 100 kV/cm and 17.2 pm/V at 100 kV/cm, respectively.
The electric and piezoelectric characteristics of the fabricated KNN thin film were determined. It
was found that these characteristics were lower in comparison to those reported for KNN
deposited on Pt electrodes. The fabricated KNN thin film is preferentially oriented in the (110)
direction and the average grain diameter was estimated to be less than 0.1 µm. High piezoelectric
108
properties of KNN thin film can be realized when the film is preferentially oriented in the (001)
with an average grain diameter of between 0.1 and 1 µm. The piezoelectric properties of the
fabricated KNN/Ni/Ti/SiO2/Si can be further improved by optimization the KNN sputtering
conditions.
In addition to the fabrication of KNN thin film, two applications use the KNN piezoelectric
thin film as an out−of−plane actuator were proposed.
6.1.3 Proposed Nanomanipulation System
The first application of KNN thin film is to use it as an out−of−plane actuator in the
proposed nanomanipulation system. The proposed system uses manipulators for the
manipulation task while the sensing task is achieved by an AFM system. Due to the limited
environment of a commercial AFM system, the manipulators need to be 2 mm long with radii of
curvature of around of 20 nm. The manipulators were fabricated through a novel electrochemical
etching technique developed in this work. The best electrochemical etching parameters were
experimentally obtained. The position of the cathode in the experimental setup was selected to be
at the air/solution interface. The length of immersion of the wire was set to 1 mm. An applied
voltage of 4 V was used to stabilize the process. The measured etching current can be used to
predict the shape of the tip, particularly the formation of meniscuses. V−shape tips with radii of
curvature of around 20 nm and length of 2 mm were produced using the proposed technique.
The proposed etching technique contributes to enhance the fabrication of tungsten
nano−tips to access a nano−scale object that is located in a limited working area. The length of
the tip can be controlled by varying the amplitude of the dynamic oscillation applied during the
109
dynamic etching step (second step). The speed of the oscillatory motion was set to 200 µm/s
leading to a stable etching process.
The fabricated KNN thin film is proposed as a micro out−of−plane actuator for the
developed manipulators. This can be achieved by deposition of the KNN thin film on the
fabricated manipulators. The out−of−plane displacement of the manipulator is controlled mainly
by the transverse piezoelectric coefficient (d31) of the KNN film. Based on the developed KNN
thin film, a tip deflection of 69 nm can be obtained at an applied electric field of 100 kV/cm.
This value is low in comparison with the out−of−plane displacement of the commercial
nano−positioning stages. To enable the practical applications of the proposed nanomanipulation
system, the tip deflection of the manipulator should be increased. This can be achieved by
improving the d31 coefficient of the fabricated KNN thin film.
6.1.4 Proposed Micro Piezo Fan Array
Piezo fans offer a number of advantages including low acoustic noise, lower power
consumption, and simple design. In this work, micro piezo fans operating at a frequency above
the upper limit of human hearing (ultrasonic) are proposed to replace the current rotary fan in
GPU cooling systems. In addition to the ease of fabrication of micro piezo fans, they can be
integrated into any available space in a cooling system as they are small in size. The proposed
micro fans are also an environmentally friendly solution as it does not contain lead. The
fabrication cost should be low as it uses nickel as bottom electrodes. The air flow rate and the
power consumption of the proposed micro piezo fan system were estimated to be 10.14 cfm and
13.19 W, respectively, based on the developed KNN thin film. The dimensions of the piezo layer
in the micro piezo fan design can be further optimized to reduce the power consumption while
110
maintaining a large air flow rate. Also, the fabrication process of the KNN thin film should be
further optimized to enhance the piezoelectric properties of the film and thus large air flow rate
as well as low power consumption can be achieved. Micro piezo fan array is a novel solution,
which has not been discussed in the literature, and therefore the performance needs to be
experimentally validated.
6.2 Major Contributions
The main accomplishments of this thesis are as follows:
1. Fabrication of KNN piezoelectric thin film on nickel−based electrodes. This reduces the
fabrication cost of the lead−free piezoelectric thin film.
2. Fully electric and piezoelectric characterization of the fabricated KNN thin film.
3. Proposing a precision automated nanomanipulation system incorporated an AFM and
piezo−actuated manipulators. This system provides real−time feedback for the particle
being manipulated.
4. Fabrication and characterization of tungsten nano−tips. The fabricated tips can be used as
manipulators for the proposed nanomanipulation system as well as they can be used for
other applications that require conical−long−sharp tungsten tips.
5. Proposing an ultrasonic fan system based on the developed KNN thin film for cooling
applications. This is a novel solution to replace the rotary fan in GPU cooling systems,
which generates more air flow and less audible noise.
111
6.3 Future Work
Based on the results obtained in this work, the quality of the KNN thin film fabricated on
nickel−based electrode can be improved as follows:
1) The crystal orientation of the KNN thin film should be preferentially oriented in the (001)
direction to realize higher piezoelectric properties. This can be achieved by controlling
the crystal orientation of the nickel bottom electrode, which can be done by changing the
deposition temperature of the nickel bottom electrode.
2) The deposition parameters of the KNN thin film need to be optimized to improve the
quality of the fabricated film. Also, the deposition rate needs to be increased. These can
be achieved by investigating the effect of each parameter as follows:
a. The deposition temperature
b. The distance between the target and the substrate
c. The argon/oxygen concentration in the chamber
d. The pressure chamber
e. The charging power
f. The chemical composition of the target material
g. The post−annealing treatment
3) Doping the KNN thin film with suitable materials such as Li and Mn to improve the
piezoelectric properties of the KNN thin film.
Once KNN thin films, with high piezoelectric coefficient d31 of more than 100 pm/V and
leakage current density of less than 1×10-6
A/cm2, are produced, they can be used in a variety of
commercial products such as inkjet printers and in the proposed systems described in this thesis:
nanomanipulation system and the micro piezo fan array system.
112
The proposed nanomanipulation system requires that the following be pursued:
1) Square cross−sectional tungsten wires need to be used and then KNN to be deposited on
these wires. Nanomanipulation with an out−of−plane actuation can then be achieved.
2) An XY nano−positioning stage needs be developed or acquired. The manipulator with
out−of−plane actuation is to be integrated with the stage to produce a nanomanipulation
platform. Finally, this platform can be incorporated with a commercial AFM system. The
nanomanipulation platform can also be incorporated with another microscope such as
scanning electron microscope.
Finally, the proposed ultrasonic piezo fan system requires further work as follows:
1) The dimensions of the micro piezo fan array are to be optimized. This will reduce the
equivalent capacitance of the piezo layer and thus reduce the power consumption.
2) Prior to fabrication of the micro piezo fan, the piezoelectric properties of the KNN thin
film need to be further improved.
3) The piezo fan array is to be fabricated and experimentally assessed.
113
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Appendix A: Characteristics of the Macro and Micro Piezo Fans
The flow rate at different distances between the maximum tip deflection of the piezo fans
and the heat sink was studied under low operating voltages. Table A.1 summarizes the
characteristics of the micro piezo fan operating at 20 kHz and the macro piezo fan operating at
60 Hz. The corresponding power consumptions are also presented in the table.
In this study, the piezoelectric properties including the transverse piezoelectric coefficient
(d31= 200 pm/V) and the dielectric constant (ε= 862) were based on published material properties
for KNN thin film [45]. The rest of the set was assumed to be similar to that of KNN bulk
ceramics [130].
At an applied voltage of 5 V, the total volumetric air flow rate and the total power
consumption for the micro piezo fans were estimated to be 9.24 cfm and 2.66 W, respectively.
The air flow rate was estimated 2 mm under the maximum tip deflection point in this case
scenario. By reducing the applied voltage to 2.5 V and reducing the distance between the
maximum tip deflection and the location of the estimated air velocity to 0.5 mm, the air flow rate
and the total power consumption were estimated to be 10.14 cfm and 0.66 W, respectively. This
validates the solution of reducing the distance between the maximum tip deflection and the heat
sink in order to increase the generated air flow rate and to reduce the power consumption.
It is worth pointing out that piezoelectric thin film can stand higher electric field in
comparison with that of piezoelectric bulk ceramic. It can be observed that it is possible to obtain
large flow rate with low power consumption by reducing the distance between the fan tip and the
heat sink.
130
Table B. Characteristics of the piezo fan designs
Characteristics Piezo fan at 60 Hz Micro piezo fan at 20 kHz
Piezoelectric material PZT−5A KNN
Elastic layer dimension 50×10×0.1 mm 1.7×0.3×0.037 mm
Piezo layer dimension 18.5×10×0.3 mm 0.8×0.3×0.005 mm
Capacitance per fan 9.42 nF 0.366 nF
Operating frequency 60 Hz 20004 Hz
Applied voltage 110 V 17 V 10 V 5 V 2.5 V
Applied electric field 367 V/mm 3400 V/mm 2000 V/mm 1000 V/mm 500 V/mm
Total power consumption 0.26 W 30.74 W 10.64 W 2.66 W 0.66 W
Maximum tip deflection 11.5 mm 0.513 mm 0.3016 mm 0.1508 mm 0.0754 mm
Flow rate at 2 mm* 15.72 cfm 35.06 cfm 19.74 cfm 9.24 cfm 5.10 cfm
Flow rate at 1 mm − − 27.19 cfm 11.55 cfm 6.47 cfm
Flow rate at 0.5 mm − − 30.31 cfm 19.27 cfm 10.14 cfm
* Volumetric air flow rate estimated at 2 mm under the maximum tip deflection