DESIGN, FABRICATION AND CHARACTERIZATION OF

NANOMETER-SCALE VARIABLE-GEOMETRY

MIM TUNNEL JUNCTIONS

by

ELIZABETH A. MCKINNEY, B.S.E.E.

A THESIS

IN

ELECTRICAL ENGINEERING

Submitted to the Graduate Faculty

of Texas Tech University in Partial Fulfillment of the Requirements for

the Degree of

MASTER OF SCIENCE

IN

ELECTRICAL ENGINEERING

i^^roved

December, 2004

ACKNOWLEDGEMENTS

I would like to thank all of the individuals that have helped me in my academic

endeavors over the last two years: Dr. Gale for his thorough and insightful advice,

technical support, encouragement and the opportunity to be a part of this research project,

Dr. Temkin for his constructive criticism and technical advice and the Maddox lab

research facility and students for their technical advise and support. I have a special

thanks for Dr. Dallas and Judy Patterson for giving me the inspiration to complete my

degree and guiding me toward a field that I love. I would also like to thank my friends

and family for their unyielding love and support: Stephen Frisbie, Thomas Knapczyk,

Robert Anderson, Mary Donahue, and Adam Williamson for the many laughs and great

friendship, Rosalyn Manor for being a great teacher, an inspiration and a role model, my

parents, in-laws and grandparents for believing in me when times were tough. And most

of all, my husband Patrick for his understanding, support and unselfish nature, for his

unconditional love for me and my children, for being the best dad I know, and lastly, my

children, Kylee and Brayden, for the never-ending joy they bring to my life.

TABLE OF CONTENTS

ACKNOLEDGEMENTS ii

TABLi: OF CONTEN PS iii

ABSTRACT v

LIST OF FIGURES vi

CHAPTER

I. INTRODUCTION AND BACKGROUND 1

Motixation 1

Magnetoresistive Devices 2

Magnetic Readheads 2

Magnetic Random Access Memory 3

Relation of Magnetoresistance to MIM TJ 4

Quantum Theory 5

Wave properties of matter 7

II. NON-MAGNETIC TUNNEL JUNCTION DEVICE DESIGN 10

MIM Tunnel Junction Function 10

MIM Turmel Junction Structure 10

Device Layout and Fabrication 12

Fabrication Process Design 12

Thermal Oxidation - Process No. 1 14

E-Beam Evaporation - Process No. 2 16

Lithography 1 - Process No. 3 17

Anodization - Process No. 4 20

iii

Lift-off- Process No. 5 and 6 20

III. ANODIZATION OF ALUMINUM 24

Chemical Reaction 25

Anodization Procedure 27

Barrier Aluminum Oxide Characteristics 28

Film Growth Experiment 28

Film Thickness Determination 33

Profilometer Measurements 33

Interferometeric Imaging 34

SEM Imaging 36

IV. ELECTRICAL CHARACTERIZATION AND EXPERIMENTS 44

Measurement Setup 44

Probe Station 45

Keithley 2400 and LabVIEW 46

MIM Current-Voltage Characteristics 47

TJ Area Comparisons 50

Turmel Mechanism Determination 54

Oxide Thickness Dependence 58

V. CONCLUSIONS AND DISCUSSION 64

Project Summary 64

Future Work 65

REFERENCES 67

IV

ABSTRACT

There is growing interest in a phenomenon known as magnetoresistance in such

applications as high-density hard disk drives and magnetic read-heads. The structure

used in these applications is a tunnel junction containing ferromagnetic materials and an

insulating barrier tunnel oxide. The turmel oxide is typically an Aluminum oxide (AI2O3)

film. The growth process for anodic AI2O3 was developed and characterized. Non­

magnetic tunnel junction test structure were designed and fabricated to test the electrical

characteristics of the tunnel oxide. Current-voltage plots obtained from devices of

different oxide thickness and area were used to determine the tunneling mechanism and

to investigate possible fringe effects.

LIST OF FIGURES

1.1 Magnetic Tunnel Junction Layers 1

1.2 Magnetic Recording Process 3

1.3 MRAM Structure 4

1.4 Barrier Penetration'' 9

2.1 Cross-section of the MIM tunnel junction. Sizes not shown to scale 11

2.2 Plan \ lew of device (left) and cross-section of device (right) 11

2.3 Table of Process Steps (top) and Process Flow (bottom) 13

2.4 SEM Image of Si02 15

2.5 Photo-mask for Lithography Step No. 1 19

2.6 Lift-off Process Flow 21

2.7 Photo-mask for Lithography No. 2 22

2.8 Plan View Image of Device 23

3.1 Anodization Process Configuration 25

3.2 Ionic Current at Sample Surface 26

3.3 Anodization Growth Response for Vanod of 20 V 29

3.4 Anodization Current Response Modeling for Vanod of 20 V 30

3.5. 25-Voh Current Response 30

3.6 30-Volt Current Response 31

3.7 35-Volt Current Response 31

3.8 40-Voh Current Response 32

3.9 45-Vok Current Response 32

3.10 Schematic of a Vertical Scan Interferometer.'° 34

VI

3.11 Interferometric Measurements 35

3.12 Barrier oxide - Al Interface After Anodization 36

3.13 SEM Image of Al - Barrier Oxide Interface 36

3.14 SEM Image - 20 V Sample 37

3.15 SEM Image - 25 V Sample 38

3.16 SEM Image - 30 V Sample 38

3.17 SEM Image 35 V Sample 39

3.18 SEM Image - 40 V Sample 39

3.19 SEM image - 45 V Sample 40

3.20 Anodization Oxide Growth Rate 41

3.21 Thickness Measurement Comparison 43

4.1 Picture of Measurement Setup 45

4.2 Image of Probed Sample 46

4.3 LabVIEW GUI 47

4.4 I-V Characteristic Curve (oxide thickness - 30 nm) 48

4.5 Log plot of Current (30 nm) 49

4.6 I-V curves for devices of different areas (30 nm) 50

4.7 Current density versus voltage (30 nm) 51

4.8 Projected Current Values Using Measured Values (30 nm) 53

4.9 Projected current for 5 device areas (30 nm) 53

4.10 Band Structures of MIM Tunnel Junctions 54

4.11 Fowler-Nordheim Plot (30 nm) 56

4.12 Fowler-Nordheim Plot of Multiple Area Devices (30 nm) 57

Vll

4.13 I-V for devices of different TJ areas (15 nm) 58

4.14 Current density versus voltage (oxide thickness - 15 nm) 59

4.15 Projected current for 4 device areas (15 nm) 59

4.16 I-E curves for 15 and 30 rmi thick barrier oxides 60

4.17 AFM Profile of Barrier Oxide Surface (15nm) 61

4.18 Fowler-Nordheim Plot of Multiple Area Devices (15 nm) 62

Vll l

CHAPTER 1

INTRODUCTION AND BACKGROUND

Motivation

Currently there is growing interest in a phenomenon known as magnetoresistance

that is used in the development of a new type of non-volatile memory and also in new

read head technologies.

Magnetoresistance (MR) is the modulation of current through materials based on

the magnetic orientation of the materials. This was first demonstrated in a device

developed by IBM in the 1970s called a Magnetic Turmel Junction (MJT). An MJT is

multi-layer stack of magnetic and non-magnetic material. The figure below shows the

MJT device stack. It includes a 'free' ferromagnetic layer, a turmel barrier, a 'fixed'

ferromagnetic layer and a pinning anti-ferromagnetic layer.

. ^rromagnetic Layer M.OiBarri-

Fixed 1-erromagnetic Layer Pinnin" AF Layer

Figure 1.1 Magnetic Turmel Junction Layers

An anti-ferromagnetic layer with strong exchange coupling is used in contact with

the fixed layer, which pins the orientation of the layer in one direction (indicated by

arrows above). The orientation of the free ferromagnetic layer can be changed by an

applied magnetic field (indicated by arrows in figure above). The tunnel barrier used

between the layers is AI2O3.

The orientation of the free layer can be changed by an external magnetic field,

which changes the spin direction in the material. Electrons can only tunnel through the

barrier into an empty state with the spin direction.^ So, when the magnetic spin

orientations of the ferromagnetic layers are the same the tunneling current is high (low

MR) and when the layers have the opposite orientation the tunneling current is low (high

MR). The MR of the MTJ is defined as MR = - ^ ^^^. MTJs with this geometry are

reported as having MR values of 20-50%.'

Magnetoresistive Devices

Applications for this technology include magnetic read heads for high-density

hard disk drives, magnetic field sensors and magnetoresistive random access memory

(MRAM). Magnetic read heads and MRAM make use of the Magnetoresistive Tuimel

Jimction multi-layer device.

Magnetic Readheads

Read head devices for hard disk drives have adopted this technology to increase

the areal density and thus the bit density on the hard disk. Hard disk bits are laid out in a

linear configuration in rings or tracks around the disk. A single bit is a block of magnetic

material with a magnetic orientation. The orientation determines the state of the bit. The

figure below is a representation of the write/read configuration of the hard disk.^

MR sensor

1 Shield

Read head (MR)

^W Track t' width

/ Transition Magnetization

Magnetic recording medium

Figure 1.2 Magnetic Recording Process

The resistance of the magnetic tunnel junction changes as the magnetic read head

passes over the bit. The orientation of the free layer within the timnel junction is aligned

to the orientation of the bit, thus changing the resistance of the MJT. The development of

magnetic read heads has made it possible to increase the linear and track densities on the

hard disks making high-density hard disk drives possible.

Magnetic Random Access Memory

MRAM is similar to hard disks in that data is stored in magnetic cells. The state

of the cell is changed by magnetic field applied by current flowing near the cell in bit and

word lines. Both bit and word lines must be active to produce a saturating field large

enough to change the state of the free magnetic layer in the Magnetic Tuimel Junction.

The state is determined by flowing a small current through the MJT to determine the

resistance of the jimction. The configuration of the memory is shown in the figure below

from the IBM research website.

Reading a bit

bit line

word line

Writing 1 Writing O'

wordhrte

Figure 1.3 MRAM Structure

Magnetic Random Access Memory (MRAM) is currently in development using

this same technology. MRAM has many advantages over conventional memory devices.

MRAM has proven to be 6 times faster than DRAM (dynamic) that requires a constant

refresh and as fast as SRAM. MRAM can also be significantly more dense than SRAM.

The main advantage MRAM is its non-volatile nature. It retains data when the power is

removed. The implementation of this technology could make computers faster, reduce

power consumption and also eliminate the boot-up process by retaining the memory

states after turn-off. MRAM is also faster and less expensive than present non-volatile

Flash memory.

Relation of Magnetoresistance to MIM TJ

A Magnetic Tuimel Junction relies on the change in the magnetic orientation of

the ferromagnetic layers, which in turn changes the resistance of the junction. The

jimction also relies on a process known as tunneling. The layer between the magnetic

layers is an insulator and that prevents the two magnetic materials from physically

contacting and aligning magnetic orientations and also serves as the tunnel barrier.

Electrons quantum mechanically tunnel from one ferromagnetic layer through the tunnel

oxide to the other ferromagnetic layer. This layer determines the reliability of the

Magnetic Tuimel Junction, which depends on the physical quality and electrical

properties of the tunnel oxide. The oxide must be relatively thin, thin enough for

tunneling to occur and free of pinholes that would short the junction. The electrical

tunneling characteristics of the oxide should be well understood to prior to investigating

magnetoresistive affects in the film to avoid confusion between magnetic and non­

magnetic effects. Therefore, the project presented in this thesis is a precursor to a project

that will include Magnetoresistive Tunnel Junctions. This project has several main

objectives which include: establishing a grow process that produces a high quality tunnel

barrier oxide and determining the electrical characteristics of the grown films. This

includes designing, building, and testing a non-magnetic test device. A process was

developed to grow AI2O3 using an electro-chemical process. A Metal-Insulator-Metal

(MIM) tunnel junction device was designed and fabricated. The test device was tested

using a measurement system developed our group developed and the data was analyzed

to determine the conduction mechanism present in the films of different oxide thickness,

as well as investigate the presence of fringe affects in the device. Tunneling of electrons

through dielectric material is based on the principles of Quantum Mechanics.

Quantum Theory

In this project electrons from a metal material must move through the insulating

material to the other metal material. To understand the theory behind this project, it is

important to understand how conduction occurs in insulating material. Conduction in

materials was first explained using classical physics and was later modified using

quantum physics. Conduction in metal is fairly simple and was described classically as

electrons moving through metal as a sea of free electrons and was later elaborated on

using quantum theory with the addition of the concept of quantized energy states.

Understanding the movement of electrons through an insulation material is vital to

understanding the theory behind this project. Conduction in insulators is not possible in

classical physics without the breakdown of the material. Quantum theory makes it

possible for electrons to appear on the other side of the insulator, otherwise knowTi as

tunneling through the insulating material. In order to understand this phenomenon, a

short introduction to quantum theory is required.

Quantum physics originated following the ultraviolet catastrophe. This

phenomenon describes the disagreement between the classical physics Rayleigh-Jeans

law describing blackbody radiation intensity versus wavelength and experimental data

taken of blackbody radiation. Following this in 1900, Max Planck developed an equation

that fully describes blackbody radiation. During this work he assumed that the oscillating

molecules have only discrete energy values equal to E^ = nhf, where n is a positive

integer known as the quantum number, h is Planck's constant, and/is the natural

frequency of the molecule oscillation. Every energy level is considered the quantum

state. He also proposed that when a molecule moves from one energy level to another,

the molecule emits or absorbs a photon of energy,

E„=hf. (1-1)

Planck's concept of quantized energy was revolutionary to physics and later research

based on this concept was used to explain physical phenomenon that could not be

explained by classical physics."*

In 1921 Einstein extended quantization to describe the photoelectric effect, were

he suggested that light waves were made up of photons. This concept was also used by

Niels Bohr to model the atom. The Bohr model states the electrons occupy discrete orbits

around the atom nucleus. This is very different from the classical model of the atom that

says that the electrons constantly give off radiation and eventually spiral into the nucleus

of the atom. In the quantum model radiation is only given off when an electron moves to

a lower energy state. The change in energy of the electron is equal to h*f.

From this concepts Louis de Broglie suggested that since photons have particle

and wave properties that maybe all forms of matter have both properties. During de

Broglie's Nobel Prize acceptance speech he suggested that electrons must also have wave

properties. To investigate this assertion, the double slit experiment was applied for

electrons. This experiment showed that electrons do if fact exhibit wave like properties.

Wave properties of matter

The results of the double-slit experiment show unmistakable evidence of the wave

property of electrons. An interference pattern is produced on the other side of the sUts. If

the electrons only had particle properties the pattern would just be the addition of the

distribution fi-om the individual slits.

The wave property of matter is described by a complex-valued wave function, \}/.

The wave function gives all of the information that can be known about the particle. The

absolute square, | |% of the wave function describes the probability of finding a particle

at a given point and time. The wave equation for a free particle moving along the x-axis

can be written as,

i/{x)=Asm\-^\ = Asm{kx) (1.2)

and the probability density, the probability per unit volume is | y/(x) p.^

For every system there are boundary conditions that exist for the system that

defines a set of wave functions. Each wave function describes the wave properties for a

particle with a given energy. These wave functions must also satisfy the Schrodinger

equation. One method in quantum mechanics is to obtain a solution to the equation,

which will give the allowed wave functions and energy states of the system."*

The Schrodinger equation for a particle moving along the x-axis that is

independent of time is

-^^--(E-U),. 0.3)

Equation 1.3 can be solved and the allowed wave functions for the system can be

determined if the value of the potential energy U(x) at the boundaries of the system are

known. Wave functions from one boundary to the next must be continuous. This is the

most important concept for the concept of electrons tunneling through a potential barrier.''

The figure below illustrates the barrier penetration situation. The wave function

in the region of the barrier exponentially decays and if the barrier is sufficiently thin so

that the wave function does not decay to zero then the wave fimction will continue on the

other side of the barrier with smaller amplitude.

Barrier V/( V) = A,e~'"J

Amplitude A,^

•- Tunnelinu

Figure 1.4 Barrier Penetration^

This means that there is finite probability that an electron that hits this barrier can be

found on the other side of the barrier. Contrary to classical physics that states, when a

particle hits this energy barrier the particle would be reflected and could not cross

through the barrier.

The probability that electrons will turmel through the barrier is exponentially

dependent on the barrier height and thickness. There are several different types of

tunneling that is discussed fiirther in Chapter 4. Experimental data obtained from the

MIM Tunnel Junctions leads to Fowler-Nordheim tunneling as the conduction

mechanism in the films. The MIM TJ device description and fabrication process is

described in the following chapter.

CHAPTER 2

NON-MAGNETIC TUNNEL JUNCTION DEVICE DESIGN

A Metal-Insulator-Metal (MIM) tunnel junction was developed to determine the

physical and electrical properties of a barrier oxide, AI2O3. In this chapter, a description

of the device function and the physical design will be discussed. The fabrication process

will also be described. This includes, mask design and layout, thermal oxidation,

lithography, metal deposition and anodization.

MIM Tunnel Junction Function

The fimction of the MIM tunnel junction is to allow the characterization of the

AI2O3 film in a simple device structure. A tunnel junction is a device made up of a three

fimctional layers: metal layer 1, dielectric material (in this case AI2O3), and metal layer 2.

An electric field created in the oxide by a voltage is applied across the two electrodes.

When this field approaches the breakdown field of the oxide quantum mechanical

tunneling as described in detail in Section 1.3.3, will begin. The tunneling current from

this device will be measured and characterized and the conclusions taken from this

project will be used in a subsequent project that uses AI2O3 in a magnetoresistive tunnel

junction for the use in Magnetic read head or MRAM applications.

MIM Tunnel Junction Structure

As stated above the tunnel junction is made up of three layers, the bottom

electrode is a 75 nm thick Al, the dielectric material AI2O3 with a specified thickness, and

the top electrode is a 75 nm Al layer. The TJ is fabricated on a Si wafer substrate with a

10

surface Si02 insulating layer. Figure 2.1 is an illustration of the device stack, not shovm

to scale.

Tunnel Oxide AI2O3 (10-30 nm)

Al 2 (75 nm). ^

Al (75 nm;>

Figure 2.1 Cross-section of the MIM tunnel junction. Sizes not shown to scale.

Figure 2.2 shows the device plan view. In the geometry of this device the area of

the top electrode defines the tunnel junction contact area. The barrier oxide and bottom

Al contact are common to the devices and will not affect the tunnel current. The device

layout was designed to incorporate top Al electrodes of different areas. The electrode

areas are: 100x100, 75x75, 50x50, 25x25, 10x10 ^m.

Figure 2.2 Plan view of device (left) and cross-section of device (right).

The different areas were included to investigate the effect of contact area on the

ttinneling current, as well as explore the possibility of perimeter/fringe effect. It could

also be used later to estimate or project the tunneling current of ttinnel junctions of

different areas that have the same oxide thickness. The perimeter affect and projection

evaluations will be discussed in detail in Chapter 4.

11

Device Layout and Fabrication

The fabrication of the MIM Tunnel Junction uses standard semiconductor

processes on 2" Si wafers. A 2" wafer will contain a 25x25 array of the set of 6 devices

shown in the plan view of Figure 2.2. This 2" wafer contains 3,750 individual tunnel

junctions with the same oxide thickness. There are five main process steps (1) fabrication

process design, (2) thermal oxidation, (3) bottom contact deposition, (4) oxide patterning

and growth, and (5) top contact lift-off.

Fabrication Process Design

The first step in the fabrication of the device is determining the process flow. It

requires some design thought. Each of the processes must work in harmony with the

others and the design of the device must include features not only necessary for device

function, such as known tunnel junction contact areas, but also for fabrication like

photomask alignment marks or considering features necessary for a later process. The

process steps are determined and checked for compatibility. Figure 2.3 below describes

the process steps for each layer in the device.

12

LAYER No.

T- C

M

3 4

5

No.

1 2 3

4 5 6

PROCESS NAME Purpose

Wet oxidation E-Beam Evaporation Lithography 1

Anodization Lithography 2 E-Beam Evaporation

Grow Si02 Deposit Al Define areas of AI2O3 growth

AI2O3 growth Pattern PR for Al dep. Deposit Al and dissolve PR

Note: 5 & 6 together describe a process called lift-off

Si substrate

Oxidation

Deposition 1

Lithography 1 PR

-AJJ

Anodization

PR

Al 1

^ Tunnel Oxide AI2O3

Remove PR ^ Tunnel Oxide AI2O3

Lithography PR

A l l

Tunnel Oxide A1,0,

Figure 2.3 Table of Process Steps (top) and Process Flow (bottom).

13

Now that the processes are knovra for each device layer, the lithography

photomask must be designed and created for the two lithography steps in the process

flow.

Thermal Oxidation - Process No. 1

The substrate that the MIM Tunnel Junction was fabricated on was a 2" Si wafer.

The Si wafer is used to support the device and the Si02 layer of 1 [xm in thickness is the

electrical isolation from the Si. Therefore, the orientation of the crystal lattice, resistivity

and doping of the Si wafer have no consequence on the function of the device.

The process used to grow the thick Si02 is wet thermal oxidation. This process

uses the principles of diffusion to grow the oxide. In the oxidation process the clean Si

wafers are loaded into a furnace, heated to 1200°C and O2 molecules are introduced, and

for wet oxidation water vapor will be introduced. The O2 molecules diffuse through any

oxide already on the Si surface to react with the Si atoms to form Si02. The growth rate

of the oxide slows as the oxide thickness increases as described in the Grove-Deal model,

ti+At„,=B(t + r), (2.1)

where the thickness of the oxide U has a quadratic relationship to the oxidation time t.

Oxides grown using the wet oxidation process are growth much faster and are less dense.

Most often thick (100-1000 nm) wet thermal oxides are used for electrical isolation.

Oxide grown without the introduction of water vapor, dry thermal oxidation, is more

dense and typically used for gate and tunnel oxides.

14

The oxidation process used for this device is outlined below and can be seen in

more detail in the appendix under process checklists.

1. Load Si wafers

2. Turn-on furnace and water hot plate

3. Adjust final temperature to 1100°C

4. Turn on O2 feed at 600°C

5. Begin timing at 1100°C for 1 hr

6. Turn off furnace

7. Turn off02 feed at 600°C

8. Cool dowTi to room temperature

9. Unload wafers

A Scanning Electron Microscope (SEM) image of the Si substrate and Si02 layer is

shown below.

Si02 {

lum EHT=17 00kV

I I W0= 3

Sign«l A " InLons Date 4 May 2004

Pho toNo= ie44 Time 11:04 00

Figure 2.4 SEM Image of Si02

15

The next step in the fabrication process is the deposition of the bottom Al contact,

layer 3 in Figure 2.3. In the anodization process it is necessary to make electrical contact

to the entire wafer surface. Therefore the bottom contact will be a "blanket deposition of

Al, meaning that the entire wafer surface will coated with Al. The process used to

deposh the Al metal layer is Electron-Beam (E-Beam) Evaporation.

E-Beam Evaporation - Process No. 2

E-Beam Evaporation is a process used to deposit metal and sometimes insulators

onto wafer samples. The system is comprised of a vacuum chamber, an evacuation

system, and a control system. The sample and target metal are held at a moderately high

vacuum (8x10' Torr) in the vacuum chamber. The chamber uses a two-stage evacuation

procedure where a roughing pump will bring the chamber to low vacuum (3x10"^ Torr)

from atmosphere and then a Cryo pump is used to bring the chamber to a high vacuum

level at ~8xlO" Torr. The control system is used to control the material specific power

output program, deposition rate and film thickness. The power output program is

responsible for controlling the power output by adjusting the current supplied to a

tungsten filament. The electrons will boil off the filament and a focused electron beam is

guided by magnetic fields onto the target metal held in a graphite crucible. Once the

melting point is reached, the metal will evaporate and travel across the vacuum chamber

and accumulate on the wafer surface as well as on the walls of the chamber. When the

program is ran it automatic mode the power is adjusted so that the specified deposition

rate is maintained. The power will then ramp down to zero once the specified thickness

is reached.

16

The deposition desired for this process step is a blanket deposition, meaning that

Al is deposited on the entire Si02 layer on the Si wafer. A brief process for deposition is

outlined below and the fiill process can be found in the appendix under process

checklists.

1. Load samples and target metal into chamber

2. Close chamber

3. Rough to 3x10"^

4. Pump using Cryo to ~8x 10'

5. Set program material, rate and thickness

6. Deposit

7. Cool Down

8. Remove Samples

A 75 nm thick Al film was formed using the E-Beam system. The deposition rate

was held at 1 A/sec at a pressure of 1x10" Torr due to a chamber leak.

The next process step is to section off areas of the device where AI2O3 will be

grown using anodization (Figure 2.3). Photolithography is the process used to pattern the

sample.

Lithography 1 - Process No. 3

Photolithography is a process used to transfer a pattern, typically squares and

rectangles, to a semiconductor sample. The sample is coated whh photoresist, baked,

exposed to UV light, and then developed in a chemical developer.

17

Photoresist is applied to the center of the sample and rotated in a Spinner at 2000-

4000 RPM (depending on application), spreading the photoresist evenly across the

sample surface. Photoresist is a fluoropolymer that is sensitive to radiation or light (in

our case). Light strengthens or weakens chemical bonds depending on the type of

photoresist. For positive photoresist, the chemical bonds are weakened and become

soluble in a developer, and for negative photoresist the opposite affect occurs.

The sample is baked to evaporate the solvent and to strengthen the chemical

cross-links in the photo-resist. This ensures that the unexposed areas will withstand the

chemical development process.

A photo-mask is placed directly on top of the sample and is held in contact with

the sample and then exposed to UV light. UV light shines through the photo-mask onto

the photoresist. The chemical bonds will be broken in the exposed regions. The photo­

mask used in this process is a glass plate with transparent and non-transparent regions.

Light passes through the transparent region and is blocked by the non-transparent region,

a chrome metal pattern.

Lastly the sample is placed in a chemical solvent and in this case the exposed

regions of photoresist are removed.

Lithography step 1 was used to open up holes in the photoresist and expose the

underlying Al layer in these areas to the chemicals used in the anodization process. In

order to do this lithography process a photo-mask must be designed and fabricated. A

mask computer file was generated that defines the pattern for the lithography step. This

file is then used to make the photo-mask. The software used to generate the photo-mask

18

was LASI 7. Figure 2.5 below is (left) a view of one device rectangle and (right) a view

of the full wafer mask files.

ai">^- |«]»^. . - '«]f-Lui a — * " 0»Bi>M»'

Figure 2.5 Photo-mask for Lithography Step No. 1.

A mask file was also generated for the lithography step 2 in the same manner.

Lithography step no. 2 will be discussed later in the lift-off process section.

The lithography recipe used in this process is briefly outlined below. The

photoresist used was Shipley Microposit SI813 at an approximate thickness of 1 ^m and

the developer was MP Developer.

1. Spin on at 3700 RPM for 30 sec

2. Bakeat 115°Cfor 1 min

3. Exposure: Expose for 7 sec

4. Develop: 24 sec

5. Rinse: DIH2O

6. Hardbake: Bake at 120°C for 20 min

The next process step is to grow the AI2O3 oxide in the regions of the Al not covered by

photoresist (Figure 2.3).

19

Anodization - Process No. 4

The anodization process is the key process in the fabrication of the tunnel junction

device. The oxide layer is the focus of the project and the method by which the oxide is

grow will affect the function of the device. Anodization can produce the high quality

barrier oxide needed for this project. The anodization process was developed in the

Maddox lab as a crucial part of this project.

The following chapter will describe this process in detail as well as the

characterization of the anodic oxide films. Followdng the growth of the anodic tunnel

oxide as described in chapter 3, the photoresist was removed and the next process was

started. The last layer of the device, the top electrode, was fabricated in two process

steps (processes 5 and 6, Figure 2.3) called lift-off.

Lift-off- Process No. 5 and 6

This two-step process combines a lithography step and a deposition step. The end

result is a patterned metal layer. Photoresist is patterned on the sample and then Al is

deposited on the wafer. The Al will deposit on top of photoresist and on the oxide area

where the photoresist was removed. After the deposition the photoresist is dissolved in

acetone, leaving the Al deposited on the oxide surface and removing the Al on top of the

resist, thus called lift-off. Below is an illustration of the two steps to lift-off.

20

Pattern PR PR

- A U s^

Tunnel Oxide AljO^

Deposit All

Tunnel Oxide AljOj

Figure 2.6 Lift-off Process Flow.

For this process it work correctly the Al on the photoresist and the oxide can't be

a continuous layer and Al can not have a good step coverage (no deposition on the

sidewalls of photoresist.) This should not occur using E-Beam evaporation to deposit the

Al layer.

Electron-beam evaporation produces a deposition with virtually no vertical

sidewall coverage. This can be a disadvantage when complete coverage of all features is

needed or it can be an advantage, as in our case of liftoff (discussed in a later section),

when no sidewall coverage is desired.

There are two main factors involved in this phenomenon. At high vacuum levels,

the evaporated atoms travel in a straight line from the crucible to the nearest surface

where the atoms condense on that surface. This is shown by the large mean free path of

approximately 64 m calculated from

1 A = - (2.2)

4*V2*;r* — *r^ kT

21

Where r (1.82 A) is the mean radius of the atom, P (1x10'^ Torr) is the chamber pressure,

k is Bohzmann's constant and T (373 K) is the temperature. This means that the atoms

vsdll on average travel 65 m before the atoms collide and change direction. So, there

should be no atoms moving in arbitrary directions that may eventually land on the

vertical sidewalls of the wafer features.

Also the evaporated atoms diverge from the crucible in a 180° steradians cone, but

the largest angle of the incident atoms is 6 ° from the vertical axis. So, at this angle very

few atoms with hit the vertical sidewalls.

These factors help the process of lift-off where it is key that no metal is present on

the Photoresist sidewalls. Also, the lithography process used eliminates the hardbake

step at the end, which leads to rounded photoresist edges. The resulting sharp angle of

the photoresist at the edge ensures discontinuous coverage.

The lithography process is the same (except the hardbake step is removed) and

deposition is also the same process as described earlier. The mask files for lithography

set 2 are shown below where (left) is a view of one device rectangle and (right) is a view

of the full wafer design.

Figure 2.7 Photo-mask for Lithography No. 2.

22

This mask would be aligned directly on top of the oxide features. The top

electrode squares should be centered on top of the oxide rectangle. The mask was

designed with alignment marks (squares located in each comer of mask) to ease the

alignment of the two layers. The image below is the top view of the completed device

taken from a optical microscope.

Figure 2.8 Plan View Image of Device

The device fabrication and design consisted of 6 process steps containing

oxidation, deposition 1, litho 1, anodization, litho 2 and deposition 2. The anodization

process used to grow the AI2O3 tunnel oxide is described in the next chapter.

23

CHAPTER 3

ANODIZATION OF ALUMINUM

Anodization is an electro-chemical process that produces an AI2O3 (oxide layer)

when Aluminum atoms react with oxygen atoms in an electrolytic solution in the

presence of an electric field.

Anodic oxide has two characteristic structures, porous oxide and non-porous

barrier-type oxide. A porous Al oxide is grown in an acidic electrolyte solution such as

sulfuric acid. Barrier-type oxides are grown in neutral electrolytic solutions such as

ammonium phosphate, borate and tartrate. Porous Al oxide is produced by a growth and

etch process that produces an ordered array of pores in the oxide layer. This oxide is a

thick oxide (up to 100 |im) and is often used as a protective coating on machined Al

parts. The barrier oxide is strictly a growth process and has electrical properties suitable

to be used as the dielectrics in capacitors.^ It contains no pores and is usually at least 100

times thinner than porous oxide.

Depending on the growth conditions, barrier oxides can be amorphous or

crystalline. The anodization procedure will be different depending on the desired

structure of the oxide, amorphous or crystalline. By changing the structure of the native

oxide present on the sample, the structure of the grown film will be changed. To produce

a crystalline oxide film the sample is annealed prior to anodization.

For crystalline oxide films an initial bake at ~550°C prior to anodization is added

to the procedure above. This bake produces a slightly thicker native oxide that contains

seed crystals for which the barrier oxide will grow. The field that the oxide supports is

higher for crystalline films. Thus, a higher voltage must be applied to begin conduction

24

and for the use as a tunnel junction eventually containing magneto resistive layer this

characteristic would lead to higher film resistivity and thus lower signal levels. For an

application that requires a crystalline film fiarther research is necessary to determine the

anodization growth characteristics of the appropriate current density and the approximate

voltage/thickness. For this project the aluminum oxide film will have an amorphous

structure.

Chemical Reaction

In anodization the sample is placed in an electrolytic solution, ammonium

pentaborate in H2O (borate-H20). The sample is coimected to the positive terminal, the

anode, of a power supply; a non-reactive metal plate is cormected to the negative

terminal, the cathode. An electric field is produced across the anode and cathode by an

applied voltage. The diagram below shows the experimental setup for the anodization

process.

Glass Beaker •

Wafer (Anode)

Metal (Cathode)

Electrolyte Solution

Figure 3.1 Anodization Process Configuration.

25

Current flows in the form of ions through the electrolyte solution. An electrolyte

is a chemical that when dissolved in water produces positively and negatively charged

ions called cations and anions. So, the anions would flow toward the sample (anode) and

cations would flow toward the metal plate (cathode). A negative potential builds at the

sample surface due to the movement of positive ions away from the sample and an

increase in negative ions at the sample surface. This causes Al ^ ions from the sample to

diffuse across the forming oxide and react with 0~" ions in the electrolyte solution. O '

ions also diffuse across the oxide layer and form oxide at the Al/oxide interface. The

diagram below illustrates the ion transport across the oxide film.

Al Oxide O 2-

Al 3+

Electrolyte

H2O

H2O

i3+ •

Figure 3.2 Ionic Current at Sample Surface.

In neutral solutions the ion transport number of Al "" is nearly 1, where Al^" ions

at the oxide/solution interface contributes most to the forming of the oxide. In acidic

solutions where electrolysis occurs the ion transport is 60% oxygen and 40% Al ions.^

The overall chemical reaction taking place during anodization is :

2A1 + 3H2O •» AI2O3 + 3H2 (3.1)

As described above, reactions occur at the metal/oxide interface and the oxide/electrolyte

interface at the anode. The reaction at the metal/oxide interface is^

2A1 + 30^' •» AI2O3 + 6e- (^-2)

26

The reaction at the oxide/electrolyte interface is^

2A13V 3H2O ^ AI2O3 + 6H^ (3.3)

The overall reaction in the solution produces a hydrogen gas evolution^:

6H^+6e"-*3H2 (3.4)

The composition of A1203 is 62% Al atoms and 38% oxygen atoms.

Anodization Procedure

During the process both the constant source current and the voltage are monitored

using a Keithley 2400 Source/meter that is interfaced and controlled using its GPIB

capabilities in conjunction with LabVIEW computer software. The current density used

during the process will affect the rate at which the oxide grows. The thickness of the

oxide is directly proportional to the final voltage level reached. Growth of the Al film

begins to occur around 1-2 V due to the native oxide present on the surface. As the

voltage is increased beyond this the thickness of the oxide is increased and E-field in the

oxide remains constant. When the voltage is kept constant the ionic current stops due to

a loss of potential difference. One advantage to this process is the ability to control the

thickness of the oxide by the voltage. The adjustment of the final voltage will determine

the final thickness of the oxide.

The procedure for growing amorphous aluminum oxide is outlined below:

1. Connect sample (anode) and metal (cathode) to power supply with clips

2. Place in electrolyte solution - 0.1 M ammonium pentaborate (borate-H20)

3. Anodize at constant current density of 1 mA/cm^ to the desired final voltage

4. Remain at final voltage for 60 sec.

27

Barrier Aluminum Oxide Characteristics

Barrier-type oxide grown on aluminum in a neutral electrolyte solution should not

contain pores and should be uniform in thickness due to the uniform voltage drop across

the sample surface. The oxide should have a density close to 3.17 g/cm^ and a refractive

index n = 1.767-1.772.^

The film needs to be uniform and free of pinholes. Pinholes are areas where the

film thickness is zero and the top and bottom aluminum electrodes are in direct contact.

The presence of pinholes would short the device and any tunneling current present would

be to weak to detect in comparison to the direct conduction. The thickness uniformity

could also affect the tuimeling current due to varying dielectric thickness. The uniformity

of the film was investigated in SEM micrographs and the presence of pinholes will be

evident in the I-V characteristics.

Film Growth Experiment

To determine the proper procedure for anodic oxide growth the following

experiment was performed. Test films were grown on aluminum and the film

characteristics and growth characteristics will be discussed.

A 1000 A Al layer was deposited by E-Beam Deposition onto a Si wafer substrate

and Positive Shipley SI813 photoresist was patterned using the standard

photolithography on the Al layer (masking some Al and exposing others to film growth).

This procedure was followed by a 20-minute hard bake at 120 °C.

28

The samples all had an exposed Al surface area of 2 cm^ and a suggested

anodization current density of 1 mA/cm^. Thus, a 2 mA constant current source was

supplied and the voltage compliance was set at the desired final anodization voltage,

Vanod- 20, 25, 30, 35, 40 and 45 V values were used to generate a group of samples with a

range of film thickness.

The voltage and current response during the anodization process remained

constant for each sample. The data graph for a 20V compliance setting depicts this

response.

~\ 2 0 -

1 8 -

1 6 -

1 4 -

Vol

tage

(V

)

.1

.1

.1

..

6 -

4 -

2 -

0 -

1 1 1 ' 1 1

~1 / /

/

/ \

I ^^

1 ' 1 ' 1 1

- 0 0020

-0.0015

-00010

- 0 0005

- 0 0000

1 1 1 1 1 r 1 • 1 • 1 1 0 50 too 150 200 2SU JUU

Time (sec)

Figure 3.3 Anodization Growth Response for Vanod of 20 V.

During the anodization growth the voltage lineariy increases with constant current

supplied (film growth region). When the voltage reached the specified compliance value

the current drops exponentially and approaches an asymptotic value. The current

response can be modeled by a 2"" order exponential decay. Though it has no theoretical

basis and a first order decay could be explained by the loss of ion motion through the

oxide during the process. The 2"'' order decay fits for all anodization samples at different

29

Vanod values. The curve fit for a 20V samples is shown below including the model

equation and constants.

0 0020

< ^ 0,0010-(U

fc o

0 0005

0 0000

Current Response Exponential Curve Fit

Data Model

Chl"2 R'2

yO Al t1 A2 12

3ata1 0 ExpDec2

= 60511E-10 = 0 98007

0 to 1 00441 5 60658 0 00042 147 26776

±0 08905 10 07071 t4 7909E.6 ±1 82473

yO + A1e''(-x/t1) + A2e"(-x0/t2)

100 — I —

150 200 250

Time (sec)

Figure 3.4 Anodization Current Response Modeling for Vanod of 20 V.

A graph of the response for each of the samples for different Vanod values is

shown below.

26-

24

22-

20-

18

16

14

12

10

8

6

4

2

0-

-2

0.0022

- 0.0020

-0.0018

0.0016

00014

O 0 0012 5

B 00010 3-

>

0.0008

0.0006

. 0.0004

- 0 0002

50 -T

150 100

Time (sec)

200

Figure 3.5. 25-Voh Current Response.

30

32

30

28

26-

24-

22-

20

18-1 0)

^ 16

o >

14

12-

10-

8

6

4

50 —r-100 150

Time (sec)

— I — 200 250

0 0005

0.0004

-0.0003

-0 0002 X >

0.0001

00000

Figure 3.6 30-Volt Current Response.

0)

•5 >

00022

- 0 0020

0.0018

00016

00014

o 0.0012 ^

00010 ^

> 00008

0.0006

0 0004

00002

100 150

Time (sec)

Figure 3.7 35-Voh Current Response.

31

4 0 -

30

(U 20

ss >

/

/

• /

/

; /

' 1 1 ' 1 -• 1 ' 1

1 ' 1 • i ' 1 ' 1 1 1 ' — 1 — 1

0 5 0 100 150 200 250 300

- 0 0 0 2 0

-0 0015

o c —t

-00010 s (A)

-0.0005

-0 0000

Figure 3.8 40-Vok Current Response. Time (sec)

50

40

30-

S" 20 o >

10

1 ' 1 ' 1 ' 1 1 1 1 1 1 r

— I 1 1 • 1 1 1 ' 1 ' 1 — 50 100 150 200 250 300

Time (sec)

00020

0.0015

-0.0010 o c

3

0.0005

-0.0000

Figure 3.9 45-Voh Current Response.

32

The response curves are very similar. It is evident from each that the voltage

increases and the current remains constant while the oxide is growing and when the

specified final voltage is reached the current decreases exponentially. Also, most of the

graphs show an increase in voltage at approximately 1 volt, which corresponds to a native

oxide thickness of ~1 nm. The 30 and 35 Volt anodization begins at ~4 volts because the

process was stopped with an oxide thickness around 5-6 nm and then the process was

restarted.

Film Thickness Determination

Determination of the thickness of the film is necessary to determination of the

relationship between final anodization voltage VANOD and the thickness. The thickness of

the anodic film is directly proportional to the final anodization voltage. A growth rate

ranging from 1.2-1.4 nm/volt can be expected for anodization of aluminum. ' The

thickness can be estimated by^,

c/ = 1.24xF^^o^+2.0. (3.5)

Through experimental data measurements a model for the relationship will be

presented. Several means for determining the thickness were used including profilometer

measurements, interferometric microscope measurements, SEM images and capacitance

measurements.

Profilometer Measurements

The first attempted method used the Dektak profilometer, which physically scans

the surface with a stylus. This method gave inconsistent resuhs for the thickness and

33

much lower values than estimated values for thickness. The profilometer scanned the

surface and on occasion would output a step transition height of around 7 nm.

Interferometeric Imaging

An interferometric microscope was used next to optically image and profile the

surface. The interferometer used was the Veeco NT 1100 Interferometric microscope. A

interferometric microscope otherwise known as a optical profiler constructs surface

features from the coherence peaks of two interfering light wave fronts.

DclccliM . \ l i a \

kiiiiiiialor

Diyili/Ld lnUii.sil>

)ala

Magnilkai iod Sclccior

Light Source

.Mi rail Interferometer

Sample

Figure 3.10 Schematic of a Vertical Scan Interferometer.

Light is coUimated and split by a beam splitter where one beam is reflected off the

sample surface and the other beam is reflected off of a reference mirror. The beam

recombines and creates a pattern of interference fringes. The intensity of the fringes is

measured along the vertical axis. At the position of the coherence plane where the

intensity is highest, the optical path length is zero. By combining many intensity

34

measurements the surface of the sample can be constructed by referencing the reference

10 mirror. The figure below shows the images captured using this method.

^Veeco X Profile X: 0J56 mm

Oft) q

OfiO -

OX' -

0 Ic -

i^V^^^^^^^

^wpl '

F

~ i — I — I — 1 — 1 — I — I — I — [ — r

00 01 o ; o j 01 05 0& o : 08 0^ 10 11 1.: u u

Y Profile X: 30 8 urn

fx. 542.9 Jtii ^ ( Y -3 6nm J

X Y Ht Dist

Angle

115 0.33

-2 69

- mm mm

- am - mm

"

Title: Subregion

Note: X offset: 16 " Y offset: 141

>00 TOO

Figure 3.11 Interferometric Measurements.

The vertical transition from Al to AI2O3 is shown at ~9nm and the thickness

estimated from equation 3.1 was 26.8 nm. A conclusion was taken from this

measurement and the previous measurements. During anodization the oxide consumes a

portion of the Al and grows not only up from the sample surface but down as well. The

illustration shows the cross-sectional profile of the grown oxide.

35

Al Oxide

Figure 3.12 Barrier oxide - Al Interface After Anodization.

This accounts for the 9nm thickness measurement. This quantifies the growth of the

oxide in the upward direction from the original Al surface.

SEM Imaging

The next method used to accurately measure the film was to take SEM images of

the film cross-section. The phenomenon illustrated above is viewed in the image below.

Figure 3.13 SEM Image of Al - Barrier Oxide Interface.

This SEM image was taken using the LEO Supra 35 without the optimal settings.

The SEM images a sample by focusing an electron beam on the area to be imaged that

emits secondary electrons off the surface of a sample and uses those electrons to

construct the image. To get high quality images it is important to think about the

materials of the sample and the way they interact with the focused electron beam. In this

case the materials to be imaged are aluminum and aluminum oxide. Aluminum is a high

z-number material that emits many electrons and oxide is an insulator that under the e-

beam can accumulate charge on the surface and this charge shows up as a cloudy white

haze. But this oxide is so thin that the electrons penetrate the oxide and image the Al

underneath. In the image above the Al layer is light in color due to the greater number of

36

emitted secondary electrons and the oxide is darker in color due to a fewer number of

electrons reaching the Al layer underneath and back through the oxide and very few

electrons are emitted from the oxide layer itself. The optimal SEM parameters used for

imaging the sample are listed below:

• Accelerating Voltage: 17 kV

• Detector: In Lens

• Working Distance: 3-5 mm

• Aperture: 20 |j.m

A stage angle of 60° from the vertical was used to image the sample. The marked

values shown in the images below must be divided by the Cos(60) to get the actual oxide

thickness. The following images are a sample of the images used to determine the

thickness versus voltage relationship.

lOOnm EHT = 17.00 kV

I WD = 6 mm

Signal A = InLens

Photo No. = 2120

Date :3 Jun 2004 Time :1 3:01:68

Figure 3.14 SEM Image 20 V Sample.

37

Figure 3.15 SEM Image - 25 V Sample.

Figure 3.16 SEM Image - 30 V Sample.

38

Figure 3.17 SEM Image - 35 V Sample.

Figure 3.18 SEM Image - 40 V Sample.

39

Figure 3.19 SEM image - 45 V Sample.

Many SEM images were taken for the samples and measurements from the

images were compiled to form the data graph shown below. The data labeled 'thickness

measurements from SEM pics' shows the average thickness value as well as the deviation

in measurements shown in the error bars. A linear curve fit was performed on the results

'Linear Fit of ThicknessDataC', showoi in red, and a characteristic equation was found.

The equation is:

:v = 1.54x-0.427. (3.6)

You can see that the growth rate was determined to be 1.54nm/volt and according to this

least squares linear fit the y intercept is -0.427. This would mean that the thickness at 0

vohs is negative .427 nm. and no native oxide is present on the surface. The

measurements on the images have some deviation and leads to possible errors in the

40

determined growth rate. A function of y = 1.53x is plotted in blue below that illustrates

another possible characteristic equation.

- Thickness Measurements from SEIVI Pics - Linear Fit of ThicknessData_C -y= 1.53*x + 0 Linear Regression for ThicknessData_C:

Y = A + B * X Weight given by ThicknessData_B error bars.

Parameter Value Error

-0.42767 1.54588

0.19848 0.00633

SD N

0.99999 0.02676 0.00261

I ' I ' ' ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

Final Voltage (V)

Figure 3.20 Anodization Oxide Growth Rate.

The experimentally determined growth rate is close to the same values stated in

literature.^ There are several sources that contribute to the difference in experimental and

cited values. Measurement errors in the collection of thickness values from the SEM

pictures are possible. The major source of difference is in the actual anodization process.

As stated above the growth rate is dependant on the current density. The Al surface area

exposed during anodization was estimated and assumed to be the same for each sample,

but slight variations could adjust the growth rate for each individual sample. If the

estimation of the surface area was lower than the actual surface area then the current

density used would be higher and the growth rate higher as well.

Capacitance measurements have also been used to determine the

thickness/voltage relationship of the oxide. By measuring devices with known contact

areas the thickness can be calculated using.

41

c = e*Sf^xA

D • (3.7)

Where e is the dielectric constant of 9.1, EQ is the permitivity of free space of 8.85 E -14

(F/cm), A is the known contact area, and D is the film thickness. These measurements

were taken using three samples at final anodization voltages of lOV, 20V, and 30V. An

LCR meter was used to measure the capacitance of the fabricated tunnel junction.

An LCR meter measures capacitance by applying an AC voltage across the

capacitor, calculating the reactance of the capacitor from x^. = ^ - and using this value in

1 x„ — ' InfC

to calculate the capacitance."

The results of the measurements are shown in the table below.

Final Anodization Voltage: 10V Oxide area measured Cap (pF) Calculated thickness (nm)

100 75 50 25

49.4 30

12.9 3.5

16.303 15.100 15.608 14.381

Table 3.1 Capacitance Measurement Summary.

1.630 1.510 1.561 1.438 1.535 Average Rate

Oxide area ^ ^ ^ ^ ^

^HP

Oxide area

^•B IHIIIh

100 75 50 25

100 75 50 25

Final Anodization Voltage: 20V measured Cap (pF) Calculated thickness (nm)

1 ^ ^ ^ ^ . 22.8 m«mmm«,«mmmi 35.322 13.4 33.807 6.1 - ' ^ ^ ^ ^ 33.006

1.97 25.550

Final Anodization Voltage: 30V measured Cap (pF) Calculated thickness (nm)

18 44.742 10.5 43.144

vtwrnmrne^-T^k 1.690 1.650 1.278 1.596/

^ ^ • 1 . 4 9 1 1.438

:^^^^^^: 4 . 4 y iip^a^H^flMi^itte»n«^w .noO

1.13 44.544 1.485 1.477 Average Rate

42

These results are in close agreement with the results obtained from the SEM

investigation. Each of the samples have a slightly different growth rate and can once

again be contributed to the variation in the surface area exposed during anodization.

The thickness of the film determined by the SEM images and the calculated

thickness from capacitance measurements are in good agreement. The figure below

shows the two data sets. The growth ratio of 1.53 nnW is also plotted.

55

50

4 5 -

4 0 -

35

30

25

20

15-

SEM determined oxide thickness Calculated thickness for TJ Capacitance

-y=1.53(x)

I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36

anod ^ '

Figure 3.21 Thickness Measurement Comparison

The anodization process is one that produces high quality oxide. One important

key to establishing a process for oxide growth is to maintain the same current density

from one sample to the next. It was seen that for the sample area used for these devices

and a current density of 2 mA/cm^ a growth rate of-1.53 nnW is achieved.

43

CHAPTER4

ELECTRICAL CHARACTERIZATION AND EXPERIMENTS

As described in chapter 1, the tunnel oxide barrier in the MIM tunnel junctions

will be used in magnetoresistive devices containing ferromagnetic layers. It is beneficial

to determine the electrical characteristics of the tunnel oxide prior to developing a more

complex device. Knowing the tunneling characteristics of the nonmagnetic device will

help to separate magnetoresistive affects from non-magnetic affects.

This chapter describes the methods used to determine the electrical characteristics

for the MIM device. Using current-voltage plots, an area/perimeter analysis was

conducted and the dominant turmeling mechanism was determined.

Measurement Setup

The electrical characterization of the MIM tunnel junctions relies on the I-V

characteristics obtained during device testing. The system used to generate the I-V

incorporates three main devices, a probe station for electrical connection to the device

electrodes, a Keithley 2400 SourceMeter for power source and measure operations, and

computer software and computer to accurately control source and measure functions the

SourceMeter. A picture of the test setup is shown below.

44

Figure 4.1 Picture of Measurement Setup

Probe Station

The probe station is the blue instrument shown in the picture above. It includes

the probes / probe manipulators, microscope, sample stage. The probes serve as electrical

connections to the source meter. The probe tips are positioned over the MIM device

using the probe manipulators. The probes are then lowered onto the MIM electrodes.

The probe has a 5 ^m tip diameter that can probe the 100x100, 75x75, 50x50, 25x25, and

10x10 i m squares. The microscope is used to view the probing area. The stage is used

to access different areas on the sample surface. The picture below is an image of a device

that is being probed on the 100x100 |.im square.

45

Figure 4.2 Image of Probed Sample.

The probes are connected to the Keithley 2400 SourceMeter.

Keithley 2400 and LabVIEW

The SourceMeter is a device that combines the function of a multimeter and a

power supply. It is a highly precise device that accurately measures currents in the pico-

Amp range. NI LabVIEW software was developed to acquire data from the Keithley

2400 and to control a basic sweep function. The LabVIEW software uses a GPIB

interface to send commands to the Keithley. The program developed sends consecutive

voltage source then current measure commands to the SourceMeter to generate an 1-V

curve characteristic of the MIM device. The program controls a voltage step fimction

that is sent to the Keithley 2400 and the current is measured after each increase in the

voltage level. The program plots the I-V curve and produces an excel file containing the

I-V data. The image below is a screen capture of the graphical user interface of the

LabVIEW program used for I-V measurements.

46

• 11 13ptApphc4tKNiFont

rMoucsrwM

M l ^-IPS'

sweep configuration Siwplype fou.™.es'

drertion

nijmb«t of ponts

step see

mxnber of points or st«p

sprang

Unear v j vokage sense range

Auto T - |

current sense range

Auto t r |

langng

Auto

speed

Slow

•e j

v l '/ - l?5

start source protection

^l.OOe+0 V/A ijj-3.0000 V/A

souce compience stop

y|l.0OE+O V/A i l.OOOO V/A

Source Delay \ (tkne betiween source and measure) i before setting up the custorn delay turn

off auto deUy otherwise its mear«igless

autodeiay custorn delay

240.0000n-

lO .e iSOy-

-80.0000n =

•3 .0 -2 5 • : 0 - I

Bs

5 -1,0 -0.5 0.0 0.5 I

voltage (V) 0

T

Save to He?

itatui cod»

' ) " ^)|o

voltage (V)

• ; 081060

-;; 027030

-1 972970

-1 <11B920

-1.864870

-1 SI0810

-1 756760

-1 70Z700

-1,648650

-1 S94600

-1 540540

-1 486490

cw

|-:.164254E-5

|- l .7476S6£-S

|-1.389476£-5

|-1 0<*7<M6E-5

| -e 630156E-6

| -6 738r39E-6

| -5 2eSl68E-6

| -4 024092E-6

|-3,079853E-6

|-2,342445E-fc

| -1 77:^?7E-6

| - 1 334697E-6

Figure 4.3 LabVIEW GUI

The program can perform I-V or V-I measurements. Minimum and maximum

source values in volts or amps are indicated on the GUI, as well as the number of steps or

step size of the voltage source. A time delay can be added between source value and

measure operations to give the device adequate response time.

MIM Current-Voltage Characteristics

A series of I-V curves were obtained for the 100x100, 75x75, 50x50, 25x25, and

10x10 nm squares of two samples of different oxide thickness. The measured I-V curves

are characteristic of a MIM tunnel junction. The graph below shows an I-V curve for a

device with an oxide thickness -28 im.

47

6.5E-09 6.0E-09 5.5E-09 5.0E-09 4.5E-09

- - 4.0E-09 ~- 3.5E-09 I 3.0E-09 I 2.5E-09 " 2.0E-09

1.5E-09 1.0E-09 5.0E-10 O.OE+00 -5.0E-10 6 1 2 3 4 6- 6 7—8 9—«—«—43—lb

Voltage (V)

Figure 4.4 I-V Characteristic Curve (oxide thickness - 30 nm)

This graph shows current versus voltage for a device with an oxide thickness of

approximately 30 nm with a turmel junction area of 100x100 |j,m. This I-V is

characteristic of a MIM turmel junction. As the field increases to approximately 3

MV/cm, conduction begins and continues with a non-linear increase indicating the

presence of quantum mechanical tuimeling. The tunneling mechanism will be identified

later in this chapter.

The graph above shows five data sets. The data set labeled "Sweep 1" was the

first voltage sweep applied to the device. Sweeps 2 through 5 were consecutive sweeps

following the first. There is a large deviation in the I-V curve from sweep 1 to sweep 2.

The sweeps following sweep 2 only deviate slightly. This deviation is due to a

stabilization of the film by initial charge motion called polarization that can be attributed

to many possible sources. These include any charge motion including, but not limited to,

ion motion, electron motion through the conduction band or hole motion through the

48

valence band, filling and emptying of traps, molecular polarization, dipole reorientation,

injection or removal of charge in surface states at the metal/insulator interface.'

The plot of Log current versus voltage further shows the deviation in the current

from sweeps 1 Eind 2.

-11.0 i ^ 8 10

Voltage (V)

Sweep 11

- Sweep 2

12

Figure 4.5 Logplot of Current (30 nm)

In order to produce elecfrical conduction analysis, it must be determined which I-V

curve is proper, ft is sufficient to use curves in which the majority of polarization has

occulted within the device, ft is evident that the majority of the polarization occurs in the

first voltage sweep. Very little polarization occurs in sweeps following the second

sweep. Thus, all subsequent data presented and analysis performed for this project will

be constructed using the second voltage sweep.

49

TJ Area Comparisons

Current versus voltage data was taken on devices of different tunnel junction

areas with the same oxide thickness. It is intuitive that a larger area would constitute a

larger current when a constant current density is maintained. This model of the current,

I = JA, where the current is proportional to the device area times the current density, can

be used. The graph below shows the current/voltage relationship for devices with

different device contact areas.

3.0E-09 r

2.5E-09

2.0E-09

Z 1.5E-09 c I 1.0E-09 o

5.0E-10

O.OE+00

-5.0E-10

-100x100 urn square

- 75x75 um square

- 50x50 um square

Voltage (V)

Figure 4.6 I-V curves for devices of different areas (30 nm).

The model states that the devices should have the same current density. The

graph below shows a plot of current density versus voltage. This graph shows that the

current densfties are not equal. The current density will remain constant but there are

other factors that can affect the current. One possibility is the presence of fringe effects

on the tunnel junctions that would lead to a perimeter effect.

50

E <

ity

<fl c 0) a c 0) k 3

o

0.35

0.3

0.25

0.2

0.15

0.1

0 05

0

-0.05

50x50 um Square

— 75x75 um Square

100x100 um Square

Voltage (V)

Figure 4.7 Current density versus voltage (30 imi).

The original model for the current through the turmel junction was modified to

include a perimeter term. The modified model can be written as,

/ = b^ + c i ' . (4.1)

Where c is a constant related to the current density on the edge of the tunnel junction, b is

a constant related to the current density on the interior of the edges of the tuimel junction,

and A and P are the area and perimeter of the TJ, respectively.

Using this model and data from two devices of different area and the same

thickness, it is possible to project the current of another ttmnel junction device of

different area with the same thickness, ft is necessary to determine a relationship

between the two data sets. Two devices are related by the length of one of the sides of

the ttinnel junction with device 1 having a side length of I and device 2 having a side

length of kL. Devices 1 and 2 have areas and perimeters of, A^=L', P^=4L and

A^ = k^L^ = eA , P. = kAL = kP^ respectively.

51

Using equation 4.1, the current for each device can be written as.

I,=bA,+cP, (4 2)

and

/ , = hA, + cPj = hk-A^+ckP. (4 3)

Solving for b in 4.2. plugging b into 4.3, and then solving for c the equations for b and c

are found to be,

/ - c P A, (4.4)

and

The b and c values calculated using the two data sets are used in equation 4.3 with

the proper scaling factor k. The graph below shows a set of I-V plots. The data for the

100x100 |j,m square and the 50x50 iim square were used to project the I-V curve for the

75x75 |j,m square device. The projected curve is shown in the bold line.

52

(V)

'rent

3

u

3.5E-09

3.0E-09

2.5E-09

2.0E-09

1 5E-09

1.0E-09

5.0E-10

O.OE+00

-5.0E-10

- " Expected Value of 75x75 um Square

100x100 um square

75x75 um square

- 50x50 um square

/

— ^ ^

^ ^ ^ ^ ^ ^ Tl

6 7 8 9 10 11 12 1

Voltage (V)

Figure 4.8 Projected Current Values Using Measured Values (30 nm).

This model can be used to project the currents of different device areas as shown

in the graph below. The 75x75 im and 50x50 ^m squares were used to project the

current for the other squares, 100x100, 25x25, and 10x10. The projected curves are

shown in yellow and the measured data curves are shown in their respective labeled

colors.

5.0E-09

4 5E-09 4-1

4.0E-09

3 5E-09

< . 3.0E-09 -

g 2.5E-09

O 2.0E-09

1.5E-09

1.0E-09

5.0E-10

O.OE+00

-10x10 um Square

-25x25 um Square

-100x100 um Square

-75x75 um Square

-50x50 um Square

Project Current Values

9 10

Voltage (V)

11 12 13

Figure 4.9 Projected current for 5 device areas (30 nm).

53

Tunnel Mechanism Determination

It is now necessary to determine the conduction mechanism of the Metal-

Insulator-Metal Tunnel Junction. There are three main types of conduction through

insulators, Poole-Frenkel conduction, schottky conduction and quantum mechanical

tunneling. Poole-Frenkel requires impurities in the material that serve as hopping centers

for the electrons that usually occurs in when metal particles are in the films. Schottky

conduction is the thermionic emission of electrons in which current is dependent on

temperature. There are two types of tunneling, direct and Fowler-Nordheim (FN)

tunneling. Direct tunneling occurs when the applied voltage is lower than the barrier

height at the metal/insulator interface. Fowler-Nordheim tunneling occurs in the

triangular region of the band structure when the applied bias is greater than the barrier

height at the interface. This is considered the conduction mechanism for the MIM tunnel

junctions. The diagram below illustrates the band diagrams of the tunnel junctions for

unbiased, direct ttmneling and FN tunneling conditions.

O.

"FM

Al

^ "FM

V,

Al

i

bias,

.

Al

e "

A l A

V, bias

Al

Un-biased junction

Vu.=0

Direct Tunneling Fowler-Nordheim Tunneling

Figure 4.10 Band Structures of MIM Tunnel Junctions.

54

The leftmost figure is the band structure of the tunnel junction with no applied bias.

The barrier height OB is equal on both sides of the junction. This barrier height remains

the same even after a bias is applied. When a bias is applied the fermi energy level

increases on one side of the junction forming a triangular shaped band structure. In the

direct tunneling condition the applied bias Vbias is less than the barrier height and

electrons tunnel through the rectangular region. In the Fowler-Nordheim condition the

Vbias is greater than the barrier height and electrons turmel through a thinner triangular

region. The barrier height at the AI-AI2O3 interface is cited in literature at 2 eV.'^

Measurable tunneling currents begin at a bias of ~10V with a current amplitude of a few

pico-Amps following the initial polarization of the anodic film for a sample with a 30 nm

thickness. The Keithley 2400 noise level is in the range of pico-Amps. Therefore, any

direct tunneling that occurs within the oxide is at a level too low to detect using the

present measurement scheme. In order to explore tunneling at lower fields, a device

capable of accurately measuring femto-Amps should be used and the present

measurement setup should be improved.

A ttmneling Vbias equal to lOV, which is larger than the barrier height of 2V, leads

to the conclusion that the ttmneling mechanism for this MIM tunnel junction is Fowler-

Nordheim. FN ttimieling current through the device is directiy proportional to the electric

field squared. The characteristic equation for tumieling current density given by the

Wentzel-Kramers-Brillouin (WKB) approximation, which ignores the temperature

influence and the lowering of the barrier due to the Schottky effect, i s ' ' '

J = fiEj cxpi-£j EJ Acm-, " - ^

where the material-dependent constants /? and Eo are

55

y = 1.541310 -6

and

^ =6.82810'

v'w^w

'm„^

(1/^) (4.7)

\f^oj

i 2 (4.8)

The presence of FN tunneling can be verified with a Fowler-Nordheim plot. With

algebraic manipulation of eq. 4.6, a slope-intercept line equation, y = b + ax, can he

written as

Ln J

\^o.x J

E

E (4.9)

where y = Ln ( J ^

\^ox J X = , Z) = Ln / ? , and Of = -E^.

E„,

Equation 4.9 defines the Fowler-Nordheim plot. If this plot is linear then it can be

concluded that the conduction mechanism through the device is Fowler-Nordheim

tunneling. Below is a FN plot for data obtained from a device area of 100x100 |im with a

thickness of approximately 30 imi.

cm)

A/V

'

< o UJ 3 c —1

-40.5 -,

-41.0

-41.5

-42.0

-42.5

-43.0

-43.5

./1.4 n -

, . • •

^'"•^«..,^^ y = -5.150E+07X - 2.877E+01 ^"**s..,,_^ R2 = 9.995E-01

: — 100x100

Linear (100x100)

2.30E-07 2.40E-07 2.50E-07 2.60E-07 2.70E-07 2.80E-07 2.90E-07

1/Eox(cmA/)

Figure 4.11 Fowler-Nordheim Plot (30 nm).

56

an The blue graph is the FN plot and the green graph is a linear fit to the data with

R' value of .9995. The plot below is a FN plot for devices of different areas. For each

device the R value is .999 or better.

^ -41.0 E ^ -41.5 < ~=- -42.0

5 -42.5 O ~> -43 0 c

_ l -43.5

-44.0

^ ^ - - - .

^***'>.^, ~"'"\ ^"" - ^^ .

"^"V.^^ X .

100x100

50x50 um 25x25 um

^ -- ^ "x

2.30E-07 2.40E-07 2.50E-07 2.60E-07 2.70E-07 2.80E-07 2.90E-07

1/Eox(cm/V)

Figure 4.12 Fowler-Nordheim Plot of Multiple Area Devices (30 nm).

The slope and intercept values of FN plot can be determined by the linear fit. With

the slope and intercept values it is possible to determine the barrier height at the material

interface given that the effective mass of an electron in the material is knovra. Little is

known about the effective mass in AI2O3. The barrier height in AI2O3 was found to be

1.5-2 eV using internal photoemission measurements.'^ So from eq. 4.7 or 4.8 the

effective mass of AI2O3 can be found. Remember from eq. 4.9 that slope a is equal to Eo

and the intercept b is equal to Ln(^). An average value of £<, = 4.9012 10 was obtained

from 10 independent FN plots. This value was used to calculate an approximate value for

the effective mass in equation 4.8, E^ = 6.82810 . r... \ m„

\^o J ^ 2 U 4.901210'. The

57

effective mass of an electron in AI2O3 is w„, = 0.152667w„. The coefficient values

obtained from the Fowler Nordheim plots are in good agreement with data presented in

(13) for similar oxide thickness and the same final anodization voltage of 20V.

Oxide Thickness Dependence

The area/perimeter analysis and tunneling mechanism analysis presented for this

oxide thickness was also performed on a sample with a thickness of 15 nm and a final

anodization voltage of lOV. The graph below is the I-V characteristics for this oxide.

3 3.5 4 Voltage (V)

Figure 4.13 I-V for devices of different TJ areas (15 nm).

The graph below shows a plot of current density versus voltage. This graph also

shows that the current densities are not equal. The plot also indicates the presence of a

perimeter affect.

58

u.a

0.8 E 0 7

io.6 •f 0 5 O 0.4

!:i 0.3

£ 0.2

3 0.1 O

0 -0.1

-25x25 um 50x50 um

75x75 um

100x100 um

= : i / /

~ziiji 11 /

/ / /

^ • ^ ^

! ?-% 3 3 5 4T5 6

Voltage (V)

Figure 4.14 Current density versus voltage (oxide thickness - 15 nm).

Using the same area/perimeter model for the current, the projection of the currents

for the 100x100 p,m and 50x50 [im squares was produced. The plot below shows the

measured current (respective colors) and projected current (yellow) versus the applied

voltage.

9.E-09

8.E-09

7.E-09

< 6.E-09

^ 5.E-09

t 4.E-09

O 3.E-09

2.E-09

1.E-09

O.E+00

-25x25 um

- 50X50 um

- 75X75 um

- 100X100 um

Projected Current Values

2.5 3 3.5 4

Voltage (V)

4.5

Figure 4.15 Projected current for 4 device areas (15 nm).

The voltage where the conduction begins is much lower, as to be expected for a

tunnel junction with thinner barrier oxide. But the electric field value where ttmneling

begins is should be the same for tunnel junctions of different oxide thickness. The figure

59

below shows an increase

barrier oxide devices.

in current at ~ 3 MV/cm for both the 15 nm and 30 nm thick

9.00E-09

8.00E-09

7.00E-09

^ 6.00E-09

^ 5.00E-09

g 4.00E-09

O 3.00E-09

2.00E-09

1 .OOE-09

O.OOE+00

15 1.75 2 2.25 2.5 2.75 3 3.25 3.5 3.75

E Field (MV/cm)

Figure 4.16 I-E curves for 15 and 30 nm thick barrier oxides.

The rate of increase or slope of the exponential is much greater for the thin oxide.

Meaning, for a given Electric field the current density is greater for the thinner oxide.

After examination of the oxide surface structure, it was concluded that there is a greater

probability of defect assisted current flow in the thinner oxide in the form of leakage

current. The oxide surface is not smooth. There are surface pits measured at a depth of

7-8 nm from measurements performed using an Atomic Force Microscope (AFM).

Therefore, there is a greater possibility that these pits could traverse the full oxide

thickness forming pinholes or create tunneling hotspots in the thin oxide. Figure 4.17 is

one of the AFM measurements obtained from the 15nm thick barrier oxide shown below.

60

Figure 4.17 AFM Profile of Barrier Oxide Surface (15nm)

One of the assumptions used in standard Fowler-Nordheim theory is that the surface

is flat and smooth'^. The RMS roughness for the thin oxide was determined to be 2 nm.

The turmeling characteristics for the 15 nm thick oxide are not as well behaved and as

stable as the 30 nm thick oxide. This instability is evident in the Fowler-Nordheim plot

below for the 15 nm thick tunnel junction barrier oxide. The intercepts and slopes of the

FN plot vary for tunnel junctions of different area. The FN plots for the 30 nm thick

oxide were consistent for different tunnel junction areas. This inconsistency in the

tunneling measurements for the 15 nm thick oxide is attributed to the surface structure of

the oxide films.

61

2.65E-07 2.75E-07 2.85E-07 2.95E-07 3.05E-07 3.15E-07 3.25E-07 1/Eox (cm/V)

Figure 4.18 Fowler-Nordheim Plot of Muhiple Area Devices (15 nm).

It is also possible that there is a conduction mechanism that begins to contribute to

the total current in tunnel junctions with thinner barrier oxide. The Fowler-Nordheim

tunneling theory presented assumes that the device is at zero temperature'^ Meaning that

it assumes that the tunneling current is independent of device temperature. It is possible

at such high electric fields to see thermal emission, in addition to field emission, of

electrons through the barrier in the form of Schottky conduction. Further experiments

will be required to determine if the higher current density could be attributed to this

mechanism.

I-V sweeps were obtained for many tunnel junctions with an oxide thickness of 15

nm and 30 nm with TJ areas of 10x10, 25x25, 50x50, 75x75 and 100x100 ^m. 30 TJ of

each device areas were measured for the 30 nm thick oxide film. 10% of these devices

had top and bottom Al contacts that were shorted. For tunnel junctions were I-V sweeps

were obtained on the same probe station setup had a 5% deviation in the max current.

From this one could conclude that the I-V data from device to device was repeatable.

Devices with an oxide thickness of 15 nm had the same repeatability and the deviation

62

nm was 15% in the max current. A much higher percentage of devices failed at the 15

thickness. Approximately 50% of the device measured failed due to shorted Al contacts.

The need to decrease the surface roughness of the tunnel junctions is apparent and is a

future objective for the project.

The Metal-Insulator-Metal Tunnel Junctions were characterized to determine

tiie dominant conduction mechanism through the AI2O3 barrier oxide. I-V curves were

acquired using measurement instrumentation designed for the project including a

LabVIEW-based data acquisition system. The I-V data was analyzed to investigate a

possible perimeter affect and to determine the dominant conduction mechanism. A slight

dependence on the perimeter was found in the tunnel junctions and a model was used to

project the tunneling current for devices of various sizes. The dominant conduction

mechanism found for the MIM TJ was Fowler-Nordheim tunneling for the device with a

barrier oxide of 30 nm. A combination of conduction mechanisms is expected for the

thinner (15nm) barrier oxide and future work will include the determination of this

conduction means.

63

CHAPTER 5

CONCLUSIONS AND DISCUSSION

Project Summary

The primary goal for this project was to develop a process for growing

reproducible high-quality AI2O3 barrier oxides and characterize the elecfrical properties

of the oxide. In order to accomplish this task a test device was designed. The scope of

the project included the full design, fabrication and characterization of the Metal-

Insulator-Metal Tunnel Junction test device.

The Tunnel Junction device design included the ability to test devices of different

areas, as well as the option to produce devices containing the desired AI2O3 barrier

thickness.

The fabrication of the MIM TJ devices included the design of the fabrication

process flow. The photomasks used in fabrication were designed in LASI 7 and

fabricated on high quality glass masks. Standard semiconductor processing techniques

were used to pattern the metal layers and to define regions for anodic oxide growth. The

process for growing anodic AI2O3 films was developed for this project. The growth

process was described in terms of the growth rate of the oxide films.

An instrumentation system was established to characterize the electrical

properties of the AI2O3 films. This system included the use of a probe station equipped

with an optical microscope and a data acquisition system comprised of computer

LabVIEW software that controls a Keithley 2400 Source/Meter. This system was used to

64

generate current versus voltage curves for TJ devices of different areas and different

thickness.

The I-V curves generated during testing were used to analyze the electrical

characteristics of the device. Comparisons between data collected on devices with the

same barrier oxide thickness but different device area showed that the current through the

jimction was slightly dependant on the device area. A model was suggested for this

dependence and the model was implemented in projecting the current for different device

areas.

The electric field regime where current through the device begins suggests that

Fowler-Nordheim conduction is present in the device. Using the Wentzel Kramers

Brillouin (WKB) approximation for Fowler-Nordheim theory a model was used to verify

the presence of Fowler-Nordheim tunneling. When the Fowler-Nordheim plot of the

tunneling data is linear the conduction mechanism is dominated by Fowler-Nordheim

tunneling. Thinner oxide samples show evidence of another mechanism contributing to

the conduction through the MIM Tunnel Junctions.

Future Work

This project was designed as a stepping-stone for a bigger project that will include

the addition of magnetoresistive layers to the AI2O3 barrier oxide layer. As described in

chapter 1 magnetoresistive devices use this barrier oxide layer. The barrier layer must be

thin, uniform and free of pinholes. The ftiture of this project will include scaling the

thickness of the barrier oxide to below 10 mn. as well as attempting to deposit an

65

aluminum layer that will be totally consumed during the oxidation process. For example

depositing a 6 nm Al film and anodizing the film to a thickness of-10 nm.

Improvements in the uniformity of the AI2O3 barrier oxide layer are needed. Possible

fabrication modification might include a pre-anodization anneal of the bottom Al contact,

using a slower Al deposition rate to improve the surface roughness of the Al contact and

also slowing the growth of the AI2O3 film by reducing the current density used during

anodization.

Following this achievement, the magnetoresistive tunnel junction (MJT) will be

designed to include a pinned ferromagnetic layer and a floating ferromagnetic layer. In

free ferromagnetic layers the magnetic orientation of a film can be changed by applying a

saturating magnetic field and will exhibit a hysteretic property, meaning that the film will

remain this orientation after the field is removed.^ The orientation of the film can be

fixed by placing an anti-ferromagnetic layer in contact with the ferromagnetic layer.

Determining the materials and the processes for depositing the materials will be included

in the future work of the MTJ development, as well as testing the MJT. Using a four-

point probe configuration the resistance of the MJT can be plotted with respect to an

external field.

This work would then lead to the development of a Magnetic Readhead device.

66

REFERENCES

1. J. M. Slaughter, E. Y. Chen, R. Whig, B. N. Engel, J. Janesky and S. Tehrani, JOM-e 52, No. 6 (2000).

2. B. J. Jonsson-Akerman, R. Escudero, C. Leighton, S. Kim, Ivan K. SchuUer and D. A. Rabson. Appl. Phys. Lett. 77, 1870 (2000).

3. C. H. Tsang, Et. Al, IBM J. Res. Develop. 42, No. 1, Jan. 1998.

4. Serway, Physics: For Scientists and Engineers, 5th Ed., Vol.2, Saunders College Publishing, Orlando, Fl, 2000

5. Campbell. The Science and Engineering of Microelectronic Fabrication, Oxford University Press, New York, 2001.

6. R. D. Knight, Physics: A Contemporary Perspective Vol. 2, Addison Wesley, New York, 1997.

7. http://www.mc.mat.shibaura-ft.ac.jp/~plaza/chap219.html

8. http://electrochem.cwru.edu/ed/encycl/art-a02-anodizing.htm

9. T. W. Hickmott, J. Appl. Phys. 93, No. 6, 3461 (2003)

10 M Zecchino, A. G. Olsak, "High Speed Surface Measurement with Lateral Scanning White Light Interferometry" (AN502), Veeco Instruments, Inc., 2650 E. Elvira Road, Tuscon,AZ 85706 (2001)

11. http://lpcl.clpccd.cc.ca.us/lpc/physics/pdf/relation.pdf

12. A. K. Jonscher, Dielectric Relaxation in Solids (Chelsea Dielectrics, London, 1983),

p. 22

13. T. W. Hickmott, J. Appl. Phys. 87, No. 11, 7903-12 (2000)

14 B Domenges,H. Murray, P. Schwindenhammer and G.Imbert,Semicond.Sci.

Technol., Vol. 19, No. 2, 191-197 (2004)

15. Y. Khlifi, K. Kassmi, L. Roubi, R. Maimouni, M.J. Condensed Matter, Vol. 3. No. 1,

1 June 2000

16. R. G. Forbes, J. Vac. Sci. Technol. B 17, 526 (1999)

17. A. M. Goodman, J. Appl. Phys. 41, 2176 (1970)

67

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