Krishna Valleti Phone: +91 44 22575876
Dept. of Physics Email: [email protected]
IIT Madras [email protected]
Chennai 600 036, INDIA.
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Personal Details:
Name : Mr. Krishna Valleti
Date of Birth : 20th March 1979
Nationality : INDIAN
Languages : English, Hindi, and Telugu
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Academic:
* PhD – Indian Institute of Technology Madras (2003 - 2
(Awaiting the award of degree)
Title of the PhD thesis: “Investigations on an innovative
magnetron cathode and on tanta
* CSIR (JRF) – July 2002.
* MSc (74.9%) – Sri Venkateswara University (1999 - 2
* BSc (76.7%) – Jawahar Bharathi (1996 - 1999), Kava
Research Interests:
* Thin film technology (Magnetron sputtering, Design o
* Hard coatings
* Nano technology and nano materials
Work Experience:
* Worked as a Junior Research Fellow in Sensor Elec
Application Centre, ISRO, Ahmedabad, INDIA (Nov, 2
* Since 2003 pursuing research on
(i) Design and evaluation of a rotatable cylindric
cathode for coating inner surfaces of cylindrical ob
----------------------
008), Chennai, INDIA.
rotating cylindrical
lum based hard coatings”
001), AP, INDIA.
li, AP, INDIA.
f cathodes)
tronics Division at Space
002 – July, 2003).
al magnetron sputtering
jects, and
(ii) Development of refractory metal nitrides for mechanically hard and
corrosion resistant applications (Ta, TaN, and TaAlN).
Work summary:
The research programme leading to award of Ph.D. degree is particularly
concerned with the designing of a cylindrical magnetron cathode of enhanced
efficiency for depositing refractory metal (Ta) and metal nitrides (TaN) on the
inner/outer surfaces of cylindrical objects or batch of tools coating or large area
planar coatings. The salient findings of the doctoral programme are as follows.
* A cylindrical magnetron cathode is designed (in rotating magnet geometry)
with a means to collect optical emission spectra from the gas discharge (for
monitoring discharge constituents; ratio of reactive gases).
* The designed cylindrical magnetron is capable of achieving ~81% target
utilization and high uniform films (only ±3% thickness variation from the
average value).
* Ta and TaN thin films are grown using planar and cylindrical magnetron
deposition techniques for mechanical hard coating applications.
* The effect of deposition parameters on structural, electrical and mechanical
properties of Ta and TaN thin films are studied in detail.
* The importance of pulsing the target power in cylindrical magnetron
sputtering has been emphasized.
* The advantages of using cylindrical magnetron in rotating magnet geometry
(stationary target and stationary substrate) are highlighted.
* TaAlN thin films are grown (by incorporating Aluminum into the Ta or TaN
thin films using co-sputtering) to study the oxidation effects of Ta or TaN thin
films with time.
(Detailed synopsis of the research is presented at the end)
Publications:
1) “Pulsed DC magnetron sputtered tantalum nitride hard coatings for tribological
applications”. Aditya Aryasomayajula, Krishna Valleti, Subrahmanyam
Aryasomayajula, Deepak G. Bhat. Surface & Coatings Technology 201
(2006): 4401.
2) “The effect of arc suppression on the physical properties of Low temperature DC
magnetron sputtered tantalum thin films”. A. Subrahmanyam, Krishna Valleti,
Shrikant V. Joshi, and G. Sundararajan. Journal of Vacuum Science and
Technology A 25 (2007): 378.
3) “Studies on pulsed rotating cylindrical magnetron sputtered tantalum thin films”
Krishna Valleti, A. Subrahmanyam, Shrikant V Joshi, 50th Annual Society
of Vacuum Coaters (SVC) Technical Conference proceedings (2007):
485.
4) “Growth of nano crystalline near α-phase Tantalum thin films at room
temperature using cylindrical magnetron cathode”. Krishna Valleti, A.
Subrahmanyam and Shrikant V. Joshi. Surface & Coatings Technology 202
(2008): 3325.
5) “Studies on phase dependent mechanical properties of DC magnetron sputtered
TaN thin films: Evaluation of super hardness in orthorhombic Ta4N phase”.
Krishna Valleti, A. Subrahmanyam, Shrikant V. Joshi, A. R. Phani, M.
Passacantando and S. Santucci. Journal of Physics D: Applied Physics 41
(2008): 045409.
6) “The rotating cylindrical magnetron cathode: An improved design and
performance evaluation by Ta and TaN thin film deposition”. Krishna Valleti,
A. Subrahmanyam, Shrikant V. Joshi, Plasma sources science and
technology (under review)
7) “Studies on pulsed cylindrical magnetron sputtered nano crystalline Tantalum
nitride thin films”. Krishna Valleti, A. Subrahmanyam and Shrikant V. Joshi
(under process)
Patent:
A patent titled “Improved cylindrical magnetron cathode and a process for
depositing thin films on surfaces using the said cathode” by A. Subrahmanyam,
Krishna Valleti, Shrikant V. Joshi, and G. Sundararajan has been filed recently
(No. 21/DEL/2008, dated January 3rd, 2008).
International conferences attended/presented:
1) “Studies on Pulsed DC magnetron sputtered Tantalum thin films for hard coating
applications: Effect of substrate temperature”. Aditya Aryasomayajula,
Subrahmanyam Aryasomayajula, Krishna Valleti, Deepak G. Bhat, 5th
international surface engineering congress during May 15-17, 2006
held at Seattle, Washington, USA.
2) “Effect of grain size on mechanical properties of Pulsed DC magnetron sputtered
Tantalum thin films”. Krishna Valleti, A. Subrahmanyam, Shrikant Joshi, GVN
Rao, Eighth International Conference on Nanosutructured Materials,
August 20-25, 2006 held at IISc Bangalore, INDIA.
3) “Studies on pulsed rotating cylindrical magnetron sputtered tantalum thin films”
Krishna Valleti, A. Subrahmanyam, Shrikant V Joshi. 50th Annual Society
of Vacuum Coaters (SVC) Technical Conference, April 28- May 3, 2007
held at Louisville, Kentucky, USA.
“Awarded as the Best poster in the Conference”
Work shops attended:
1) “Awareness workshop on the facilities of UGC-DAE consortium for
scientific research” held at Dept. of Physics, Pondicherry University,
Pondicherry during November 04 - 05, 2004.
2) National workshop on “Advanced techniques for characterization of
nanomaterials (XRD, SEM/EDS, SPM)” held at Dept. of Physics, University
of Pune, Pune during June 28 – July 2, 2005.
3) National workshop on “Plasma Science and Technology: Industrial
Applications and Diagnostics” held at Birla Institute of Technology, MESRA,
Extension Centre, Jaipur during August 30 – September 2, 2005.
Thin Film Deposition Techniques used:
* Sputtering and Magnetron Sputtering
* Thermal Evaporation
* Electron Beam Evaporation
* Pulsed LASER Deposition
* Chemical Vapor Deposition
Known Material Characterization Skills:
* X-ray diffraction analysis
* Four probe, Vander-paw, and Hall Effect analysis
* Langmuir probe analysis
* Atomic Force Microscopy
* Scanning Electron Microscopy
* Energy Dispersive Analysis of X-rays
* Rutherford Back Scattering Analysis
* X-ray Photoelectron Analysis
* Nano-Indentation
* Adhesion Tester
* UV-VI-IR Spectro photometer
SYNOPSIS OF Ph.D. THESIS
TITLE OF THE Ph. D. THESIS: “Investigations on an innovative rotating cylindrical
magnetron cathode and on tantalum based hard coatings”
1. OBJECTIVES OF THE STUDY
Coating uniform thin films on inner surfaces of tubular objects for strategic and
commercial applications is a challenge. Among the several techniques being employed
presently for the large area coatings and coatings on tubular objects for both protective
and hard coatings, cylindrical magnetron cathodes are being used widely (Holland and
Linnenbrugger, 1993; Vigilante and Mulligan, 2006; Cardarelli et al., 1996). Though
there are several designs presently in use, large scope exists for improvements in the
design of cylindrical magnetron cathodes. [
It is well known that the magnetic field geometry in the cathode design plays an
important role. In the patent issued to Hawton, Jr et al. (1976), the cylindrical magnets
are oriented symmetrically about the axis (of the cylinder) resulting in a toroidal
magnetic field confinement; however, this configuration gives non-uniform target
consumption. An elongated magnet assembly used length wise within the cathode and the
target is rotated has been proposed by Boonzenny et al. (1990). Since the coolant and the
electrical contacts are attached to the rotating target, this design is prone for frequent
vacuum seal breakdowns.
For the specific application where it involves high temperatures and corrosive
environments (like gun barrels), only refractory materials like tantalum (Ta), rhenium
(Re) and niobium (Nb) are the inevitable choices. Tantalum exists in two phases, stable
bcc structure (α phase) and unstable tetragonal structure (β phase). The α-Ta being highly
stable up to 2996° C and ductile, it is the desired phase. In the thin film form, at normal
conditions of deposition, Ta always grows in an undesired β phase (Read and Altman,
1965). The α phase is achieved by several ways: doping impurities (like N2, O2, etc.),
using seed layer, using substrate biasing, maintaining substrate temperature above 300°
C, etc. In all these processes, the only parameter that is modified is the energy of the
depositing adatom.
Like Ta, Tantalum nitride (TaN) also exhibits several stable and meta-stable
phases. The phase composition is largely dependent on the partial pressure of the
nitrogen during the thin film growth (Arranz and Palacio, 2005). In both the cases (i.e.,
for Ta or TaN), the mechanical properties are largely dependent on the phase
composition. The available data on the hardness of different TaN phases are rather
limited and very few phases with the mechanical properties are identified. In particular,
the main aims of the doctoral programme are to:
(i) design and fabricate a cylindrical magnetron cathode for improved / enhanced
performance in terms of target utilization and in giving more uniformity over the coating
surface, and
(ii) to investigate growth and mechanical properties of Ta and TaN thin films (in
view of applications) by planar and rotating cylindrical magnetron sputtering technique.
2. SUMMARY OF THE WORK
2.1. Cylindrical magnetron cathode design
It is well known that the race track in the magnetron cathode is dictated by the
confinement of the electrons due to the mutually perpendicular electric and magnetic
fields. In order to achieve a proper plasma confinement, the magnetic field is optimized
such that the radius of the electrons is greater than the thickness of the cathode sheath
(cathode dark space) and it should not alter the path of the heavy argon ions significantly.
Thus, in the present study on cylindrical magnetron cathode design, an emphasis has been
laid onto the optimization of the magnetic field strength and its field profile over the
surface of the magnetron cathode by experimenting with different permanent magnet
configurations.
The cylindrical magnetron cathode is designed in three permanent magnet (Nd-
Fe-B) configurations. The geometry of the outer body of the cathode is designed
accordingly to house the magnets. All the three deigns are fabricated and tested for their
performance by depositing copper and tantalum thin films. The evaluation parameters
that are taken into consideration are: (i) target utilization, (ii) uniformity of the grown
thin films, (iii) process stability, and (iv) electron confinement efficiency. Here the target
utilization is measured by using the simple relation that governs the weight of the target
material. 100 weightInitial
weight)final- weight(Initial (%)n utilizatioTarget ×=
In the first configuration, the magnets (disk shaped - 20 mm dia and 20 mm thick)
are fixed on the top and the bottom endplates of the cathode body. This design leads to a
single plasma confinement zone (figure 1(a)). In the second configuration, the magnet
structure is made up of alternate stacks of magnets (disk shaped - 20 mm dia and 20 mm
thick) interspaced with Teflon (20 mm dia and 20 mm thick). The magnet geometry is
mounted inside the cathode body. This magnet configuration results in a 2N (N-number
of magnet stacks) number of ring profiles (figure 1(b)).
In the third configuration, an innovative design, the bar magnets are stacked onto
a mild steel block such that the north or south pole will be seen on the outer surface of the
magnets; this magnet geometry is isolated from the coolant and it is rotated. The plasma
is shown in figure 1 (c). The outer body of the cathode is designed with coaxial
cylindrical hollow tubes such that the coolant is circulated in the annular region between
the two copper tubes.
(a) (b) (c)
Fig. 1: (a) configuration – 1 (b) configuration – 2 (c) configuration – 3
Among the three configurations, the third configuration with vertical plasma
confinement has the following advantages:
(i) the target utilization is 81% ,
(ii) the uniformity in the grown thin films is ~ 95 %,
(iii) cathode is provided with a means for optical emission monitoring which is
important in large area or tubular inner surface coating processes, and
(iv) glancing angle deposition of adatoms (> 50 %).
It is well known from the literature that the glancing angle deposition of thin films
leads to a new thin film growth process (Hawkeye and Brett, 2007)). It is anticipated that
the present innovative design, because of the rotation of the vertical plasma confinement,
gives rise to oblique incidence of adatoms. With this hypothesis, Ta and TaN thin films
are grown using the rotating cylindrical magnetron (R C-Mag.) cathode. The hypothesis
is confirmed by comparing the physical properties of Ta and TaN thin films grown by R
C-Mag. with those grown by conventional planar magnetron cathode.
2.2. Investigations on Ta thin films
Kelly et al. (2003) reported that pulsing the target power results in an increased
energy reaching to the growing thin film (substrate). From the literature it is also known
that the much desired α-Ta can be grown if an additional energy is supplied to the
growing thin films. An attempt has, therefore, been made to alter the energy of the
adatoms (i.e., to the growing film) by using the target power in different power modes
and by growing films using different cathodes (planar and cylindrical). The substrates
used are: polished 316 stainless steel, single crystal silicon (100) and soda lime glass. All
the Ta thin films grown are studied for their physical and mechanical properties.
2.2.1. Planar magnetron sputter deposited Ta thin films
The Ta thin films are grown using planar magnetron sputtering technique in
different power modes (DC, arc suppressed and pulsed). The temperature of the substrate
is varied from room temperature (27° C) to 400° C in all the modes of the thin film
deposition. All the depositions are carried out at constant power mode (12 W/cm2); the
chamber pressure (with argon gas) is maintained constant at 5× 10-3 mbar, the sputter
time is 30 minutes and the target to substrate distance is 60 mm. The main results are as
follows.
• The tantalum grown using normal DC magnetron sputtering technique shows a
phase transition above 300° C.
• In the case of arc suppression mode and pulsed mode (40 kHz) (Fig.2), the α
phase formation is found to occur at 200° C.
• The surface morphology of the grown Ta thin films is observed to change at phase
transformation point.
• The α phase formation at relatively low temperature in arc suppression and pulsed
modes is attributed to the increased energy of the adatoms reaching the substrate.
Fig. 3: XRD patterns of pulsed planar magnetron grown Ta thin films
Fig. 2: XRD patterns of 40 kHz pulsed planar magnetron grown Ta thin films
2.2.2. R C-Mag. Sputter deposited Ta thin films
The Ta thin films are grown at room temperature (without any intentional heating)
using R C-Mag. cathode (designed and fabricated in the first part of the work). The
pulsing frequency of the target power has been varied from 0 (DC) to 100 kHz in steps of
25 kHz. All the other growth parameters are as described in the section 2.2.1. The main
results of R C-Mag. grown Ta thin films are as follows.
• Up to 75 kHz frequency all the thin films are composed of β-phase Ta only
(figure 3).
• At 100 kHz near α-phase Ta (~ 68 %) thin films are formed.
• The observed α-phase growth at higher frequencies could be due to: (i) changes in
plasma parameters (electron temperature, ion densities, etc.), (ii) an increased
stress in the films, and (iii) the influence of the microstructure of the grown films
due to the innovative design of the cathode.
• The mechanical hardness of the films increases up to 75 kHz (a maximum of 21
GPa at 50 kHz) and starts decreasing from and above 75 kHz (lowest of 12 GPa at
100 kHz).
• At 100 kHz a bunching effect has been observed in the hardness values, probably
due to the presence of mixed phases of different hardness values.
• From the surface morphology, it is observed that there is a clear change in the
grain structure when the phase change has taken place. From the figure 4, it is
also noticed that the phase distribution is uniform and the grains size (composed
of many micro crystallites) is of the order of 200 nm in α-phase.
• The individual phase hardness values are calculated by using rule of mixtures
(iso-strain model). A highest hardness of 12 GPa is observed for α-Ta thin films
in nano crystalline form (grain size – 15 nm).
Fig. 4: The surface morphology of Ta thin films grown using R C-Mag.
2.3. Investigations on tantalum nitride (TaN) thin films
TaN is a rich compound with many phases (depending on the nitrogen
concentration) of different physical and mechanical properties (bcc α-Ta, solid–solution
α-Ta (N), hexagonal γ-Ta2N, hexagonal ε-TaN, etc.). In spite of its many applications, a
very limited work has been executed on TaN thin films to correlate the phase to the
corresponding mechanical properties. In the present study an attempt has, therefore, been
made to analyze the mechanical hardness of individual phases. The TaN thin films are
grown by varying the reactive to sputter gas ratio (R) using reactive planar and
cylindrical magnetron sputtering techniques in different power modes as in the case of Ta
thin film deposition. The substrates used are: stainless steel (polished: < 1 µm roughness),
single crystal Silicon (100) and glass. All the grown thin films are characterized for their
physical and mechanical properties.
2.3.1. Planar magnetron sputter deposited TaN thin films
TaN thin films are grown by reactive DC magnetron sputtering technique. The
ratio of the reactive gas (nitrogen) to the sputter gas (Argon) R has been varied from 0.04
to 0.30. Initially, the vacuum chamber is evacuated to a base pressure of 2×10-6 mbar for
all depositions. During the deposition (with the flow of argon and nitrogen), the chamber
pressure is maintained at 5×10-3 mbar. All the depositions are carried out for 30 minutes
keeping the effective target power density, substrate temperature and the substrate to
target distance constant at 12 W/cm2, 300° C and 60 mm respectively. The important
results are:
• with increasing R, multiple phases in all the TaN samples are observed,
• the quantitative phase analysis (concentration) of TaN thin films is carried out by
studying the Ta 4f7/2 binding energy (XPS), and
• the XPS of all the TaN thin films show a broad peak (21 eV to 27 eV) is de-
convuluted into five significant peaks of TaN with binding energies close to 24.7
eV, 23.7 eV, 23.3 eV, 22.3 eV and 21.6 eV.
Correlating the observed XPS with XRD results, the different phases and their
compositions in individual TaN samples have been evaluated, and the mechanical
hardness (obtained from the nanoindentation measurement) of TaN thin films are
summarized in Table 1. In the sample grown at R = 0.1, a bunching effect has been
observed. This observed multiple hardness values may be a result of multiple phases
coexisting in the film.
It is well known that the rule of mixtures gives the resultant hardness of a thin
film consisting of phases of near equal volume fractions. In the present study the rule of
mixtures is used to get the corresponding hardness values of individual phases.
Table 1: The summary of the phase composition and mechanical properties of TaN thin films grown using
planar magnetron sputtering (DC and arc suppression modes)
2.2.2. R C-Mag. Sputter deposited TaN thin films
TaN thin films were grown using reactive R C-Mag. cathode in pulsed mode (100
kHz). The samples were grown at varying nitrogen to argon pressure ratio (R) from 0.1 to
0.7 keeping the pulsing frequency constant at 100 kHz. The films were also deposited by
varying pulsing frequencies (25, 50 and 100 kHz) of the target power keeping the R
constant at 0.1. All the TaN thin films were grown at an ambient temperature in a
constant power mode (18 W/cm2) with a fixed working gas pressure (5× 10-3 mbar) for
90 minutes. The target to substrate distance was kept constant at 60 mm for all the
depositions.
The XRD analysis (figure 5) shows a phase composition similar to 300° C DC
planar magnetron sputter grown TaN thin films at increased nitrogen to argon ratio (i.e.
the phases of 0.5 R samples in C-Mag. have similar phases to that of 0.1 R planar
magnetron deposited films). This confines the enhanced momentum and energy reaching
to the substrate in the R C-Mag. deposition technique. These improved properties in R
C-Mag. cathode might be attributed to the glancing angle deposition, increased high
energy reflected neutrals, and pulsing frequency (increased energy flux to the growing
thin film). The surface morphology of the 0.5 R grown samples is shown in figure 6. The
surface morphology clearly indicates the preferred oriented growth of thin film.
20 30 40 50 60 70 80
x 1.0 0.10
(110) - a
2θ (degree)
x 1.7 0.30
SS
x 4.20.50
(110) - a
(111
) - c
(220) - c(020) - c
(200) - d(100) - b
Inte
nsity
(arb
. uni
ts)
x 2.5R = 0.70(100) - b (200) - d
R = 0.5
Oriented growth of film
500 x 500 nm2C. Mag
3. R
A A
low-
App
Boo
large
F C
for t
of R
M
prop
Scie
Fig. 5: XRD pattern of TaN thin films grown by R C-Mag. where, a - TaN0.1 (Cubic), b - TaN0.8 (Hexa.), c - Ta4N (Ortho.) and d-TaN (Cubic)
EFERENCES CITED
rranz and C Palacio (2005) Composition
energy nitrogen implantation: a actor analy
lied Physics A, 81, 1405-1410.
zenny, Alex, Hoog and Josef T (1990) Rota
area coating, United States Patent, No. 5,09
ardarelli, P Taxil and A Savall (1996) Tan
he chemical process industry: Molten salts el
efractory Metals & Hard Materials, 14, 365-
M Hawkeye and M J Brett (2007) G
erties, and applications of micro- and nanos
nce and Technology A, 25, 1317-1334.
Fig. 6: Surface morphology of TaN thin film grown using R C-Mag.
of tantalum nitride thin films grown by
sis study of the Ta 4f XPS core level,
ting cylindrical magnetron structure for
6,562.
talum protective thin coating technique
ectro coating as a new alternative, Int. J.
381.
lancing angle deposition: Fabrication,
tructured thin films, Journal of Vacuum
Hawton Jr, John T, Shumate and William G (1976) Cylindrical magnetron sputtering
source, United States Patent, No. 4,179,351.
J Holland and A Linnenbrugger (1993) Cylindrical magnetron sputtering for the
production of wear-resistant and durable overlays of uniform thickness for journal
bearings for application in high performance combustion engines, Surface and Coatings
Technology, 60, 541-544.
P J Kelly, C F Beevers, P S Henderson, R D Arnell, J W Bradley and H Backer
(2003) A comparison of the properties of titanium based films produced by pulsed and
continuous DC magnetron sputtering, Surface and Coatings Technology, 174-175, 795-
800.
M H Read and C Altman (1965) A new structure in tantalum thin films, Applied
Physics Letters, 7, 51-52.
G N Vigilante and C P Mulligan (2006) Cylindrical magnetron sputtering (CMS) of
coatings for wear life extension in large caliber cannons, Materials and Manufacturing
Processes, 21, 621-627.