Krishna Valleti Phone: +91 44 22575876

Dept. of Physics Email: krishna@physics.iitm.ac.in

IIT Madras kvalleti@gmail.com

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)

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Fig. 6: Surface morphology of TaN thin film grown using R C-Mag.

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ting cylindrical magnetron structure for

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