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Surface & Coatings Technolog

Nucleation and growth of DC magnetron

sputtered titanium diboride thin films

S.K. Mishra*, P.K.P. Rupa, L.C. Pathak

National Metallurgical Laboratory, Jamshedpur-831 007, India

Received 4 October 2004; accepted in revised form 9 October 2004

Abstract

Titanium diboride (TiB2) films deposited on different substrates at room temperature using DC magnetron sputtering had good adherence.

The studies using transmission electron microscope (TEM) and atomic force microscope (AFM) showed fabrication of smooth titanium

diboride films with very low surface roughness values. Island formation during nucleation and growth of these films could be observed in

scanning electron microscopy study. The nano-crystallinity of these films was confirmed from the AFM investigation, which also revealed

layered growth of these materials. These conducting films showed micro-hardness in the range of ~2850 Hv0.015 on Si and resistivity in the

range of 200�10�6 V cm.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Thin films; Titanium diboride; Hard coatings; Nucleation and growth

1. Introduction

Titanium diboride is becoming an important ceramics

due to its high hardness, strength and excellent chemical

resistances to corrosive environments even at elevated

temperatures. It also has high oxidation resistance, and

high thermal and electrical conductivity [1,2]. All these

combinations of properties suggest enormous potential of

TiB2 thin films in wear, abrasion resistance and oxidation

resistant applications. Due to its high electrical conductiv-

ity, it has also the potential for use as interconnects for

semiconductor applications [3]. The excellent tribological

properties (low friction coefficient with low wear rates) of

TiB2 indicates its potential as hard solid lubricant wear

resistant coating on different industrial parts as well as on

surgical tools. So far, several methods namely sputtering,

arc evaporation, electron evaporation, plasma spray,

plasma-enhanced chemical vapour deposition (PECVD),

chemical vapour deposition (CVD) have been reported to

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.10.006

* Corresponding author. Tel.: +91 6572270709; fax: +91 6572270752.

E-mail address: suman@nmlindia.org (S.K. Mishra).

deposit titanium diboride thin films [4–8]. Amongst them

the sputtering process is one of the techniques which yield

best-quality thin films with excellent adhesion character-

istics. Titanium diboride being conducting, thin films could

be easily deposited by DC sputtering, which is the simplest

among all the sputtering techniques. The deposition of

titanium diboride thin film by DC magnetron sputtering

and growth under different deposition condition were

studied by Berger et al. [9], where texture growth of

TiB2 thin film with respect to substrate biasing was

investigated. It was observed that initially film grow

randomly and later texturing took place. However, reports

on different stages of thin film in the early stages of

microstructural growth behaviour are not addressed. In this

paper, we present the results on deposition, nucleation and

growth of titanium diboride thin films by DC magnetron

sputtering technique.

2. Experimental

Titanium diboride films were deposited by a DC

magnetron sputtering unit (HHV, India) using a single

y 200 (2006) 4078–4081

S.K. Mishra et al. / Surface & Coatings Technology 200 (2006) 4078–4081 4079

target fabricated from the TiB2 powder prepared by a SHS

route (magnetothermic reduction of Ti and B oxides)

[10,11]. The sub-micrometer sized (~500 nm) single-phase

TiB2 powder obtained after leaching out the reaction

byproduct (MgO) in acidic medium, was palletized into 50

mm disc with 3 mm thickness and subsequently sintered in

a graphite furnace at 1800 8C in flowing argon atmosphere

to ~95% of theoretical density. The system was evacuated

to a pressure of 2�10�6 mbar. Thin films were deposited

in Argon atmosphere on Si, glass and stainless steel

substrates at room temperature and at a pressure of

4�10�3 mbar using a DC power of 120 W for 60 s to

20 min. Prior to deposition, the substrates were ultrasoni-

cally cleaned twice by acetone, dried and immediately put

into the sputtering chamber for evacuation. The deposition

was carried out at room temperature (25 8C). However, theincrease in substrate temperature after 20 min of film

deposition was observed to be ~40–45 8C due to plasma

Fig. 1. TEM micrograph and corresponding SAED of the TiB2 films

heating of the substrate. Insignificant variations in sub-

strate temperatures were observed when deposition was

carried out for lesser times.

Titanium diboride films were simultaneously deposited

on amorphous carbon-coated copper grids for the TEM

(Phillips EM 200, Netherlands) investigations. Standard

method of arc evaporation was used for carbon deposition

on soap solution-coated glass substrates. Floated carbon

films were cleaned in dust-free distilled water and carried

to the copper grids. The phase analyses and microstructural

studies of the deposited films were carried out using X-ray

diffractometer (Siemens, Germany) and Scanning electron

microscope attached with KEVEX EDX (JEOL, JSM 840,

Japan), respectively. Atomic force microscope (Seiko SPA

400, Japan) was used to investigate the topography of the

deposited films and the microhardness was measured at 15

g loads using Leica micro-hardness tester (VMHT Auto,

Germany).

deposited for (a) 120 s, (b) 270 s, (c) 300 s, and (d) 20 min.

Fig. 3. The SEM image of the film surface adhered to the glass slide

showing the initial stage of deposition.

S.K. Mishra et al. / Surface & Coatings Technology 200 (2006) 4078–40814080

3. Results and discussions

The deposited TiB2 films were amorphous and structure-

less in the initial stages of deposition and as the film

thickness increased some crystallinity appear. The TEM

studies of the film deposited for 60 s did not show any

structure and were completely amorphous. Whereas, some

fine structures appeared after 120 to 180 s of deposition and

island formation was detected in 270 s deposited films (Fig.

1a,b). All those films showed hallow in selected area

electron diffraction (SAED) patterns, which represented the

characteristic of amorphous material. The films deposited

for more than 300 s showed very fine-grained morphology

with polycrystalline type of ring in SAED patterns (Fig. 1c).

Though grain size increased with the deposition time, the

20-min deposited films still showed nano-crystalline grains

with sharp rings in SAED patterns (Fig. 1d). The EDX

analyses of the grains showed the presence of Ti, B could

not be detected due to limitation of the instrument. SAED

analysis and the XRD analyses of the 20-min deposited

films confirmed the formation of TiB2 phase in those films.

Similar observations were also made in the SEM studies

of the film deposited on glass substrates. Initially, no

structure could be seen and was charging for films deposited

for 60, 120 and 180 s. The film deposited for 270 s showed

globular structures (Fig. 2a). In the initial stages of

deposition, discontinuous film formation due to island type

of growth was evident from the charge accumulation in films

on glass substrate and once the films became continuous no

charging was observed during SEM study. Film deposited

for 300 s showed larger islands coalescing with each other

(Fig. 2b). The film deposited for 330 s again showed a very

Fig. 2. SEM micrograph of the TiB2 film deposited on glass at (a) 270 s, (b

smooth film with very fine islands (Fig. 2c). This is due to

island growth to continuous film. The sample deposited for

20 min did not show any structure; it was a smooth film. As

the film thickness increased, the island size became finer

with the formation of continuous films and ultimately

disappeared with the formation of smooth films (observed

in 20-min deposited films). The cross-sectional view of the

TiB2 film deposited on glass substrate (Fig. 2d) confirmed

the globular kind of growth in the initial stages, which

became smooth with increase of thickness. The 20-min

deposited thin film was taken out from the glass substrate,

which showed globular type of growth in the surface adhered

to the glass substrate (Fig. 3) and showed no structure on the

top surface. This clearly suggests the island type of

nucleation and growth phenomenon of TiB2 films, which

is in agreement with the proposal of others on PVD of thin

) 300 s, (c) 330 s, (d) cross-sectional view of 20-min deposited film.

Fig. 4. The AFM picture of the titanium diboride film deposited for 20 min: (a) lower magnification showing grains in the range of 20 nm, (b) atomically

resolved image of the grain A [of (a)].

S.K. Mishra et al. / Surface & Coatings Technology 200 (2006) 4078–4081 4081

films [12]. First, three-dimensional nuclei were formed

randomly and rapidly approached a saturation density with

a small amount of deposit. These nuclei then grew to form

observable islands whose shapes were determined by

interfacial energies and deposition conditions. The growth

was diffusion controlled and as they came closer the island

size increased, the larger ones grew by coalescence of the

smaller ones by mass transfer through diffusion process.

After that island density decreased monotonically at a rate

determined by the deposition condition and resulted into

continuous networked structure and the islands were

flattered to increase surface coverage. The coalescence was

very rapid and resulted in continuous film formation.

The AFM studies on the film deposited on glass/Si

substrate clearly showed the fine grains in the range of 20 nm

(Fig. 4a) and the surface roughness Ra value for the

200�200-Am scan area was found to be ~10 nm indicating

very high smoothness of the 20-min deposited films. Atomic

resolution on a grain showed layered type of growth (Fig.

4b). In these films, two types of grains were found. For the

grain A (of Fig. 4a), the atomic spacing matches with (hkl)

values of (004) and (103) planes for TiB2.5 composition,

whereas the other grains B showed the atomic spacing for the

TiB2 composition. The layered type of growth indicated two-

dimensional nucleation mechanism during nucleation and

growth of the films. The deposited films were found to be

hard and the micro-hardness values were measured in the

range of ~2850 Hv0.015 and the resistivity of the 20-min

deposited showed was found in the range of 100–200�10�6

V cm. The high mechanical strength and excellent resistivity

of these films may be used as wear resistant hard protective

coatings and as interconnects for electronic applications.

Further investigations on the deposition parameters and its

properties are under progress.

4. Conclusion

The nucleation and growth study for the deposition of

TiB2 films have been carried out, which indicated island

growth and coalescence phenomenon was responsible for

the growth of these films. The films were polycrystalline

and had very fine grains in the range of 10–20 nm with

average roughness of ~10 nm. The AFM study showed

the atomic resolution of the film. The hardness of the film

was found to be very high having average hardness of

~2850 Hv0.015.

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