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Surface & Coatings Technolo

Stress and surface morphology of TiNiCu thin films:

effect of annealing temperature

Yongqing Fua,b,*, Hejun Dua,b, Sam Zhangb, YanWei Gub

aAdvanced Materials for Micro and Nano Systems Programme, Singapore-Massachusetts Institute Technology (MIT) Alliance, 4 Engineering Drive 3,

Singapore 117576, SingaporebSchool of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798, Singapore

Available online 26 November 2004

Abstract

TiNi-based films sputtered at room temperature are amorphous; thus, postsputtering annealing is a must because shape memory effect

only occurs in their crystalline form. It is suggested that the lowest possible annealing temperature be used in a bid to conserve thermal

processing budgets, and to minimize thermal stresses and possible interfacial reactions between film and its substrate. In this paper,

Ti49.5Ni47.5Cu3 (at.%) films with a thickness of 3.5 Am were deposited on Si substrate by cosputtering of TiNi and Cu targets at room

temperature, then annealed at different temperatures from 430 to 650 8C. Phase transformation behaviors, crystalline structure, residual stress

and stress evolution of the films were systematically studied. At the gas pressure of 0.8 mTorr, the residual stress in the as-deposited films

was 260 MPa, compressive. A minimum annealing temperature (450 8C) was necessary for film crystallization; thus, large thermal stress

could be released significantly due to martensitic transformation. With increase of annealing temperature, crystallite and martensite plate sizes

in the film increased; thus, both recovery stress and stress-increase rate increased, while the transformation temperatures shifted to higher

values. The surface roughness increased drastically with increase of annealing temperature in correlation to martensitic transformation.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Annealing temperature; TiNiCu thin films; Surface morphology

1. Introduction

Thin film shape memory alloys (SMAs) attracted much

attention as a promising and high-performance material in

the field of micro-electro-mechanical system (MEMS)

applications [1–6]. However, residual stress and stress

evolution in the films could pose potential problems in

applications, as it may influence not only adhesion

between film and its substrate (cracking, buckling or

delamination), but also deformation of MEMS structure,

mechanics and thermodynamics of transformation and

superelasticity effects [7]. Deposition conditions, postde-

position thermomechanical treatment and composition in

TiNi films could have important consequences with respect

to the development of residual stress [8,9]. TiNi films

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

doi:10.1016/j.surfcoat.2004.10.107

* Corresponding author. Department of Engineering, Cambridge

University, Trumpington Street, Cambridge, CB2, 1PZ, UK.

E-mail address: yf229@cam.ac.uk (Y. Fu).

deposited at room temperature are usually amorphous;

thus, postsputtering annealing (usually higher than 400 8C)is a must because SMA effect only occurs in materials of

crystalline form. Martensitic transformation and super-

elasticity of TiNi films are sensitive to postsputtering

annealing and/or aging temperature and duration [10,11];

thus, postsputtering annealing should be handled with care.

It is suggested that the lowest possible annealing temper-

ature be used so as to conserve thermal processing budget

and more importantly minimize the possible film–substrate

interfacial reactions [12,13]. High-temperature and long-

term postannealing process should be avoided since it

could trigger dramatic changes in microstructure (i.e.,

precipitation), mechanical properties and shape memory

effects [14]. However, as revealed from this study, the

residual stress in the films could increase dramatically with

annealing temperature until above a temperature of 450 8C.At the same time, surface roughness increases significantly

as annealing temperature increases above 450 8C. Those

gy 198 (2005) 389–394

Y. Fu et al. / Surface & Coatings Technology 198 (2005) 389–394390

have seldom been reported previously. In this paper,

Ti49.5Ni47.5Cu3 (at.%) films with a thickness of 3.5 Amwere prepared by cosputtering of TiNi and Cu targets at

room temperature and annealed at temperatures from 430

to 650 8C. Phase transformation behaviors, crystalline

structure, residual stress, stress evolution as well as

evolution in surface morphologies were systematically

studied.

Fig. 2. Stress evolution curve vs. temperature for the annealed films

showing shape memory effects.

2. Experimental

Films of Ti49.5Ni47.5Cu3 (at.%) were deposited on 4-in.

(100)-type silicon wafers (with a thickness of 0.45 mm) at

room temperature by magnetron sputtering of a Ti55Ni45

(at.%) target (RF, 400 W) and a pure Cu (purity of 99.99%)

target (2 W, DC). The deposition lasted for 4 h and the films

obtained a thickness of 3.5 Am. During the deposition, the

substrate holder rotated for uniformity. The substrate-to-

target distance was 100 mm, and argon pressure was about

0.8 mTorr. After the deposition, the films were annealed at

temperatures from 430 to 650 8C in a vacuum at 1�10�7

Torr. The film composition was determined using energy

dispersive X-ray microanalysis (EDX). The measurements

were conducted in five different regions on each sample for

an average. The film crystalline structure was characterized

using glazing incidence X-ray diffraction (GIXRD) at an

incident angle of 18. The wafer curvature was measured by a

Tencor FLX-2908 laser system, and the residual stress was

calculated from the change of radius of curvature before and

after deposition [7,9,14]. The residual stresses were plotted

as a function of annealing temperature, from which the

martensitic transformation information was obtained. The

heating and cooling rate was set at 1 K/min. The surface

Fig. 1. Stress evolution of TiNiCu film annealed up to 650 8C using

curvature measurement method.

morphology of the film was characterized with an atomic

force microscope (AFM, SFT-9800, Shimazu).

3. Results and discussion

XRD confirms that the as-deposited films are amor-

phous. The residual stress in the as-deposited films is

compressive in nature at room temperature, at a magnitude

of 260F10 MPa. Since the deposition takes place at room

temperature, there being little thermal effect, the residual

stress should be mainly intrinsic. At low gas pressures, the

Fig. 3. XRD analysis of TiNiCu films annealed at different temperatures.

Fig. 6. Phase transformation temperatures for films annealed at different

temperatures.

Fig. 4. Residual stress and recovery stress for films annealed at different

temperatures.

Y. Fu et al. / Surface & Coatings Technology 198 (2005) 389–394 391

sputtered ions undergo very few collisions with gas

molecules (thus, little loss of kinetic energy) before arriving

at the growing film. This gives rise to significant atomic

peening and enhancement of mobility of adatoms through

kinetic energy transfer. Consequently, intrinsic compressive

stress generates in the films.

To estimate the crystallization temperature, one sample

has been annealed in the stress measurement chamber in

nitrogen flow. Fig. 1 plots the stress evolution up to 650 8C.In the beginning, below 150 8C (from a to b), the net

compressive stress increases linearly, indicating that thermal

stress is at play: compressive stress results because the film

expands more than four times that of the substrate [the

coefficient of thermal expansion (CTE) of TiNiCu film, af, is

Fig. 5. Stress increase rate (or differentiated stress) vs. temperature for films

annealed at different temperatures.

Fig. 7. TEM photos showing the grain sizes of films annealed at different

temperatures.

Y. Fu et al. / Surface & Coatings Technology 198 (2005) 389–394392

about 15.4�10�6/K and that of Si substrate, aSi, is 3�10�6/

K [9]]. However, above 120 8C (i.e., after b), the tensile

component prevails such that the net stress becomes less and

less compressive as a result of densification with increasing

temperature [7]. Further heating to about 435 8C (point c),

crystallization of TiNiCu film occurs and densification ends.

After c, heating-generated stress (compressive) prevails,

resulting in an almost linear increase in net stress. From d to

e, cooling-induced thermal stress (now tensile) increases

linearly with decreasing temperature. At e, martensitic

transformation starts (Ms). With further decrease in temper-

ature, significant decrease in stress occurs (from e to f).

Fig. 8. AFM surface morphologies of as-deposited and

From Fig. 1, the crystallization temperature should be

around 430 to 440 8C. Thus, one sample was annealed at

430 8C in high vacuum and the stress response of the

annealed film with temperature from room temperature to

130 8C is plotted in Fig. 2. The film has a residual stress of

about 550 MPa in tensile. With increase of temperature, the

stress curve falls almost onto a straight line (no hysteresis

loop), indicating that only thermal stress occurred during

heating and cooling. The XRD results also confirm that

amorphous phases are dominant in this film (Fig. 3).

Therefore, the real crystallization temperature for the film

is probably higher than 430 8C. The thermal stress generated

annealed TiNiCu films at different temperatures.

Fig. 9. Surface roughness (Ra) values of TiNiCu films annealed at different

temperatures.

Y. Fu et al. / Surface & Coatings Technology 198 (2005) 389–394 393

during cooling from 430 8C to room temperature (20 8C)can be estimated from the following equation:

rth ¼ dr=dT � DT ¼ Ef= 1� yfð Þ½ � aSi � afð Þ � DT ð1Þ

where a is the coefficient of thermal expansion, E is the

Young’s Modulus in austenite, t is the Poisson Raito, Si is

the silicon substrate and f the film. As the first-degree

approximation, taking the Ef as 80 GPa [10], and aSi and af

as 3�10�6/K and 15.4�10�6/K, respectively [9,15], and tfas 0.3, the calculated thermal stress rth is 569 MPa. This is

very close to the measured residual stress of 550 MPa as

indicated in Fig. 2 for the film annealed at 430 8C. As such,it is reasonable to infer that the stress left over is mainly

from the thermal effect. Such a high residual stress would be

detrimental to any potential application.

XRD results shown in Fig. 3 confirmed that martensite

(B19V phase, monoclinic) is dominant in all the films

annealed at temperature at and above 450 8C, and film

crystallinity increases with the annealing temperature. No

apparent precipitates can be observed. For films annealed at

and above 450 8C, the stress evolution curves vs. temper-

ature display a closed hysteresis loop (Fig. 2). During both

heating and cooling, a one-stage phase transformation can

be observed, corresponding to transformation between B19Vmartensite (confirmed by XRD results) and austenite. Upon

heating, a sharp increase in stress is seen corresponding to

phase transformation from martensite to austenite. With

further heating, thermal stress evolves and the stress

decreases slightly. During cooling, martensitic transforma-

tion occurs and the stress relaxes significantly correspond-

ing to formation of twining structure. The residual stress

(the stress value at room temperature) and the recovery

stress (the difference between the maximum and minimum

stresses during transformation) are obtainable from Fig. 2

and the results are plotted in Fig. 4. With the increase of

annealing temperature, the recovery stress increases while

residual stresses remain insignificant (b50 MPa). Fig. 5

shows the stress-increase rate (i.e., the differentiated stress

vs. temperature curve derived from Fig. 2) during heating.

The stress-increase rate corresponds to the generation rate

for actuation (or actuation speed). With the increase of

annealing temperature, the maximum stress-increase rate

increases, which shows quick response or large actuation

speed. The stress-increase-rate curve also reveals one-step

phase transformation during heating, and the transformation

temperatures shift to higher values with increase of

annealing temperatures. The transformation temperatures

obtained from the hysteresis loops in Fig. 2 are plotted in

Fig. 6. The transformation temperatures shift to higher

values with increase of annealing temperature. Fig. 7(a) and

(b) shows TEM photos of the TiNiCu films annealed at two

different temperatures, 450 and 550 8C. After annealing at a

temperature of 450 8C, the film shows fully crystalline

structure and the average grain size is about 50 nm. Further

increase in annealing temperature to about 550 8C, the grainsize increases to about 300 to 400 nm. The higher the

annealing temperature, the higher fraction of the crystallized

phases and larger grain size and martensite plate size, thus,

the more significant martensitic phase transformation is

[11,16] (i.e., larger recovery stress, quicker response and

higher phase transformation temperatures).

During annealing, stress evolution is governed by the

competition among: (1) residual stress in as-deposited films,

which is large compressive stress in this study; (2) thermal

stress changes (compressive stress during heating, while

tensile during cooling); (3) tensile stress due to densification

of structure and film crystallization; (4) relaxation of stress

due to martensitic transformation; (5) internal stress changes

due to reduction of defects, or formation of precipitates,

which is rather difficult to evaluate. Results showed that an

appropriate annealing temperature is needed to promote the

film crystallization; thus, the phase transformation can occur

above room temperature and the large thermal stress

generated during cooling can be released significantly. If

there is no martensitic transformation above room temper-

ature, or the films are not fully crystallized, no or only

partial stress relaxation occurs, causing the large deforma-

tion of structure. Thus, an appropriate and correct selection

of annealing temperature is very important.

The annealing process significantly influences surface

morphology of the deposited films. Fig. 8 shows the AFM

morphology of the films annealed at different temperatures,

and Fig. 9 shows the corresponding surface roughness

results (Ra) obtained form AFM analysis. The as-deposited

amorphous films show quite smooth surface as revealed

from AFM observation in Fig. 8(a), and the film surface is

shiny and reflecting to naked eyes. After being annealed at

430 8C, the film surface still shows fine and smooth

morphology (see Fig. 8(b)), and the roughness even

decreases slightly (Fig. 9), which could be explained from

the densification of amorphous film structure at a higher

temperature.

Fig. 10. AFM morphology of the film annealed at 450 8C with a scan size

of 500�500 nm.

Y. Fu et al. / Surface & Coatings Technology 198 (2005) 389–394394

With the increase of annealing temperature to 450 8C, thefilms become significantly rough and coarse with large hilly

and trench structures, and surface roughness increases

significantly as shown in Figs. 8(c) and 9. The film surface

appears to be slightly opaque and cloudy. With further

increase of the annealing temperature, the roughness

increases even further and then remains at a high value

afterwards. However, these hilly and trench structures are

much larger than the grain size obtained from TEM analysis.

This indicates that some long-range order structures (much

larger than grain size) are present in their martensite phases.

Fig. 10 shows a small-scale AFM morphology of the film

annealed at 450 8C, clearly revealing the fine grain size

identical to TEM observation. In situ AFM analysis with

change of temperatures showed that when heated to higher

temperature, the film surface became smoother and those

hilly and trench structures disappear. After cooling back to

room temperature, the film surface becomes rough again

with almost the identical original surface morphology. The

similar results have been reported in Ref. [17]. Therefore, it

can be confirmed that the significant increase in surface

roughness is due to dramatic surface relief or self-

accommodation caused by martensitic transformation (i.e.,

formation of twining martensite structures, such as marten-

sitic pins, rods, branches, plates, etc., upon cooling).

However, if martensitic transformation is below room

temperature or only R-phase transformation occurs, it may

not be possible to cause such a significant surface rough-

ening effect. Depending on the temperature, the TiNiCu film

surface can have either shiny or cloudy appearance, and the

change of the phases may also cause the change in reflective

index. This interesting phenomenon might be used for many

potential optical applications and devices, such as optical

switches, or micro-mirror arrays [17].

4. Conclusions

The following conclusions can be drawn from this study:

(1) With a gas pressure of 0.8 mTorr and target–substrate

distance of 100 mm, the residual stress in the as-

deposited TiNiCu films is compressive (260 MPa).

(2) A minimum annealing temperature (450 8C) was

necessary for film crystallization; thus, large thermal

stress could be released significantly due to marten-

sitic transformation.

(3) With the increase of annealing temperature, crystallite

and martensite plate sizes in the film increased; thus,

both recovery stress and stress-increase rate increased,

while the phase transformation temperatures shifted to

higher values.

(4) The surface roughness increased drastically with the

increase of annealing temperature in correlation to

martensitic transformation.

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