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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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