Biomedical Engineering Reference
In-Depth Information
stress, and superior fatigue property, and so on, which make them more suitable for micro-
actuator application. However, the transformation temperatures of TiNiCu films decrease
slightly, and the transformation becomes weaker with the increase in Cu contents, in terms
of recovery stress, maximum recovery strain and heat generation, and so on. Also, the film
becomes brittle when Cu content is more than 10 at.%.
Generally speaking, the constrained films have smaller hysteresis as compared with
freestanding films, and the films with larger compressive stress could have much smaller
(even almost zero) hysteresis compared with films with large tensile stress. Therefore,
selection of a suitable substrate (with larger thermal expansion coefficient than TiNi film)
could help generate large compressive stress, thus a smaller hysteresis. An alternative is to
use an external heat sink. TiNi-based films can be deposited on a suitable substrate with
good thermal conductivity, such as Cu plate, thus significantly improving thermal dissipa-
tion and working frequency. However, this brings in more critical issues, such as integra-
tion and compatibility with MEMS batch process, residual stress, and adhesion.
Adhesion and Interfacial Analysis
When TiNi films are deposited on Si substrate, there exist interfacial diffusion and chemi-
cal interactions at the interface, whereby titanium and nickel silicides may form during high-
temperature deposition or post-deposition annealing. These interfacial reaction products could
be complex, heterogeneous, and metastable (Stemmer et al., 1997; Wu et al., 2001). Because the
thickness of TiNi film required in MEMS applications is usually less than a few microns, a rela-
tively thin reaction layer could have significant adverse effect on adhesion and shape memory
properties. TiNi film adheres well to silicon substrate provided it is clean and prechemically
etched. TiNi films deposited on a glass substrate can be easily peeled off, which is quite use-
ful to obtain freestanding films. In MEMS processes, there is a need for an electrically and
thermally insulating or sacrificial layer. Thermally grown SiO
2
is often used as this sacrificial
layer. However, the adhesion of TiNi films on SiO
2
layer (or on glass and polymer substrate)
is poor owing to the formation of a thin intermixing layer and a fragile and brittle TiO
2
layer
(Fu et al., 2003). Upon a significant deformation or during a complex interaction involving
scratch, this layer is easy to be broken, thus peels off. Wolf et al. (1995) proposed a two-step
deposition method to solve this problem: predeposition of 0.1
μ
m TiNi film on SiO
2
at 700°C to
promote interdiffusion of elements, followed by bulk film deposition at room temperature. Fu
et al. (2004) reported that the addition of Si
3
N
4
interlayer between film and Si substrate did not
cause much change in phase transformation behavior as well as adhesion properties. There is
significant interdiffusion of elements and formation of Ti-N bond at the Si
3
N
4
/TiNi interlayer.
If compared with poor adhesion of TiNi films on SiO
2
interlayer, Si
3
N
4
interlayer seems to be
a good choice for an electrically insulating and diffusion barrier layer in respect of adhesion
properties. Adhesion of TiNi films on polysilicon and amorphous silicon layers is also quite
good.
Stability, Degradation, and Fatigue
Stability and fatigue have always been concerns in development of TiNi thin films for
applications. Fatigue of TiNi films is referred to as the nondurability and deterioration of
the SME after many cycles. The repeated phase changes will alter the microstructure and
hysteresis of the transformation and in turn will lead to changes in transformation tem-
peratures, transformation stresses, and strains. The performance degradation and fatigue
of thin films are influenced by a complex combination of internal (alloy composition, lattice
