Compared with electrostatic, piezoelectric and bimetallic micro-actuators which are current interest, titanium-nickel (TiNi) thin-film SMA micro-actuators offer many advantages such as the large recovery force and low driving voltage. Despite these apparent advantages, successful implementation of TiNi SMA thin-film micro-actuators presents tremendous technical challenges, such as deposition of the film, mechanical designations, compatibility with micro-machining process, etc. _{2 x y}...[ Read more ]

Compared with electrostatic, piezoelectric and bimetallic micro-actuators which are current interest, titanium-nickel (TiNi) thin-film SMA micro-actuators offer many advantages such as the large recovery force and low driving voltage. Despite these apparent advantages, successful implementation of TiNi SMA thin-film micro-actuators presents tremendous technical challenges, such as deposition of the film, mechanical designations, compatibility with micro-machining process, etc.

By studying residual stress in thin film, such as SiO₂, Si_xN_y, Cr and TiNi, it is found that the residual stress in different material is different. The residual stress in thin films could be adjusted not only by using different process conditions but also by changing the underlying layer structures. For TiNi thin film, the residual stress of amorphous TiNi thin film deposited on Cr/poly-silicon/silicon-oxide/Si is smaller than one deposited on Cr/silicon-nitride/silicon-oxide/Si. Moreover, large stress would induce many problems in device fabrication, such as cracks formation in thin film, thin film delamination, device failure.

Applied external stress affects shape-memory behaviour of TiNi material. Effects of residual stress induced by multi underlying layers on TiNi thin film microstructure and characterization will be investigated. The micro-structures and the phase transformation temperatures of sputtered TiNi thin films, both free-standing and attached on different underlying multi-layer substrates have been studied. Differences in the micro-structures, such as the lattice constants and relative concentrations of TiNi, Ti₂Ni and TiNi₃ phases, were observed among the free-standing and the attached films, among the films attached on different underlying multi-layers and among films with different relative orders of aging and release. Not surprisingly, the corresponding phase transformation temperatures are also different. It is proposed that both underlying-layer and process induced stress significantly affect the micro-structures, hence the phase transformation characteristics, of the resulting shape-memory thin films.

Based on studied stress effect on microstructure and characteristics of TiNi thin film, the first attempt to fabricate the sputtered TiNi thin film micro-tweezers without requirement of manual training is reported. The basic idea in the designation and fabrication is that utilizing the residual stress in structural layer and cycle thermal stress realizes repeat deformation of the device. The measurements of the characteristics of the devices demonstrate successful designation and implementation of the training-less TiNi thin film micro-actuators.