Relevant Investigation Of NiTi Shape Memory Alloy Thin Films In MEMS Applications

More and more attention is being given to NiTi shape memory alloys due to its excellent Shape Memory Effect (SME) and Superelastic Effect (SE). The shape memory alloy tin films are becoming one of the most potential microactuating materials, due to their large recoverable strain, strong output force, low driving voltage, good biology compatibility, etc. However, there are some basic questions need to be studied for device application.Firstly, NiTi thin films were prepared by sputtering. Resistant-Temperature (R-T) curves, tensile, bugling and indentation tests were adopted to describe the superelastic characteristics of thin films. The results show: Tensile, bugling and indentation tests can explain the superelastic characteristics and mechanical parameters visually. Furthermore, because the bugling test can express the force state of actual thin films, the mechanical properties of thin films can be reflected veritably.Secondly, thermal-transformation characteristics of thin films were studied. The results show: The crystallized NiTi thin films can be prepared in situ heating at 330℃, which avoids the drawbacks caused by high annealing temperature. The sputtering process suited for microdevice was optimized, and the best technique is: sputtering with heated substrate, 300℃;sputtering power is 200w; Ar pressure is 6×10-4 Torr.After discussing the style of texture with XRD and pole figures, we also calculated the transformation strains of polycrystalline NiTi thin films by MATLAB. The analysis about texture shows: The NiTi thin films deposited in situ heating has A (110) texture. Strong texture can be gotten with proper substrate temperature, lower sputtering power and less Ar pressure. The strong (110) texture will bring more strain than weak (110) texture, and the sample without texture has more strain than that with texture. Hence, it is necessary to control texture in preparing NiTi thin films.Before fabrication, the bridge structure was simulated with finite element analysis software—ANSYS, and the internal stress of thin films was investigated. The results show: the thermal stress in interface is 169Mpa, and there are stress grads in thickness direction. The stress distribution of A and M are different each other, which affect the driving effect of thin films.At last, with the results above and MEMS technique, SMA micro temperature switch was fabricated. The results show: The monolayer NiTi bridge structure has more deflection than NiTi/Si structure, and the maximal recoverable strain is 0.5%; the working temperature of MEMS microswitch is in 20℃～70℃. With an up-and-down temperature, the working state of device tend to stabilization, and the thermal hysteresis is about 15℃.