A new mechanical characterization method for thin film microactuators and its application to NiTiCu shape memory alloy

Abstract

In an effort to develop a more full characterization tool for shape memory alloys, a new technique is presented for the mechanical characterization of microactuators and applied to shape memory alloy (SMA) thin films. , A test instrument was designed to utilize a spring-loaded transducer in measuring displacements with resolution of 1.5 [mu]m and forces with resolution of 0.2 mN. Employing an out-of-plane loading method for freestanding SMA thin films, strain resolution of 30 [mu]E and stress resolution of 2.5 MPa were achieved. This new testing method is presented against previous SMA characterization methods for purposes of comparison. Four mm long, 2 [mu]m thick NiTiCu ligaments suspended across open windows were bulk micromachined for use in the out-of-plane stress and strain measurements. The fabrication process used to micro-machine the ligaments is presented step-by-step, alongside methods of fabrication that failed to produce testable ligaments. Static analysis showed that 63% of the applied strain was recovered while ligaments were subjected to tensile stresses of 435 MPa. In terms of recoverable stress and recoverable strain, the ligaments achieved maximum recovery of 350 MPa and 3.0% strain independently. No permanent deformations were seen in any ligament during deflection measurements. Maximum actuation forces and displacements produced by the 4 mm ligaments situated on 1 cm square test chips were 56 mN and 300 [mu]m, respectively. Fatigue analysis of the ligaments showed degradation in recoverable strain from 0.33% to 0.24% with 200,000 cycles, corresponding to deflections of 90 [mu]m and forces of 25 mN. Cycling also produced a wavering shape memory effect late in ligament life, leading to broad inconsistencies of as much as 35% deviation from average. The effect of stress-induced martensitic twinning that leads to less recoverable stress and the shape memory behavior of long life devices is addressed. Finally, a model for design of microactuators using shape memory alloys is presented to illustrate how results obtained from these tests can be interpreted and applied to the creation of MEMS devices.