Invar thin films for MEMS bistable devices

Bistable elements are beginning to appear in the field of MEMS (microelectromechanical systems) as they allow engineers to design sensors and actuators which require no electrical power to function. Our research group previously used highly compressive thermal oxide and polyimide thin films to fabricate no-power bistable vacuum sensors. In this study we investigate a new thin-film candidate material, invar, which shows promise for the design of no-power temperature sensors and thermal actuators. Invar thin films were deposited using a PVD (physical vapor deposition) sputtering system and then elementary analysis was performed using EDX (energy dispersive x-ray analysis). Elementary analysis showed our thin films to be 54.95% Fe, 41.50% Ni, and 3.55% Oxygen. This composition proved to be slightly different than the bulk invar target, which was 36% Ni and 64% Fe. The coefficient of thermal expansion (CTE) of the thin invar films were then investigated using a KLA-Tencor FLX-2320 Stress Analysis System. It produces an experimental CTE average for sputter-deposited invar to be 1.376ppm/°C (32°C to 120°C). This result was encouraging, as it competitive with published values for bulk invar. Finally this paper presents strategies for tuning the CTE values of the film by co-sputtering Ni and Fe discrete targets to produce the alloy thin film.