Anelastic Creep Phenomena in Thin Metal Plated Cantilevers for MEMS

Abstract

ABSTRACTDuring the past several years, we have developed high displacement sensitivity tunneling accelerometers using surface micromachining and metal electroplating techniques. These devices consist of a Au tunneling tip fabricated below a 1-2 μm thick metal cantilever beam of electroplated Ni or Au. A thin film of e-beam evaporated Au on the underside of the cantilever serves as the tunneling counter electrode. In operation, a 100mV bias is applied across the tunneling gap. A larger turn-on voltage is also applied between the cantilever and a control electrode, located on the substrate, to deflect the cantilever and maintain a constant tunneling current of 1 or 10 nA. Typical deflections of the end of 100 μm-long and 250 μm-long cantilevers are 0.5μm during operation. We have observed that the turn-on voltage decreases over time for most devices with a larger drop observed for the Au cantilevers. In all cases, the initial decay of the turn-on voltage was almost completely recoverable after the device was turned off for 24 hrs. This decay was not found to be strongly dependent on the magnitude of the tunneling current, but could be significantly reduced by pre-stressing the cantilever before operation. Finally, a vacuum anneal at 100°C influences the measured temperature dependence of the turn-on voltage. The observed effects appear to be consistent with fatigue and creep phenomena in the cantilevers. These effects are reversible at room temperature and are dependent on the stress and temperature history of the devices. A comparison is made between metal plated and all-Si structures.