Electrical contact resistance as a diagnostic tool for MEMS contact interfaces

The electrical contact resistance (ECR) was evaluated as an in situ diagnostic tool for the contact interface behavior of microelectromechanical systems (MEMS). Special polycrystalline silicon (polysilicon) MEMS devices fabricated by surface micromachining were used to study polysilicon/native oxide/polysilicon contact interfaces. ECR measurements obtained during monotonic contact loading and unloading and cyclic contact loading are interpreted in the context of a previous ECR theory. For monotonic contact loading and unloading, the ECR was measured as a function of apparent contact pressure and was found to be on the order of 10⁵ Ω. The fairly moderate decrease of the ECR with the increase of the contact load is attributed to the intrinsic nonohmic behavior of the native oxide film. Experimental ECR results are correlated with analytical solutions to determine the oxide film thickness. The results indicate that the oxide film remains nearly intact under monotonic contact loading and unloading. Differences in the ECR behavior during unloading are discussed in light of the statistical distribution of asperity nanocontacts and the prevailing deformation mode. During cyclic contact loading, rupture of the oxide film leads to the formation of polysilicon/polysilicon nanocontacts, which produces ECR values in the range of 10²-10³ Ω. The erratic behavior of the ECR during cyclic contact loading is related to the pronounced effects of the insulating oxide film and oxide debris trapped at the contact interface.