Failure mechanisms of a MEMS actuator in very high vacuum

The tribochemical and mechanical origins of wear of a microelectromechanical systems (MEMS) actuator (electrostatic lateral output motor) operated in very high vacuum (10−7 torr) are reported in this study. Failure mechanisms in vacuum were determined and then compared to those in dry air, which is one of the harshest environments for causing early failure. Durability in vacuum was poor, even worse than that in dry air. Poor durability in vacuum is related to the kinetics of wear and reformation of the native oxide at asperity contacts. Devices failed due to catastrophic wear in vacuum, and more wear debris was generated than in dry air. There was a fundamental difference in wear debris morphology for devices run in vacuum and dry air. In vacuum, wear debris took the form of pulled-out polysilicon grains. In dry air, wear debris was an agglomerate of smaller particles, which were largely comprised of SiO2. An oxide layer reformed quickly enough in the air mediated wear process to provide some protection, but resulted in oxygen rich wear debris. In vacuum, the passivating native oxide layer was removed exposing reactive areas on the surface, which led to junction formation at Si–Si asperity contacts. It is proposed that interfacial bonds formed at asperity contacts were stronger than the cohesive bonds within the polysilicon, which resulted in grain pull-out in an adhesive wear process.