Reliable MEMS contact through environment control

Designers of MicroElectroMechanical Systems (MEMS) avoid rubbing surfaces because of the challenges associated with reliability in such devices. Surface treatment for non-contacting MEMS is focused mainly on obtaining structures that survive handling associated with production and assembly. A lubrication approach to create reliable sliding contacts would enable a significant expansion of the design space. Interfaces that are deeply buried in complex structures present particular challenges for lubrication. Line-of-sight surface treatment approaches are not applicable to these contacts, and the chemisorbed monolayer films used extensively to date have limited mechanical durability in repetitive contact. Vapor phase lubrication (VPL) has been developed to address the shortcomings of previous lubrication methods, specifically to create a film of molecular dimensions that is replenishable during operation. This approach enables extremely long duration operation of devices with little or no wear. Molecules in the vapor phase adsorb on silicon surfaces and react to form a friction and wear reducing film. As long as the molecule is available in the environment, the lubricant film can be replenished. Micromachined silicon tribometers have been used to demonstrate the VPL concept on MEMS devices, and to compare friction and adhesion behavior with and without the wear-reducing film. Operational lifetime is increased from the order of 104 cycles with a chemisorbed monolayer alone, to in excess of 108 cycles with vapor phase lubrication. VPL has also been demonstrated on a complex polysilicon gear train, and results in a dramatic increase in operating life. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) of macro-scale wear tracks from pin-on-flat tests indicate the formation of a high molecular weight product in the wear scar. Although too small for quantitative chemical analysis, similar deposits are observed on the contacting surfaces of MEMS devices.