Performance and reliability of a new MEMS electrostatic lateral output motor

Relative humidity (RH) effects on the performance, reliability and durability of a new MEMS lateral output motor were determined in this study. The parallel plate motor design provides greater force per unit device area than comparably sized comb type actuators. Both the electrical and tribological performance of the motor were dependent on RH. Electrostatic forces acting in the capacitive device decreased (for a fixed voltage) with increasing RH, as well as under conditions where moisture condensed on surfaces in close proximity. The amount of adsorbed water vapor on interacting surfaces in the motor controlled performance and durability. In a dry 0.1% RH environment, sliding contact wear and high adhesion or cold welding of static contacts caused early failure. In moderate environments up to 50% RH, negligible wear and low adhesion were observed, and durability was over 3 orders of magnitude larger than in the dry environment. Adsorbed water films provided surface passivation and lubrication. To demonstrate the critical nature of surface chemistry and adsorbed surface films, motors failed after about 10⁵ cycles at 0.1% RH, whereas failure was never observed for motors run up to 10⁹ cycles at 50% RH. Above 70% RH, static friction (stiction) forces limited dynamic performance and reliability and caused device sticking. Adsorbed water is thus needed for surface passivation and lubrication, but too much led to high capillary forces and stiction. By controlling and balancing surface chemistry and surface forces by adjusting RH, excellent performance, low friction and wear, and excellent durability were attained in MEMS