Vibration Isolation and Shock Protection for MEMS.

Abstract

Forces arising from environmental sources have profound influence on the functioning of microelectromechanical (MEMS) devices. Two examples include mechanical vibration and shock, which can significantly degrade the performance and reliability of MEMS. Mechanical vibrations can generate unwanted device output, and shock loads can permanently damage device structures. Thus, there is strong motivation to understand and to mitigate the adverse effects of shock and vibration on MEMS devices. The effects of mechanical vibrations and the means to mitigate them are not well understood. Herein, we present detailed analyses that identify how vibration degrades device performance, especially for MEMS gyroscopes. Two classes of gyroscopes are studied and modeled in detail: Tuning fork gyroscopes (TFG) and vibrating ring gyroscopes (VRG). Despite their differential operation, all capacitive TFGs are affected by vibration due to nonlinear characteristics of their capacitive drive/sense electrodes, while some TFG designs are shown to be more vibration-tolerant than others by >99%. By contrast, VRGs remain immune to vibration effects due to the decoupling of vibration excited modes and sensing modes. Overall, vibration effects in gyroscopes and other MEMS can also be reduced by integrating a vibration-isolation platform, and TFG’s vibration sensitivity is improved by >99% using a properly-designed platform. Prior shock protection in MEMS has utilized two strategies: optimizing device-dimensions and hard shock stops. While both strategies afford protection, they also incur a trade-off in shock versus device performance. Two new shock-protection technologies are developed herein: (1) nonlinear-spring shock stops and (2) soft-coating shock stops. The nonlinear springs form compliant motion-limiting stops that reduce impact. Similarly, soft coating stops utilize a soft thin-film layer on an otherwise hard surface to increase the surface compliance and energy dissipation. Both solutions decrease the impact forces generated between the device mass and the shock stops, and enable wafer-level, batch fabrication processes compatible with microfabrication techniques. Simulation and experimental results clearly demonstrate that both solutions offer superior shock protection compared to conventional hard shock stops. Following testing of more than 70 devices, we observe a twenty fold increase in device-survival rate for devices protected either by silicon nonlinear-spring stop or by Parylene soft-coating stops.