Review of Device and Reliability Physics of Dielectrics in Electrostatically Driven MEMS Devices

After decades of improving semiconductor-device reliability, dielectric failure rates resulting from surface-charge accumulation, dielectric breakdown, and charge injection have been reduced to an almost imperceptible range. However, the influence of dielectric properties and behavior on device performance in microelectromechanical systems (MEMS) is still poorly understood and a substantial contributor to device failure. The difference is primarily due to two factors. First, the characteristic length scale and electrical stress of dielectrics in MEMS are often an order of magnitude or more than in semiconductor devices. Lateral dimensions of tens of micrometers increase the probability of including defect sites, and higher applied voltages increase the probability of dielectric breakdown. Second, dielectrics in MEMS are often designed to fulfill multiple functions, often with no equivalent in semiconductor devices. The use of dielectrics as structural material puts substantial emphasis on material properties other than the classic dielectric properties. The use of freestanding elements in MEMS causes large interfacial surfaces between the dielectric and air which in turn provides various charge trap mechanisms. The same surfaces, when allowed to come in contact, could lead to a failure mechanism called stiction, where the surfaces cannot be separated after contact. This paper provides a review of the most prevalent failure mechanisms resulting from the use of dielectrics in electrostatically driven MEMS devices and methods to characterize both their material properties and impact on reliability performance.