Dielectric charging in capacitive RF MEMS switches with silicon nitride and silicon dioxide
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Capacitive radio frequency (RF) micro-electromechanical (MEMS) switches are among the most promising applications in MEMS systems. They have been introduced in the last 15-20 years as a practical alternative over traditional semiconductor switches. Low-cost RF MEMS switches are prime candidates for replacing the conventional GaAs Field Effect Transistors (FET) and pin diode switches in RF and microwave communication systems, mainly due to their low insertion loss, good isolation, linear characteristic and low power consumption. Unfortunately, their commercialization is currently hindered by reliability problems. The most important problem is charging of the dielectric, causing unpredictable device behavior. The charging of the dielectric has been found to be a complicated process and is currently under intense research. Developing a good analytical model that would describe accumulating of charges in the dielectric and their influence on the device behavior would be the main step to achieving more reliable switches. This work intends to theoretically and experimentally investigate the dielectric charging effects of capacitive RF MEMS switches with silicon nitride and silicon dioxide as the dielectric layer. For the silicon nitride study, both MEMS switches and MIM capacitors were fabricated, and their charging behaviors were analyzed and compared. Several different dielectric stoichiometries, deposition temperatures, and thicknesses were examined in order to understand the effects of each parameter on the charging mechanisms of the dielectric. The goal was to determine the most favorable deposition conditions to induce minimum dielectric charging in silicon nitride capacitive switches. The switches were measured over a wide temperature range and the temperaturedependent behavior of the dielectric was examined to characterize and study its charging behaviors. For the silicon dioxide MEMS switches, several different actuation mechanisms were systematically analyzed, and their effects on the dielectric charging of the switches were studied. A general model of distributed charge and air gap was adopted and further developed to better explain the charging behavior of MEMS switches. The goal was to provide a deeper insight into the trapping processes in dielectric materials and their corresponding time constants. This will in turn aid in better modeling of charging processes in capacitive RF MEMS switches.