PIC/MCC simulations and measurements of microdischarges in metallic MEMS structures

Summary form only given. Micro-Electro-Mechanical-Systems (MEMS) have emerged as a promising technology for the development of miniaturized low-cost and low-power switches, actuators, and sensors. However, there are a number of physical mechanisms that need more understanding and this remains as one of the biggest hurdles to widespread commercial adaptation of MEMS-based radio frequency (RF) electronics. In this work we study the phenomenon of gas charging in micro-gaps using PIC/MCC simulations and experiments with the aim of relating it to being one of the possible failure modes of electrostatic MEMS. While gas breakdown in macro-gaps is well established and is governed by the traditional Paschen curve, the field emission of electrons from the metal cathode leads to deviation from the Paschen law in micro-gaps. There have been various experiments in the past which have observed glows or sparks in micro-gaps at voltages much lower than the macroscopic breakdown voltage. In a recent paper, Garg et. al presented the direct measurements of these field emission currents in MEMS capacitors. In this work, we perform PIC/MCC simulations using the XPDP12 augmented with field emission of electrons and collision models for Nitrogen3. The PIC/MCC simulations including field emission were performed for various gaps at atmospheric pressure with work function corresponding to Nickel cathode and a field enhancement factor (β) of 55 which is a typical value based on measurements1. The inclusion of a constant source of electrons due to field emission at the cathode leads to a self-sustaining discharge even in these small gaps. The steady state ion and electron number density profiles indicate a net positive charge in the gap. Results obtained for both Argon and Nitrogen indicate an exponential increase, with voltage, in the total charge in the gap. This trend is compared to results obtained by performing measurements for micro-gaps of air.