Physical model of dielectric charging in MEMS

Dielectric charging is a key failure mechanism in radio frequency (RF) microelectromechanical systems (MEMS). To date we lack suitable physical models for the prediction of long-term behavior of RF MEMS switches, susceptible to dielectric charging. In this paper, we present an original approach of studying dielectric charging including modeling of long-term behavior of RF MEMS switches based only on physical dielectric properties that are experimentally characterized. For the first time, we introduce explicitly the trapping kinetics in the model of charging in RF MEMS. In this study, the conduction mechanism and trapping properties of the plasma enhanced chemical vapor deposition (PECVD) SiNx and SiO2 dielectric layers used for the fabrication of RF MEMS switches are characterized. The conduction is determined from I-V sweeps in metal-insulator-metal (MIM) capacitors, whereas the trapping properties are determined from constant current injections in MIM capacitors. The SiNx material shows Poole-Frenkel conduction while Schottky conduction is found to be the best fitting model for the SiO2 dielectric material. The trapping properties of the SiNx layer is best fitted with a logarithmic model of repulsive trapping, while SiO2 fits better with an exponential model of first-order trapping. These extracted physical properties serve as input parameters to our model of pull-in voltage drift in RF MEMS switches. The results of modeling are verified against the pull-in voltage drifts measured on real fabricated capacitive-type RF MEMS switches. It is concluded that the simulated switch lifetime results are very consistent with experimental data obtained for electrostatic MEMS switches either made with PECVD SiNx or SiO2 dielectric materials.