Effects of dielectric charging on fundamental forces and reliability in capacitive microelectromechanical systems radio frequency switch contacts

Abstract

Microelectromechanical systems (MEMS) radio frequency switches hold great promise in a myriad of commercial, aerospace, and military applications. In particular, capacitive-type switches with metal-to-dielectric contacts (typically gold-on-silicon nitride) are suitable for high frequency (⩾10GHz) applications. However, the devices are known to be unstable in their performance due to parasitic dielectric charging. Although several authors have previously reported the switch failure along with shifts in pull-down and release voltages due to charging, there is some disagreement and lack of understanding among the various reports. This study uses a switch simulator capable of measuring microscale electrostatic and adhesive forces to investigate charging and its effect on reliability and fundamental forces acting within MEMS capacitive switches. An important advantage of the switch simulator is that it can be actuated with or without a bias voltage. Electrostatic force and dielectric charging increased as surfaces were worn smooth by cycling. This is because the surface smoothening decreases separation and increases the electric field strength inside the dielectric. A simple analytical model was developed using electromagnetic theory for the electrostatic force in terms of bias voltage and the areal density of parasitic charge. Using the model and experimental data, it was determined that “charging” (net charge is zero) with the same polarity as the bias voltage resulted in reduced electrostatic force (under bias voltage) when a worn-in switch was actuated repeatedly at constant bias voltage ⩾40V. Small electrostatic force under bias voltage can explain failure in the “up” position (failure to actuate and self-release). Reversing the polarity of the bias voltage between actuations prevented charge buildup and doubled the electrostatic force, which can help explain the effectiveness of bipolar actuation. The charging time constant for parasitic dielectric charge is about 30s under typical MEMS contact conditions. In the specific case of parasitic charge being present, high electrostatic and adhesion forces were measured at zero volts bias. This can explain failure in the down position under zero bias voltage due to self-actuation or adhesion.