Failure Mechanisms of Capacitive MEMS RF Switch Contacts

Abstract

Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. In particular, capacitive type switches with metal-to-dielectric contacts (typically Au- on-silicon nitride) are suitable for high frequency (≥10 GHz) applications. However, there is little fundamental understanding of the factors determining the performance and reliability of these devices. To address this void in understanding, we conducted fundamental studies of Au-on-Si3N4 contacts at various bias voltages using a micro/nanoadhesion apparatus as a switch simulator. The experiments were conducted in air at 45% relative humidity. The switch simulator allows us to measure fundamental parameters such as contact force and adhesion, which cannot be directly measured with actual MEMS switches. Adhesion was found to be the primary failure mechanism. Both a mechanical and electrical effect contributed to high adhesion. The mechanical effect is adhesion growth with cycling due to surface smoothening, which allows increased van der Waals interaction. The electrical effect on adhesion is due to electrostatic force associated with excess charge trapped in the dielectric, and was only observed at 40 V bias and above. The two effects are additive, but the electrical effect was not present until surfaces were worn smooth by cycling. Surface smoothening increases the electric field in the dielectric, which leads to trapped charge and higher adhesion. Excessive adhesion can explain decreased lifetime at high bias voltage previously reported with actual capacitive MEMS switches. Aging of open contacts in air was found to reduce adhesion. Surface analysis data show the presence and growth (in air) of an adventitious film containing carbon and oxygen. The adventitious film is responsible for aging related adhesion reduction by increasing surface separation and/or reducing surface energy. No junction growth and force relaxation with time were observed in capacitive switch contacts, as was previously observed with Au–Au contacts at low current in direct current MEMS switches.