Characterization and Adhesion of Interacting Surfaces in Capacitive RF MEMS Switches Undergoing Cycling

The performance and reliability of capacitive-type Radio Frequency Microelectromechanical Systems (RF MEMS) switches are affected by interfacial interactions such as strong attractive adhesion forces that could cause the switch to get temporarily or permanently stuck, thus rendering the device nonfunctional. Such adhesion forces could be induced and progressively increase during cycling by mechanical surface changes, such as increased real contact area or electrical effects such as increased electrostatic forces by dielectric charging. Both effects can occur simultaneously, which makes the characterization of capacitive switch behavior more complex than metal-to-metal switches. In this work, capacitive RF MEMS switches with Ti-on-silicon nitride contact surfaces were fabricated and tested for different numbers of cycles. After cycling, the Ti beam and Si3N4 dielectric contact surfaces were exposed and examined topographically (using Atomic Force Microscopy), chemically (using Time-Of-Flight Secondary Ion Mass Spectrometry) and nanomechanically (using nanoindentation) to measure surface changes induced by cycling. Such surface changes could explain adhesion behavior and failure of capacitive RF MEMS switches. Topographical roughness analysis showed that physical surface changes occurred early (during the first few hundred cycles). Chemical and nanomechanical analyses also showed surface property changes with cycling, in agreement with the roughness analysis. Therefore, RF MEMS switch surface changes occur with cycling and such changes are readily detectable and could adversely affect (increase) interactive adhesion forces and thus render the switch inoperable.