Modeling and Measurement of Thermal–Mechanical-Stress-Creep Effect for RF MEMS Switch Up to 200 °C

Abstract

High-temperature processes, such as packaging and annealing, are challenges for Radio-Frequency Micro-Electro-Mechanical-Systems (RF MEMS) structures, which could lead to device failure. Coefficient of thermal expansion (CTE) mismatch and the material’s creep effect affect the fabrication and performance of the MEMS, especially experiencing the high temperature. In this paper, the Thermal–Mechanical-Stress-Creep (TMSC) effect during thermal processes from room temperature (RT) to 200 °C is modeled and measured, in which an Au-cantilever-based RF MEMS switch is selected as a typical device example. A novel Isolation-Test Method (ITM) is used to measure precise TMSC variation. This method can achieve resolutions of sub-nanometer (0.5 nm) and attofarad (1 aF). There are three stages in the thermal processes, including temperature ramping up, temperature dwelling, and temperature ramping down. In different stages, the thermal–mechanical stress in anchor and cantilever, the grain growth of gold, and the thermal creep compete with each other, which result in the falling down and curling up of the cantilever. These influencing factors are decoupled and discussed in different stages. The focused ion beam (FIB) is used to characterize the change of the gold grain. This study shows the possibility of predicting the deformation of MEMS structures during different high-temperature processes. This model can be extended for material selection and package temperature design of MEMS cantilever in the further studies.