Simple sticking models and adhesion criterion to predict sticking effects of fixed-fixed beams in RF MEMS switch design

The fixed-fixed beam in RF switches fabricated by MEMS techniques of a sacrificial layer is inclined to stick to the substrate due to the significant capillary force during drying process. In this paper, sticking models of the fixed-fixed beam are investigated to analyze the sticking effect. Based on these models, an adhesion criterion h_c for the fixed-fixed beam is derived to determine whether the beam sticks to the substrate or not. The deduced formula shows that h_c is determined by the material property of the beam and its length L and thickness t. It is also found that the width of the beam has no effect on h_c. The theoretical models and adhesion criterion are simulated and verified by Finite Element Analysis. The effects of residual stress are also discussed to meet the uncertainty of real fabrication processes.