Self-tensioning Support Post Design to Control Residual Stress in MEMS Fixed-Fixed Beams

Abstract

Fixed-fixed beams are ubiquitous MEMS structures that are integral components for sensors and actuation mechanisms. However, residual stress inherent in surface micromachining can affect the mechanical behavior of fixed-fixed structures, and even can cause buckling. A self-tensioning support post design that utilizes the compressive residual stress of trapped sacrificial oxide to control the stress state passively and locally in a fixed-fixed beam is proposed and detailed. The thickness and length of the trapped oxide affects the amount of stress in the beam. With this design, compression can be reduced or even converted into tension. An analytical model and a 3D finite element model are presented. The analytical model shows relatively good agreement with a 3D finite element model, indicating that it can be used for design purposes. A series of fixed-fixed beams were fabricated to demonstrate that the tensioning support post causes a reduction in buckling amplitude, even pulling the beam into tension. Phase shifting interferometry deflection measurements were used to confirm the trends observed from the models. Controlling residual stress allows longer fixed-fixed beams to be fabricated without buckling, which can improve the performance range of sensors. This technique can also enable local stress control, which is important for sensors.