Control and quantification of residual stresses in anodically bonded MEMS structures

Residual stresses in anodically bonded silicon devices can result in quality control and process control deficits if the stresses are not controlled. At the same time several geometries may benefit from a controlled introduction of residual stresses. For example, long, thin structures may utilize a residual tensile stress to minimize the likelihood of buckling, while etched cavities with sharp comers can benefit from a residual compressive stress to suppress crack initiation and growth. In the present work, we quantify the residual stress fields present in silicon wafers that are anodically bonded to virgin Pyrex wafers. Anodic bonding is conducted using standard procedures as well as a proposed alternative method that utilizes differential thermal bonding to control the residual stress state. The experimental stress state is compared to theoretical finite element calculations to determine the capability of controlling stresses based on a simple thermal model.