An investigation of residual stress effects due to the anodic bonding of glass and silicon in MEMS fabrication

Anodic bonding is widely-used in the fabrication of Micro-Electro-Mechanical Systems (MEMS) devices to join silicon and glass components. The process involves the application of temperature, moderate pressure and an electric field. This paper investigates residual stresses arising during anodic bonding, focusing on the resulting induced distortions. Components of a MEMS silicon rate sensor, in which a silicon wafer is anodically bonded to Pyrex (TM) glass, were used as the vehicle for the investigation. Distortions generated by the anodic bonding process when using two different electrode configurations (point and planar) were measured using a surface optical profiler. These showed a particular pattern across the wafers for both configurations. An efficient FEM study was carried out to model the qualitative effect of the following residual stress sources; thermal stress, glass shrinkage due to structural relaxation and compositional gradients due to ion migration. Importantly, the FE model takes account the actual multi-device wafer-level configuration, as opposed to a single device. The results demonstrate that compositional gradients can make a significant contribution to the observed pattern of distortions.