Comparative study of the wafer bonding processes for MEMS devices

In this paper, a comparative study of the anodic bonding process and Au-Si eutectic bonding process for MicroElcetroMechanical Systems (MEMS) devices is presented. Especially, the residual stress characteristics are analyzed and compared, using in-situ strain gauges. The residual stress is an important issue to achieve the reliability of packaged devices. In order to fabricate packaged MEMS devices for the residual stress analysis, a Pyrex™ 7740 glass wafer and a (111) silicon wafer are used for the cavity wafer and the structure wafer, respectively. The MEMS structures are fabricated by the Sacrificial Bulk Micromachining (SBM) process, and the glass cavities are fabricated by a HF wet etching process. In the two bonding processes, the bonding temperature and pressure of the bonding chamber are set identically, for ensuring comparisons. The residual stresses of the bonded wafer are measured, using the in-situ strain gauges as well as the bow measurement of the bonded wafers. In the measurement results, the residual stress of the Au-Si eutectic bonded wafer is about one half of that of the anodic bonded wafer.