Handbook of Silicon Based MEMS Materials and Technologies

7.4.3. Properties of Thick Bonded SOI Wafers

7.4.3.1. Stress and Wafer Bow

The presence of the buried silicon oxide film significantly alters the thermal and mechanical behavior of the SOI structure compared with processing of standard bulk silicon wafers. The thermo-elastic properties of thermal oxide are clearly different from those of silicon, and the buried oxide has a significant impact on the room-temperature bow of the SOI wafer and the stress in the active device layer. The most important sources of strain are the differential thermal expansion of silicon and thermal silicon dioxide, and the intrinsic stress of the SiO² layer. In bonded SOI wafers the strain is accommodated elastically by the build-up of in-plane stresses. When the device ...