New MEMS Process Technology for Pressure Sensors Integrated with CMOS Circuits

This paper describes the newly developed processes of low temperature wafer bonding using plasma activation and deep dry silicon etching technologies. Both processes are a new type of “MEMS” (Micro Electro Mechanical System) process technology suitable for automotive pressure sensors. The conventional pressure sensor was a unified unit consisting of a silicon sensor chip and a glass stage. The diced unified unit was cut from a bonded disk of a processed silicon wafer and a glass stage substrate, and the silicon sensor chip incorporated four piezo-resistors, a diaphragm and bipolar-circuit. However, the pressure sensor had difficulty in accurately measuring pressure in the high temperature range because of the thermal strain caused by the thermal expansion coefficient difference between the silicon sensor chip and the glass stage. By changing the stage substrate material from glass to silicon, this has eliminated the difference of the thermal expansion coefficient between the silicon sensor chip and the stage, and resulted in making it possible to accurately measure pressure in a high temperature environment such as a vehicle's engine room. Furthermore, by introducing a newly developed low temperature wafer bonding process, this has led to the integration of the silicon sensor chip with a CMOS circuit. Additionally, by adopting a deep dry silicon etching process instead of the conventional wet etching process, which incidentally generated unnecessary bevel portions at the etched side walls of the diaphragm during etching, this has not generated the unnecessary bevel portions and resulted in reducing the diaphragm area and downsizing the silicon sensor chip.