Development of MEMS capacitive sensor using a MOSFET structure

The concept of a capacitive MOSFET sensor using a SOI wafer for detecting vertical force applied to its floating gate was already reported by the authors. A MOSFET is fabricated on a SOI wafer, and the box oxide under the gate is removed to release the gate structure. This sensor detects the displacement of the movable gate electrode from changes in drain current, and this current can be amplified electrically by adding voltage to the gate, i.e., the MOSFET itself serves as a mechanical sensor structure. Following this, the present paper reports the fabrication of a practical test device and its preliminary characterization. The present paper also proposes a circuitry, which converts the drain current change to the voltage change while compensating the temperature change. The performance of this circuitry is confirmed by SPICE simulation. In accelerometer application, a comparatively heavy proof mass and thin supporting beams are necessary for increasing the sensitivity. For this purpose, a fabrication process of depositing a thick mass structure using electroplating is newly proposed.