Fabrication of MOSFET capacitive sensor using spray coating method

A capacitive MOSFET sensor using a SOI wafer for detecting vertical force applied to its floating gate is herein proposed. 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. By using a SOI wafer having a thick active silicon layer, a thick gate structure is made possible, which increases the mechanical stiffness and durability, and increases the sensitivity in accclerometer application by weighting a proof mass. During the fabrication process of this sensor, it is necessary that photoresist be coated on the top-surface and sidewall of the vertical pillar structure with a high aspect ratio. To address this problem, photoresist is applied using spray coating. The uniform coating is successfully realized by adjusting the moving speed of the substrate, and the substrate temperature. A practical test device is under development using this method, and aluminum electrodes for drain, source and gate areas are successfully patterned.