An integrated air-gap-capacitor process for sensor applications

Several MOS-compatible techniques and processes for fabricating diaphragms for use in capacitive sensors have been developed and incorporated with a unique capacitance detection scheme. Surface and bulk micromachining are used to produce a flexible, conductive diaphragm after MOS device structures for circuits are built. The wafer surface forms the bottom plate of a variable air-gap capacitor, and the diaphragm constitutes the top plate that deforms under pressure. KOH etching of small holes through the silicon substrate and subsequent HF etching of a sacrificial dielectric layer through these holes form the diaphragm after circuit processing. Special one-sided etching techniques are used to protect the frontside from etchant attack. A test chip containing doped-polysilicon diaphragm air-gap capacitors and NMOS readout circuits has been successfully fabricated and tested, demonstrating the compatibility of the diaphragm sensor structure with MOS circuitry. The thicknesses of the poly diaphragms range from 0.5 to 1 mu m, and they are 100 mu m*100 mu m in size.<>