Surface micromachined micro-diaphragm pressure sensors

A micro-diaphragm pressure sensor with a silicon nitride diaphragm of 100 mu m diameter has been developed using surface micromachining (type-C). To increase the yield in the diaphragm forming process, the generation of hydrogen bubbles is reduced by eliminating the anisotropic etching of the silicon substrate and adopting a flat vacuum chamber fabricated by undercut etching of a polysilicon sacrificial layer. The sacrificial layer etching is carried out with a concentric circle around a central etch hole to reduce the stress concentration in the etching process. A tetramethyl ammonium hydroxide solution compatible with the LSI process is utilized as an etchant for polysilicon sacrificial etching. The arrangement of the polysilicon piezoresistors is determined according to stress analysis using a finite element method. The yield of the diaphragm forming process is improved to more than 90%. The pressure sensitivity of approximately 10 mu V/V/kPa is five times that of the conventional micro-diaphragm pressure sensor (type V).>