The design, fabrication and characterization of a piezoresistive tactile sensor for fingerprint sensing

Presently reported are the design, fabrication and characterization of a diaphragm-based piezoresistive tactile sensor for fingerprint sensing. A dimpled diaphragm was constructed using low-pressure chemical-vapor deposited (LPCVD) low-stress silicon nitride. A sacrificial double-layer, consisting of amorphous silicon and silicon oxide, was used to form a shallow cavity underneath the diaphragm. The depth of the cavity was designed to provide sufficient damping to eliminate oscillation of the diaphragm and made shallow to prevent damage to the diaphragm when excessively loaded. Metal-induced laterally-crystallized polycrystalline silicon (poly Si), having a gauge factor larger than that of conventional LPCVD poly-Si, was used to construct the piezo-resistors. A static response of ~0.1μV/V/Pa was measured for a 100×100μm² square diaphragm, sufficient to produce an output signal in the milli-volt range under a typical finger-induced pressure loading of 0.2 to 2.5 MPa.