Pressure sensors of CVD diamond films

Electrical and piezoresistive properties of chemical-vapor-deposited boron-doped (B-doped) p-type polycrystalline diamond films are investigated. The diamond films about 2 mu m thick were grown on a flat insulating polycrystalline diamond substrate using a conventional microwave plasma CVD system. Deposition conditions for the diamond films were carefully selected to suppress the degradations, such as the surface conductive layer, the impurity-band conduction under high B-doping concentration, and the resistive conduction across the grain boundaries. The optimized film exhibits hole conduction originated from B acceptor with an activation energy of 0.31-0.33 eV and reasonably high mobility (> 30 cm(2)/V . s at 300 K). A piezoresistor (500 mu m long and 50 mu m wide) of the p-type polycrystalline diamond film was fabricated on a diaphragm structure using photolithography and reactive ion etching in an oxygen plasma. Relative change of the electrical resistance ( R/R-0) of the p-type diamond piezoresistor is almost proportional to the applied strain. Gauge factor K for the p-type diamond piezoresistor is derived to be similar to 1,000 at room temperature and > 700 at 200 degrees C.