PHOTO-CVD SILICON-NITRIDE THIN-LAYERS AS PH-ISFET SENSITIVE MEMBRANE

Silicon nitride films have been deposited by the process,of mercury-se nsitized photochemical vapor deposition (photo-CVD), using a gaseous m ixture of SiH4 and NH3 under 253.7 nm UV light irradiation. The main a dvantages of this technique are low deposition temperature (200-400-de grees-C) and no radiation damage. The optimal conditions of the deposi t are defined in order to obtain a stable sensor with a surface sensit ive to pH: optimal temperature of 200-degrees-C, gas flow rate (SiH4/N H3) of 1/80 and deposition time of 8 min. An annealing is necessary at 400-degrees-C in N2 atmosphere for 20 s. Under these conditions, a de position thickness of 560 angstrom and refractive index of 1.97 were o btained. The silicon nitride ISFET obtained has a quasi-nernstian pH r esponse with a sensitivity equal to 53.5+/-0.5 mV pH-1, due to the NH2 sites at the surface of the silicon nitride deposit. The stability of the response when the silicon nitride surface is kept in contact with an aqueous solution is about five months.