COMPOSITION, ETCHING AND OPTICAL-PROPERTIES OF SILICON-NITRIDE FILMS DEPOSITED BY PLASMA-ENHANCED CHEMICAL VAPOR-DEPOSITION PREPARED IN VARIABLE NH3-N2 GAS-MIXTURE DILUTED WITH HELIUM

Stoichiometric silicon nitride films were prepared by plasma-enhanced chemical vapour deposition at low frequency (50 kHz) and moderate temperature (350-degrees-C) with reactive gases diluted in helium. IR spectroscopy analysis showed the strong influence of the ratio R = [N2]/[NH3+N2] on the chemical bonds and hydrogen content in the films. Optical measurements were used to determine the refractive index, the absorption and the band gap, the variations of which are mainly correlated with the presence of ammonia in the plasma. In addition, our results indicate a way of producing a low etching rate, high band gap and low sub-band-gap optical absorption for silicon nitride.