Hydrogen content and annealing of memory quality silicon-oxynitride films

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Optical measurements have been made to investigate the effects of post-deposition heating to 950° in vacuum on 750°-CVD silicon-nitride films which contain approximately 5 atomic percent oxygen. The refractive index and thick-ness, determined ellipsometrically at 5461 A for 600 A films, were unchanged by annealing within the experimental uncertainties of the measurements. Annealing, however, caused a shift of the absorption edge toward higher energy, a decrease in the etch rate, a cracking of 4500 A films on thick-Si substrates, and an increase in the peak absorption for the SiN band. These effects are interpreted as atomic reordering and densification of the films. The absence of any significant change in refractive index or in the posi-tion of the SiN band upon annealing implies that the oxygen content is unaffected by annealing. Multiple internal reflection measurements show a large hydrogen concentration (~ 7 atomic percent) initially bound to N or to Si in the films. The most striking effect of annealing is on the SiH centers which are annealed out by one hour at 900°C, and a 20 percent reduction is observed for the NH centers. Preliminary electrical measurements indicate an increase in conductance for films annealed at 800 and 850°C; and for the same applied voltage, dielectric failure is observed after annealing at 950°C. Exposure of dangling silicon and nitrogen bonds which were initially-decorated with hydrogen is suggested to explain the higher conductance after annealing.