Effects of Wet Oxidation on the Properties of Sub-10-nm-Thick Silicon Nitride Films

Capacitors were prepared by wet oxidation of 8 nm (80 Å) thick silicon nitride films which had been deposited on polysilicon substrates. The capacitance and breakdown field of these capacitors were measured as a function of the wet oxidation conditions. The properties of the polysilicon films were also investigated by varying the preparation conditions to find a better substrate for the deposition of the ultrathin silicon nitride films. These polysilicon films were prepared using low-pressure chemical vapor deposition at 560° and 625°C on silicon wafers. The films were then annealed at 900°C for 30 min after As doping by implantation with a dose of more than 2 × 10¹⁵/cm². The films deposited at 560°C have a smooth surface and a low electrical resistivity and are thus suitable substrates for the ultrathin silicon nitride films as well as suitable contact electrodes. For the capacitors with an oxide nitride composite layer, the capacitance decreases sharply, but the breakdown field increases with an increase in the wet oxidation time at 900°C. This is due to an increase in the thickness of the oxide—nitride layer. For the capacitors with silicon oxynitride layers created by removing the top oxide of the silicon nitride wet-oxidized at 900°C for more than 35 min, however, the values of both the capacitance and the breakdown field are higher than those of capacitors with an unoxidized silicon nitride layer.