Structural order in Si–N and Si–N–O films prepared by plasma assisted chemical vapor deposition process

Abstract

Local bonding modes and the nearest-neighbor environment of the constituent atoms in glow discharge silicon nitride and oxynitride films have been probed with Fourier-transform infrared spectroscopy. The silicon nitride films prepared under a wide range of (NH3+N2)/SiH4 ratio and rf power levels all show characteristic absorption bands of N–H, Si–H, and Si–N bonds. The Si–H stretch mode absorption peak shifts to higher wave numbers with the increasing number of N–H bonds in the material. This behavior is observed for all power levels and gas flow combinations. The observed universal dependence of the Si–H stretch frequency on the concentration of the N–H bonds, and the sign of the shift indicate that the change in the frequency is caused by the increasing number of nitrogen atoms at the Si site. Arguments advanced on the basis of the electronegativity of the constituents suggest that the formation of N–H and Si–H bonds at the Si–N site leads to opposite shifts in the Si–N frequency. Thermal annealing studies show that the N–H and Si–H bonds dissociate at and above 600 °C. The Si–N stretch mode absorption in the oxynitride films shifts towards the higher wave numbers with the increasing number of Si–O bonds in the material. The refractive index correspondingly approaches to that of a-SiO2. The observed one-to-one correspondence between the Si–N peak position and the refractive index decisively indicates that the oxynitride films are comprised of a complete network of bonds between the Si, N, and O without any phase separation.