Backscattering analysis of the composition of silicon-nitride films deposited by rf reactive sputtering

Abstract

Amorphous silicon‐nitride films prepared by rf sputtering of an 8‐in.‐diam silicon target in a nitrogen discharge have been analyzed using 1.8‐MeV‐⁴He‐ion backscattering to determine the dependence of film composition on the discharge pressure and rf power. Films with thicknesses between 1000 and 2200 Å were deposited on (111) silicon substrates at pressures ranging from 2 to 20 mTorr and rf power densities between ∼1 and 5 W/cm² of target area. The target‐to‐substrate spacing was held constant at 5 cm. Film composition, measured as the silicon‐to‐nitrogen ratio (N_Si/N_N), was found to be uniform through the film thickness (with a depth resolution of ∼350 Å) and across the substrate surface. However, films deposited in separate runs under closely controlled and nominally identical conditions exhibited variations in N_Si/N_N of up to 8%. The silicon‐to‐nitrogen ratio (N_Si/N_N) in the deposited films varied from approximately 0.60 to 0.80 over the range of power and pressure investigated. The observed dependence of composition on the nitrogen discharge pressure exhibited similar features at all power densities. At the lowest pressures employed (2–4 mTorr) the films were always nitrogen rich as compared to the stoichiometric compound Si₃N₄. As the nitrogen pressure was increased up to ∼8–10 mTorr the films became progressively less nitrogen rich. Beyond this point the composition tended to a value (N_Si/N_N) nearly independent of pressure, but dependent on the rf power density. The pressure dependence of composition is consistent with reaction occurring at the substrate. The tendency for films to become less nitrogen rich with increasing nitrogen pressure is attributed to a reduction in the flux of chemisorbable nitrogen species at the substrate as pressure is increased.