The most intense feature in the infrared spectrum of SiO2 is the “νas band” near 1070 cm-1. Assigned to the asymmetric stretching mode, this peak exhibits a shoulder on the high frequency side of the band. In this study, Fourier transform infrared (FTIR) spectrophotometry is used to examine the effects of overlying, unpatterned layers of undoped polysilicon (poly) and of tungsten silicide (WSi) upon 0.08 μm of themal oxide grown on silicon. For the poly studies, the oxide is grown on both sides of double-side-polished wafers. 0.11 μm of poly is then deposited on each side of half of the set; 0.25 μm, on the remainder. For the WSi studies, 0.21 μm of undoped poly is deposited onto oxide grown on single-side-polished, low-oxygen wafers. WSi is then deposited on one side.
For the poly set, the peak position (PP) of the “νas band” shifts from 1074 to 1071 to 1066 cm-1 when comparing the uncoated oxide, 0.11 μm poly/oxide, and 0.21 μm poly/oxide samples, respectively. As a result of the observed shifts, the high frequency shoulder becomes resolved as a broad, weak band centered near 1168 cm-1. Thus, the “νas band” appears to be a doublet, consisting of a weak high-frequency component that is insensitive to, and an intense low-frequency component that is sensitive to. overlying films. For the WSi set, the PP of the “νas band” shifts from 1068 to 1076 to 1083 cm-1 for the poly/oxide, WSi/poly/oxide, and annealed-WSi/poly/oxide samples, respectively. Possible causes for the doublet character of the “νas band” and mechanisms for the film-film interactions, such as hydrostatic pressure, are discussed.