The transverse optical (TO) and longitudinal optical (LO) phonons of the asymmetric stretch of the O in Si-O-Si bridging bond in thermal SiO2 show red shift with decreasing oxide thickness. These shifts are primarily due to interface effects like strain and substoichiometric silicon oxides (suboxides). A method to isolate the contributions of strain and suboxide concentration towards the observed shifts is proposed. The procedure, which utilizes simple optical model and effective medium approximation, allows estimation of the average strain and suboxide concentration in films of different thickness. Analysis of infrared spectra shows that suboxide-rich layers in low temperature dry and wet oxides are confined to a distance of similar to1.5 nm from the interface; wet oxidation may generate lower stress near the interface. (C) 2003 Elsevier B.V. All rights reserved.