Intrinsic stress and stress gradients at the SiO2/Si interface in structures prepared by thermal oxidation of Si and subjected to rapid thermal annealing

Abstract

This paper reports on a study of the intrinsic stress and strain in thin films of SiO2 prepared by the thermal oxidation of crystalline silicon. We focus on the relationship between the inhomogeneity of thin-film properties and the thermal history of the oxide film, including both film growth and thermal annealing. We show that this film can be inhomogeneous in the sense that oxide formed initially at the silicon–silicon dioxide (Si/SiO2) interface has been annealed at the growth temperature for the time required for film growth, whereas newly formed oxide at the growth interface has not been annealed for any appreciable period. We demonstrate that thermal annealing cannot completely remove the thickness dependence in the strain induced by the mismatch between the molar volumes of silicon and silicon dioxide at the growth interface, subject to constraints introduced by the chemical bonding structure at that interface. Based on laser-beam deflection and photoreflectance measurements, we show that there is alway a substantial residual intrinsic interfacial stress, and that is independent of the growth temperature, and thermal annealing. A time scale for describing thermal relaxation of stress and strain profiles is given by the ratio of process time to the viscoelastic relaxation time at the processing temperatures.