It has been demonstrated that Si-SiO2 interfaces in field effect transistors are not atomically abrupt, but instead contain: (i) an interfacial transition region similar to0.5 nm thick with an average SiO composition as well as (ii) a strained or defective region in the Si substrate that is of similar extent. The strain profile across these interfacial transition regions, compressive in the SiO2 and tensile in the Si substrate, results from a combination of growth induced strain, as well as differences between the linear expansion coefficients of SiO2 and the substrate Si. Two high-temperature transitions modify the strain profile, and the transition region bonding at the Si-SiO2 interface. The first is a visco-elastic relaxation in the SiO2 occurring at similar to1000 degreesC, and the second is associated with bonding changes within the interfacial transition region occurring at similar to900 degreesC. This article uses spectroscopic studies to identify the chemical bonding changes within the interfacial transitions region that occur after 900 degreesC annealing in an inert ambient. The physical and chemical forces that drive these changes are addressed from two perspectives: (i) reactions kinetics and (ii) bond constraint theory. Finally the effects of strain relief on device performance and reliability are discussed. (C) 2004 American Vacuum Society.