The chemical behavior of defects in SiO2/Si structures is a key to the success of Si field effect transistor (FET) technology. By exercising a high degree of control over the processing environment, it has been possible to elucidate microchemical behavior intrinsic to the SiO2/Si interface, particularly the oxide decomposition reaction Si + SiO2->2 SiO and its microstructural and electrical consequences. Initiation of this reaction is shown to occur at existing defect sites, leading to lateral growth of voids in the oxide. At earlier stages of reaction, electrically active defects are generated, following a process that can be reversed by the presence of oxygen; the detailed chemistry indicates competition between decomposition and reoxidation reactions at the defect site, i.e., the defect microchemistry emulates known macroscopic chemical reactions. These results have significance both for defect identification and control in processing. Finally, positron annihilation studies have shown strong evidence for the presence of microvoids in as grown thermal oxide, similar to the early stages of oxide void growth under inert annealing conditions; these microvoids could be yet another manifestation of the oxide decomposition reaction, here competing with direct Si oxidation. © 1990, American Vacuum Society. All rights reserved.