This chapter describes the reliability issues regarding silicon dioxide films, for example, time-dependent dielectric breakdown and leakage current at a trench corner. Thin thermally grown silicon dioxide on single crystal silicon substrate has superior dielectric characteristics, which derive from the wide energy gap and the high energy barrier height. As a feature size decreases, three-dimensional structures, such as the trench capacitor and buried oxide isolation, have been widely examined. In corner region in these structures, the thermally grown silicon dioxide becomes thinner than that on a plane surface. The oxide thickness on the protuberance is thinner than that of the rest of the other plane surface. High electric field application gives some dielectric characteristics to the thin silicon dioxide. For highly efficient development, an understanding of the fundamentals of materials, processes and machines is required in addition to the study of degradation mechanisms or reliability of the thin film insulators.