Simulation of degradation of dielectric breakdown field of thermal SiO₂ films due to voids in Si wafers

Degradation of the dielectric breakdown field of thermal SiO₂ film caused by voids that are formed during growth of silicon single crystal has been a serious problem with reliability of MOS devices. To understand the degradation of breakdown field, local thinning of oxide film grown on pits (i.e., voids exposed at the wafer surface) is simulated using a simple model, and the degradation of breakdown field expected from the thinning is compared with experimental reports. In the model, oxide film grown on inner surface of a sphere is calculated by assuming that deformation of oxide film is visco-elastic and that oxidation reaction rate is reduced by compressional normal stress acting on the Si/SiO₂ interface. The calculated results show appreciable thinning of oxide film, which explains the low breakdown field observed experimentally. It also helps to understand the unique degradation characteristics reported for pits and voids: lower breakdown field for thicker oxide film and recovery of breakdown field by chemical etching. No clear pit size dependence observed in the experiments suggests that the oxide thinning is localized at corners of voids.