A Study of Parasitic MOS Formation Mechanism in Plastic Encapsulated MOS Devices

With a view to preventing the formation of parasitic MOS in plastic encapsulated MOS devices, we studied the mechanism of the formation and found that parasitic MOS is due not only to surface mobile charge on oxidized silicon but also to bulk charge in plastic materials. Concerning an epoxy resin for semiconductor use, which is the most widely used material for plastic encapsulation, the behavior of bulk charge under pressure was investigated by electrical measurement and that under application of high voltage dc was also investigated electrically and by EPMA analysis. As a result, it was established that the majority of the mobile charge is actually certain ions with a negative charge and containing chlorine, rather than electrons or dipoles. It was further confirmed that parasitic MOS formation is extremely scant in MOS devices encapsulated with the same type of epoxy resins with little hydrolyzable chloride ion, and hence that minimizing the ionizable component in plastic materials is highly effective in preventing parasitic MOS formation. Motion of charge along the interface between silicon oxide layer and plastic could not be clarified. However, it is thought that parasitic MOS formation due to this motion can be prevented by providing an appropriate guard ring.