Thin LPCVD stacked nitride-oxide gate MISFET's offer the potential for high current drive as a consequence of the high permittivity of the nitride. However, thin nitride-oxide films have yet to be used in actual MISFET's for LSI products because thin nitride films have poor masking ability during re-oxidation and low reliability under hot-carrier stress. We have investigated improvements to thin LPCVD stacked nitride-oxide films as regards masking ability during re-oxidation and hot-carrier reliability. The method proposed to improve film quality is densification through high-temperature rapid thermal processes and its effectiveness has been tested. Simple rapid thermal annealing (RTA) did not improve the film quality at all. On the other hand, rapid thermal nitridation (RTN) on the deposited nitride film improved it considerably. The reason for the improvement by RTN was investigated.