The electric characteristics Si3N4 films the interface characteristics of Si3N4/Si formed by newly developed microwave-excited highdensity plasma are described. The leakage current through the Si3N4 films reduces 3 orders of magnitude and the lifetime improves 30,000 times compared to the conventional SiO2 films formed by a thermal oxidation. The MISFET on Si(100) surface with gate insulator of Si3N4 films have more excellent performance compared to that with conventional SiO2 films. The crystal orientation of silicon wafer surface is affected to the subnitride at Si3N4/Si interface. The subnitride at Si 3N4/Si decreases with an increase in Si atom density in the silicon wafer surface, as a result, the quantity of subnitride is lowest in Si(110) surface than any other surfaces. The valence band offset at Si 3N4/Si interface is independent of a silicon wafer surface. Si3N4 films formed by the microwave excited plasma is essential to high performance and low leakage LSI devices.