Method of making MOS transistor having improved oxynitride dielectric
High quality ultrathin gate oxides having nitrogen atoms therein with a profile having a peak at the silicon oxide-silicon interface are formed by oxidizing a surface of a monocrystalline silicon body in an atmosphere of nitrous oxide (N.sub.2 O) at a temperature above 900.degree. C. preferably in the range of 900.degree.-1100.degree. C., and then heating the silicon body and oxidized surface in an atmosphere of anhydrous ammonia to introduce additional nitrogen atoms into the oxide and increase resistance to boron penetration without degrading the oxide by charge trapping. The resulting oxynitride has less degradation under channel hot electron stress and approximately one order of magnitude longer lifetime than that of conventional silicon oxide in MIS applications.