is formed between the channel region 170 and element isolation insulating film 110 b fig7 shows the distributions of the gate threshold voltages of mosfets fabricated by cutting the semiconductor substrate 100 in this embodiment in which the semiconductor substrate 100 was not exposed to an oxidizing ambient or the atmosphere when it was transferred from the nitriding chamber 60 to the annealing chamber 70 and in a comparative example in which the semiconductor substrate 100 was exposed to an oxidizing ambient or the atmosphere as shown in fig7 in the comparative example in which the substrate 100 was exposed to an oxidizing ambient or the atmosphere when it was transferred from the nitriding chamber 60 to the annealing chamber 70 the gate threshold voltage of each mosfet fabricated from the semiconductor substrate 100 existed within the range of u2212 0 62 v to u2212 0 54 v that is the variation was very large and accordingly the yield was low by contrast in this embodiment in which the substrate 100 was not exposed to an oxidizing ambient or the atmosphere when it was transferred from the nitriding chamber 60 to the annealing chamber 70 it was possible to make the variation in gate threshold voltage much smaller than that in the comparative example and greatly increase the yield accordingly also fig8 shows the time dependent dielectric breakdown tddb characteristics of mosfets fabricated as they were cut from the semiconductor substrate 100 in this embodiment in which the semiconductor substrate 100 was not exposed to an oxidizing ambient or the atmosphere when it.