upper layer 6 having a thickness of about 850 angstroms were obtained then after an electrode wiring was formed on the same conditions as those in example 1 a side wall oxidation was carried out with respect to each of samples in examples 1 and 2 reference example 1 and comparative example 1 after the side wall oxidation the presence of voids was examined in order to examine the presence of voids each of the samples was wet etched or dry etched to remove the sio 2 film and then an apm solution hyper aqueous ammonia mixture of ammonia and hydrogen peroxide was used for carrying out a wet etching to remove the tungsten silicide layer 4 with respect to each of the samples from which the tungsten silicide layer 4 had been removed the surface and cross section of the wiring pattern were observed by means of sem as a result in both of examples 1 and 2 the surface of the polysilicon layer 3 was flat and any depressions shown in fig4 were not observed however in comparative example 1 a depression was observed in the central portion of the surface of the polysilicon layer 3 in reference example 1 although a depression was observed in the central portion of the surface of the polysilicon layer 3 the depression was smaller than that in comparative example 1 from this it was revealed that if phosphorus atoms were added at the stage that the tungsten silicide layer 4 was formed phosphorus atoms exist in the interface between the polysilicon layer.