PREPARATION OF SEMICONDUCTOR DEVICE

PURPOSE:To reduce a sheet resistance of a poly-Si film by roviding a doped poly-Si thin film on a substrate and then executing thermal processing by covering a vaporization protection film. CONSTITUTION:A field oxide film, a gate oxide film are provided on an Si substrate 1, a poly-Si is deposited by the CVD method at a low temperature, then phosphorus ion is implanted. The patterning is carried out to the poly- Si 3 and an SiO2 film 4 is deposited in the thickness of 0.1mum or more by the CVD method at a low temperature on said poly-Si. When it is heated for the period of 10-100sec at a temperature of 1,000-1,200 deg.C, phosphorus P is activated and particle size of poly-Si increases up to 0.3-0.5mum, lowering a resistance value. At this time, an SiO2 prevents vaporization of phosphorus. Moreover, since a thermal processing period is short, the doped phosphorus P does not break through the gate oxide film. With such a structure, a sheet resistance of poly-Si can be lowered easily.