PURPOSE: To facilitate forming an upper layer with a uniform thickness by a method wherein, after a required treatment is carried out with photoresist as a mask, an ashing treatment is performed and further a depressurized heat treatment is carried out.

CONSTITUTION: After a pattern of photoresist 3 is formed on a semiconductor substrate 1 and a required treatment is carried out with the photoresist 3 as a mask, an ashing treatment is performed and further a depressurized heat treatment is carried out. For instance, after an oxide film 2 is formed on the P-type Si substrate 1, the required pattern of the photoresist 3 is formed and As ions are implanted with the photoresist 3 as a mask. Then, after the photoresist 3 is removed by ashing in an oxygen plasma, a depressurized heat treatment is carried out. Then, SiO2 is deposited by a CVD method under a normal pressure to form an SiO2 layer 6. With this constitution, contaminating impurity against CVD.SiO2 created by the ashing treatment can be removed by the depressurized heat treatment, the SiO2 layer 6 can be so formed as to have the thickness at the circumference 7 same as the thickness at the center part 8.