Semiconductor device fabrication method

With a semiconductor device fabrication method, an oxide film and a thermal oxide film formed over a semiconductor substrate are etched and ions are implanted in the semiconductor substrate in a high-temperature environment with the etched oxide film as a mask. Furthermore, the thermal oxide film has high adhesion to the semiconductor substrate. As a result, even if a difference in linear expansion coefficient arises between the semiconductor substrate and the oxide film due to a change in the linear expansion coefficient of the semiconductor substrate, the oxide film does not peel off the semiconductor substrate or crack because the oxide film is formed over the semiconductor substrate with the thermal oxide film therebetween.