Method of manufacturing semiconductor device

A manufacturing method of a semiconductor device, comprising the steps of:

• (a) preparing a semiconductor substrate having an active region and a peripheral region surrounding the active region in a plan view;
• (b) selectively forming an oxidation-resistant mask layer on a main surface of the semiconductor substrate in the peripheral region;
• (c) forming a field oxide film in the peripheral region by thermally oxidizing the semiconductor substrate exposed from the oxidation-resistant mask layer;
• (d) after the step of (c), removing the oxidation-resistant mask layer;
• and
• (e) after the step of (d), forming a plurality of semiconductor regions of a first conductivity type in the peripheral region by introducing impurities of the first conductivity type into the main surface of the semiconductor substrate using the field oxide film as a mask, wherein
• in a direction from the active region to the peripheral region, the oxidation-resistant mask layer has a plurality of first portions with a first width and a plurality of second portions with a second width smaller than the first width,
• in the step of (c), a first oxide film is integrally formed with the field oxide film under the plurality of first portions, and a second oxide film thicker than the first oxide film is integrally formed with the field oxide film under the plurality of second portions,
• the plurality of semiconductor regions includes a first semiconductor region formed under the first oxide film and a second semiconductor region formed under the second oxide film, and
• a depth of the second semiconductor region is shallower than a depth of the first semiconductor region.