Semiconductor device having heat detecting element and insulating cavity and method of manufacturing thereof

A method of manufacturing a semiconductor device comprising:

• forming a silicon oxide film on a silicon substrate as at least a portion of a heat insulating layer;
• forming a silicon polycrystalline film on the silicon oxide film;
• forming a hole extending through the silicon oxide film and silicon polycrystalline film by dry etching, the hole having an opening and an interior;
• oxidizing at least a portion of the silicon polycrystalline film that is in contact with the opening of the hole so that the opening is closed or a diameter of the opening is made smaller than that of the interior of the hole;
• and
• forming a heat detecting portion on the heat insulating layer.