Thin film formation method and manufacturing method for semiconductor device

A method of forming a thin film including a first portion having a first film thickness and a second portion having a second film thickness, thinner than the first film thickness, the method comprising:

• forming the thin film having the first film thickness on a substrate;
• acquiring an interference waveform upon film formation, as a first interference waveform of reflected light, by irradiating, with laser light, the thin film upon film formation;
• etching the second portion of the thin film;
• of acquiring an interference waveform upon etching as a second interference waveform of reflected light, by irradiating, with laser light, the second portion;
• and
• calculating an interference waveform for target etchings as a third interference waveform, on condition that the second portion has the second film thickness, based on the first interference waveform wherein, in etching the second portion, stopping the etching when the second interference waveform becomes the same the third interference waveform.