Patent ID: 6897964

Claim:
A thickness measuring apparatus for measuring a thickness of a semiconductor wafer during execution of wet etching using an etchant, characterized by comprising: a measurement light source which supplies measurement light at a predetermined measurement time; light branching means for branching the measurement light from said measurement light source; light output means for outputting one component of the measurement light branched by said light branching means to the semiconductor wafer as a measurement object so as to irradiate the semiconductor wafer from a side of an etching surface to which the etchant is being supplied; light input means for inputting reflected light obtained when the measurement light irradiated from said light output means is reflected by the etchant or semiconductor wafer; reference light generating means for passing the other component of the measurement light branched by said light branching means through a reference optical path designed to be able to change an optical path length so as to generate reference light for which a reference optical path length is set; light coupling means for obtaining interference light by coupling the reflected light from said light input means to the reference light from said reference light generating means; photodetection means for detecting the interference light from said light coupling means; and thickness calculating means having raw thickness value calculating means for calculating a raw thickness value of the semiconductor wafer on the basis of an optical path length difference in reference optical path length between two light intensity peaks selected, as a wafer upper surface peak and wafer lower surface peak, from a plurality of light intensity peaks on the basis of a predetermined selection criterion, using a light intensity distribution representing a correlation between the reference optical path length set by said reference light generating means and a light intensity of the interference light detected by said photodetection means at the measurement time.