| 摘要 |
Provided is a technique for calculating a hole depth or substrate thickness with high accuracy during surface processing work, such as etching or grinding. A difference spectrum calculator calculates the difference between a spectrum acquired at one time and another spectrum acquired at a time earlier than the aforementioned time by a predetermined. The base spectra which are contained in the observed spectra but do not contribute to interference can be regarded as common to the observed spectra. Therefore, the difference spectrum is a virtually normalized interference spectrum. A Fourier transform operator performs a frequency analysis on the difference spectrum, using a Fourier transform or similar technique. In the thereby obtained signal, a clear peak originating from the interference appears at a position corresponding to the optical path length. From this peak position, an optical distance calculator determines the optical path length, calculates the hole depth, and displays the calculated result.
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