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In the depth measurement of trenches on a semiconductor substrate by means of interference spectroscopy, we sometimes cannot measure the depth when the diameter approaches the wavelength of visible light. To investigate the cause, we determined the delay properties from the reflected intensity data obtained by interference spectroscopy. Then, we confirmed that the delay time in the short wavelength is nearly equal to the time taken by the light to reach the bottom and return to the entrance. However, we found that when the trench diameter approaches the wavelength used in measurement, the change of delay becomes long.

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