PURPOSE: To measure the thickness of a thin semiconductor film in terms of time elapsed, by irradiating the surface of the film with light of more than crystal band gap energy and detecting fluorescent light diffused by minority carriers produced by the light.

CONSTITUTION: When the surface of a thin film is irradiated with a laser light pulse to produce numerous minority carriers, fluorescent light having a component of wavelength intrinsic with regard to the crystal boundary or substrate crystal is generated. Since the wavelength depends on the impurity level of the crystal, the wavelength can be previously determined. The fluorescent light is changed into an electric signal through a spectroscope and a light detector to record the change in the intensity of the light with time. The change is compared with a predetermined fluorescent light intensity curve to determined the thickness of the thin film.