Film thickness measuring apparatus employing intensity compensation

Abstract

An improved film thickness measuring apparatus including an optical measuring system, a detective photomultiplier tube, a corrective photomultiplier tube, a control circuit and a signal processing section. Measuring of the film thickness of a sample material is carried out with the use of spectral reflectivity while optical interference takes place between light beam coming from the surface of the layer structure of the material and light beam coming from the base plate constituting the sample stage. Light beam from light source is introduced into a monochromator to spectrally transform it to monochromatic light. The optical measuring system includes a chopper which serves to chop light beam fed from the light source to introduce intermittent light beam into the monochromator. The latter is provided with a rotatable diffraction grating to effect scanning of wavelength. The second optical system includes a detective semi-transparent mirror, a corrective semi-transparent mirror and an objectve lens so that a part of monochromatic light beam is introduced into the detective photomultiplier tube via the detective semi-transparent mirror. A screen for the optical projection system is provided on the front surface of the housing section to visually observe an image of the sample material in an enlarged scale.