An optical film thickness measurement method and film formation method uses a method of measuring the optical thickness of films by radiating a monitor light beam towards a substrate during the formation of a stack of films on the substrate and measuring the optical film thickness from extreme values in the resultant reflection intensity. This stack of films comprises a first film having a reflectance of at least 98% within a predetermined wavelength range and a second film formed on the first film and having an absorption coefficient of 1000 cm.sup.-1 or less within that predetermined wavelength range. The first film is measured by a first monitor light beam having a predetermined wavelength and the second film is measured by a second monitor light beam having a wavelength that differs from the predetermined wavelength range.

A method of manufacturing an optical information recording medium comprising the steps of: forming a plurality of component thin films on a substrate or on a sample at a predetermined film-forming speed and within a predetermined film-forming time in sequence; measuring spectral reflectance of the multilayer thin film formed on the substrate or on the sample; comparing the measured value of the spectral reflectance and the standard value of the spectral reflectance to detect the difference between them; compensating at least one of the film-forming speed and the film-forming time based on the detected difference; and forming a plurality of thin films based on the compensated film-forming speed and with the compensated film-forming time. Consequently, a thickness of each component thin film of the optical information recording medium can be measured easily and production loss in measuring a film-forming speed can be decreased.

The present invention relates to a non-contact, non-destructive measuring apparatus that measures thickness profile and refractive index distribution of a single or multiple layers of thin films by means of the principle of reflectometry. According to the present invention, by employing more than one narrow band-pass optical filters and a two-dimensional array of CCD sensors, and by finding an optimal solution for the nonlinear functional relationship between the thickness of said thin film or thin films and the corresponding refractive indexes by using an iterative numerical computation method, said apparatus simultaneously measures local area-wise thickness profile and refractive index distribution among others of said a single layer or multiple layers of thin films on a substrate.