Optical film thickness controlling method and apparatus, dielectric multilayer film and manufacturing apparatus thereof

An optical film thickness controlling method, comprising:

• transmitting, over a film formation period of single-layer- or multilayer-structured optical thin film made of one or more kinds of dielectric, incident monochromatic light through said single-layer or multilayer structure to measure a transmittance of said optical thin film and calculate a reciprocal of the transmittance as a reciprocal transmittance,
• regressing measured data group of two variables by a least squares method to a quadratic function before said measured data group achieves a maximum or minimum, the two variables being a film growing time of the latest surface layer film being deposited in association with increase of the film thickness thereof and said reciprocal transmittance, and
• using a film growing time at a maximum point or minimum point on said quadratic regression function as a predicted value of said film growing time when optical film thickness at a maximum or minimum on said reciprocal transmittance is achieved, the maximum and minimum of said reciprocal transmittance having a periodical distribution at every interval of optical film thickness coinciding to ¼ of wavelength of said monochromatic light based on an interference theory.