Method of measuring film thicknesses

A method of measuring thicknesses of one or more transparent films of a sample object which are stacked on a substrate of said sample object, said method comprising the steps of:

• (a) determining the number of said transparent films and optical constants of each one of said transparent films;
• (b) irradiating light of a predetermined observation wavelength range upon said sample object, measuring spectral reflectances and deriving an interference waveform;
• (c) finding the total number of peaks and valleys in said interference waveform and determining possible film thickness ranges for said transparent films based on the total number of peaks and valleys and first and second wavelengths which are respectively located in a shorter wavelength side and a longer wavelength side within said observation wavelength range;
• and
• (d) calculating a deviation between theoretical spectral reflectance and measured spectral reflectance with respect to tentatively determined film thickness values while changing said tentatively determined film thickness values each by a predetermined film thickness pitch within said possible film thickness ranges to find a film thickness combination which causes the deviation to be minimum, thereby finally determining the thicknesses as the thicknesses of said transparent films.