A method monitors a thickness of a subject film deposited on an underlying
structure, the underlying structure contains at least one thin film
formed on a substrate. The method includes determining thickness data of
the underlying structure and storing the thickness data of the underlying
structure in a thickness memory; measuring profile of optical spectrum of
the subject film on the underlying structure; reading the thickness data
of the underlying structure from the thickness memory; calculating
theoretical profiles of the optical spectrum of the subject film based
upon corresponding candidate film thicknesses of the subject film and the
thickness data of the underlying structure; and searching a theoretical
profile of the subject film, which is closest to the measured profile of
optical spectrum of the subject film so as to determine a thickness of
the subject film.