  A new approach was proposed to accurately determine the thickness of film,
especially for ultra-thin film, through spectrum fitting with the assistance of
interference layer. The determination limit can reach even less than 1 nm. Its
accuracy is far better than traditional methods. This determination method is
verified by experiments and the determination limit is at least 3.5 nm compared
with the results of AFM. Furthermore, double-interference-aided spectra fitting
method is proposed to reduce the requirements of determination instruments,
which allow one to determine the film thickness with a low precision common
spectrometer and largely lower the cost. It is a very high precision
determination method for on-site and in-situ applications, especially for
ultra-thin films.
