Method for determining the optical constants of thin dielectric films withvariable thickness using only their shrunk reflection spectra

Thickness inhomogeneities in thin films have a large influence on their opt ical transmission and reflection spectra. If not taken into account, this m ay lead to rather large calculated values for the absorption coefficient or the erroneous presence of an absorption-band tail, as well as to significa nt errors in the calculated values of the refractive index and the film thi ckness. The effect of thickness variation on the optical reflection spectru m of a thin dielectric film covering a thick non-absorbing substrate, is an alysed in detail in this paper, and analytical expressions are presented fo r such a reflection spectrum and its upper and lower envelopes. A method is suggested for determining the refractive index n(lambda) and the extinctio n coefficient k(lambda), as well as the average thickness and the thickness variation, of a thin dielectric film with variable thickness, by using onl y the two envelopes of the corresponding shrunk reflection spectrum. This m ethod is used for the geometrical and optical. characterization of thermall y-evaporated amorphous chalcogenide films, deposited on glass substrates.