Reflections from the back surface of a transparent substrate influence
the evaluation of optical constants of thin films from ellipsometric
measurements. If the thickness of the substrate is large compared with
the coherence length of the Light, the relative phase between the p a
nd s mode, which commonly is measured by ellipsometry, cannot be defin
ed properly. We show how the reflections from the back surface of the
substrate are taken into account in ellipsometric measurements by calc
ulating the intensities of reflections for arbitrary angles of polariz
ation. Applications of the new method, such as transmittance ellipsome
try, ellipsometry at the back surface of the substrate, and the determ
ination of the optical constants at the substrate-layer interface, are
compared with measurements. (C) 1997 Optical Society of America