  Reliable control of the deposition process of optical films and coatings
frequently requires monitoring of the refractive index profile throughout the
layer. In the present work a simple in situ approach is proposed which uses a
WKBJ matrix representation of the optical transfer function of a single thin
film on a substrate. Mathematical expressions are developed which represent the
minima and maxima envelopes of the curves transmittance-vs-time and
reflectance-vs-time. The refractive index and extinction coefficient depth
profiles of different films are calculated from simulated spectra as well as
from experimental data obtained during PECVD of silicon-compound films.
Variation of the deposition rate with time is also evaluated from the position
of the spectra extrema as a function of time. The physical and mathematical
limitations of the method are discussed.
