A simple method for the determination of slowly varying refractive index   profiles from in situ spectrophotometric measurements
  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. 
Daniel Poitras and Ludvik Martinu