In-situ spectroscopic ellipsometry is known to be very sensitive, non-invasive
technique for monitoring and control of thin film growth. In the production
of optical interference coatings the refractive index of the material is
usually assumed to remain constant within a single layer. Under such
assumption only the optical thickness of the layer can be efficiently
controlled. For modern complex structures, however, even insignificant
deviation from the design, due to the shift in the refractive index, can be
very detrimental to the resulting performance of the coating. Simultaneous
real-time determination of refractive index and growth rate is necessary in
order to comply with strict specifications. If the index departs from the
target value, one has to adjust process parameters and, ultimately, perform
re-calculation of the filter structure to compensate for an error. We will
review the work performed in our laboratory during last few years.
Development of several different control strategies will be discussed and
their application to the control of PECVD of optical interference coatings
demonstrated. Latest work is concerned with the advances in the closed-loop
control of the fabrication of optical thin films by in-situ multi-wavelength
phase-modulated kinetic ellipsometry using both, direct numerical inversion
algorithm for the real-time reconstruction of refractive index and layer
thickness and real-time least-squire fitting-based approach. These
techniques have been tested on quarter-wave index optical filters as well as
on inhomogeneous refractive index profiles and demonstrated efficiency and
robustness.