Deposition of optical coatings with real time control by the spectroellipsometry

Abstract

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.