Bias-Corrected Optical Measurement of Film Thickness for Vacuum Evaporation Coating

Vacuum evaporation coating is a widely used technique in production of high-tech optical devices. In particular, chalcogenide films formed this way allow to produce axicons lenses with unique characteristics. To ensure consistency of these characteristics, the coating process is automated using digital control systems. The quality of coated films can further be increased with introduction of optical thickness measurement. Due to the specifics of the process, monochrome laser interferometry methods are preferred in axicon lens production. These methods show promising results in manual and semi-manual control. However, the inherent instability of lasers can greatly hinder integration of optical measurement into existing digital control systems. In this paper we consider the development of a digital sensor for optically measuring film thickness in vacuum evaporation coating processes. We present a method for mitigating laser-induced noise by using a reference signal compensation along with hyperbolic filtering. We analyze experimental results of applying this method in vacuum evaporation coating of chalcogenide films ina UVN-71-P3 plant. The results show that the proposed method and filtering algorithms significantly reduce the impact of laser instability and allow for higher precision thickness measurement.