Real time control of plasma deposited optical filters by multiwavelength ellipsometry

Abstract

Real time control of optical filters by multiwavelength ellipsometry is presented. The filters consist of SiO2/Si3N4 multilayers deposited on transparent substrates by a microwave plasma technique at room temperature. Using four-wavelength kinetic ellipsometry measurements and standard dispersion laws, efficient algorithms have been developed to determine in real time the current refractive index (n) and deposition rate (Rd) of the different layers. The program is based on minimizing square differences between calculated and experimental ellipsometry data. From the (n, Rd) calculation, which takes less than 200 ms, a control process operating the gas flows has been implemented which allows the deposition of high quality optical filters. As an illustration, the growth of a 11-layer Bragg reflector has been controlled in real time. The deposited filter has been characterized by comparing experimental and target spectral response, both in ellipsometry and spectrophotometry modes. Transmission measurements show excellent agreement in peak position and bandwidth, with less than 2% error.