Wavefront control of UV narrow bandpass filters prepared by plasma ion-assisted deposition

Metal oxide layers produced by plasma ion-assisted deposition are extensively used for complex optical coatings due to the availability of materials, the high packing density of films, and the smooth surfaces. Stringent optical surface figure specifications necessary for both laser optics and precision optics require film stress to be well controlled and surface deformation to be corrected or compensated. In this paper, SiO2 based single cavity UV narrow bandpass filters were prepared by plasma ion-assisted deposition. The correlation between film stress, refractive index, deposition parameters, and post deposition annealing was established. The film stress was calculated based on interferometric surface deformation. The refractive index and film thickness were determined by means of variable angle spectroscopic ellipsometry. The center wavelength of the filters was obtained through spectral transmission measurement. The results suggest that the wavefront distortion of the multilayer coatings is dominated by the compressive stress of the SiO2 layers, and can be controlled and corrected by the amount of plasma ion momentum transfer, substrate temperature, post deposition annealing, and stress compensation via backside SiO2 coating. Based on the understanding of the mechanical and optical properties, the wavefront correction technique enables us to satisfy stringent surface figure specifications.