Silver film grain boundary pinning by ion bombardment decreases surface plasmon resonance absorption (Conference Presentation)

Abstract

Telescope mirrors based on highly reflective silver films must be protected from atmospheric corrosion with dielectric overlayers. Reflectivity is optimized when the silver surface is extremely smooth and uniform prior to dielectric overlayer deposition. Silver thin films were deposited on glass slides by electron beam evaporation using a custom deposition system at the University of California Observatories Astronomical Coatings Lab. The silver thin films were subsequently covered with a stack of dielectric films utilizing silicon nitride and titanium dioxide deposited by ion assisted electron beam evaporation to fabricate protected mirrors. In-situ argon ion bombardment was introduced after silver deposition prior to the deposition of dielectric films to assess its effects on the performance of the mirrors. Effectiveness of the ion bombardment was systematically studied for different holding time in vacuum, the time between the end of the silver thin film deposition and the start of the ion bombardment, related to the changes in the surface morphology of silver films and resulting reflectivity spectra. Reflectivity at wavelengths in the range of 350nm – 800nm was found to improve due to ion bombardment, which was qualitatively interpreted to result from decreased surface plasmon resonance coupling. The decrease in the coupling is explained by asserting that the ion bombardment slows down silver surface diffusion and pins grain boundaries, preventing post-deposition grain growth, forming smoother silver-dielectric interfaces.