PNNL

Abstract

The performance and durability demands for broadband high reflector coatings for the thirty meter telescope (TMT) are some of the most stringent yet to be developed. High reflector coatings must have reflectance greater than 0.95 between 310 nm and 23,000 nm and an emissivity less than 0.01 in the NIR to LWIR, and will eventually be deposited onto 1.2 m Zerodur substrates. The basic coating design consists of a silver base layer with a multilayer dielectric overcoat. The adhesion and durability of the multilayer coating depend on a thin (~ 15 Å) layer between the substrate and silver base layer and between the silver layer and first dielectric layer. This paper describes the evaluation of adhesion layer materials, coating design and initial performance results. The basic design of the coating is A/Ag/A/Si3N4/(SiO2/Si3N4)n, where A is the adhesion layer. Ni0.5Cr0.5N, Ti0.5Al0.5N, Ti0.2Al0.8N, ZrN, TiWN, and AlN adhesion layers were evaluated. The high reflector coatings were subjected to T/H (60C/90% RH), salt spray and H2S environments. While reflectance of the coating generally met specifications, UV reflectance was degraded by surface plasmon absorption. Optical performance and strategies to eliminate the surface plasmon absorption will be presented.