The optical performance of windows and domes are subject to degradation from rain and
sand erosion damage in harsh flight environments. While durable window and dome
materials, such as ALON®, spinel and sapphire are more or less impervious to rain and
sand erosion damage in the captive carry environments, the coatings use to provide antireflection
(AR) function are not. Rain and/or sand erosion damage of the outer AR
coating leads to degradation of the windows optical performance, even when the
underlying window itself is not damaged.
Surmet has been working on design and development of physical gradient index (Moth-
Eye) structures based AR surfaces etched directly into the surface of the ALON substrate.
By eliminating the need for less durable coating materials, these structures offer high
optical performance without compromising durability. The difficulty of this approach is
that the same durability that makes ALON impervious to erosion damage makes it very
difficult to etch. Processes have been developed at Surmet which facilitate the etching of
fine deep features into ALON surfaces required for broadband AR function. Recent
results will be presented.
Space based HgCdTe imaging devices, built on CdZnTe substrates, require radiation hardened anti-reflection (AR) treatments in order to withstand the rigors of the space environment. Conventional anti-reflection (AR) coatings provide adequate optical performance but are prone to delamination and degradation due to extreme temperature cycling and irradiation in space. Consequently, there is an intense need for improved AR technology that combines high optical performance with improved durability. Etching physical gradient or motheye structures directly into the CdZnTe eliminates the need to deposit additional layers of different materials onto the substrate, avoiding the possibility of delamination and cross contamination. Motheye AR surfaces, under development at Surmet Corporation, have demonstrated excellent broadband optical performance in the LWIR (7 to14 micron) waveband. Surmet's motheye technology involves direct etching of a regular pattern of fine features into the CdZnTe substrate, using standard lithography and dry etching techniques. The results from this ongoing research and development effort are discussed.
Durable coatings of silicon-carbon-oxy-nitride (a.k.a. SiCON) are being developed to protect high-speed missile windows from the environmental loads during flight. Originally developed at Rockwell Scientific Corporation (RSC) these coatings exhibited substantial promise, but were difficult to deposit. Under a DoD DARPA SBIR Phase I program, Surmet Corporation, working closely with RSC, is depositing these coatings using an innovative vacuum vapor deposition process. High rate of coating deposition and the ease of manipulating the process variables, make Surmet’s process suitable for the deposition of substantially thick films (up to 30 μm) with precisely controlled chemistry. Initial work has shown encouraging results, and the refinement of the coating and coating process is still underway. Coupons of SiN and SiCON coatings with varying thickness on a variety of substrates such as Si-wafer, ZnS and ALON were fabricated and used for the study. This paper will present and discuss the results of SiN and SiCON coatings deposition and characterization (physical, mechanical and optical properties) as a basis for evaluating their suitability for high speed missile windows application.