Long life, high performance anti-reflection treatment for HgCdTe infrared focal plane arrays

The image contrast and clarity recorded by backside illuminated HgCdTe focal plane arrays (FPAs) is strongly dependent on minimizing signal loss and detector noise caused by scattered and reflected light from the FPA window and imaging optics. Thin film anti-reflection (AR) treatments based on stacks of thin-film materials have been exclusively used to minimize substrate reflections for this application. The performance and lifetime of these thin-film AR coatings is limited, and can be inadequate for some space based applications due to the damage produced in the coatings by radiation exposure and extreme temperature variations. A new type of high performance AR treatment for HgCdTe FPAs promising very wide bandwidth operation and increased lifetime in high radiation environments is under development as part of the MDA's Space Tracking and Surveillance System, or STSS. Based on surface relief microstructures fabricated directly in the FPA window, the new textured AR treatment replaces thin-film coatings, eliminating inherent coating limitations such as stress, thermal expansion mismatch, adhesion, radiation hardness, and low laser damage thresholds. Progress on the design, fabrication, and space qualification of AR microstructures for staring format HgCdTe FPA windows, is reported here. Transmission data for FPA windows containing AR microstructures is presented, demonstrating a reduction of reflected light loss from 21% for an untreated window down to an average of less than 1% over a six micron wide spectral range in the long wave infrared region (7-13&mgr;m). The potential for AR microstructures to perform over even wider bandwidths such as the important dual-band infrared region (3-12&mgr;m), has been demonstrated. Such high AR performance is coupled with nearly un-measurable scattered light losses as recorded by sensitive instruments operated by NIST. Initial proton radiation exposure and thermal cycling tests show no damage to the microstructures and no degradation of the AR performance. Interferometer measurements of the surface flatness of FPA windows incorporating AR microstructures indicate no change from the initial surface flatness, a result that is a significant improvement over thin-film AR coatings, and one that has great potential for large format FPA fabrication.