Radiation-Hard, High Performance AR Treatment For STSS Program Visible-Light Sensors

The MDA is developing the Space Tracking and Surveillance System (STSS), a constellation of satellites designed to monitor, detect, and track ballistic missiles. Each STSS satellite employs multiple visible- and infrared-light imaging sensors. The critical tracking function is provided by the combination of a starring FPA to detect LWIR light, and a silicon-based scanning camera to detect visible light. A second visible light camera employed by each satellite is the chief component of an instrument known as a Star Tracker. Star Trackers provide attitude and position information to the satellite by recording an image of fixed star constellations, and then matching the image to star maps stored in the instruments memory. Detection of very low light levels by these visible-light cameras requires the suppression of background and stray light reflections to unprecedented levels. The AR performance of conventional thin-film coatings is inadequate to meet the STSS program requirements, and AR coatings exhibit short lifetimes when operating in space due to damage caused by thermal variations and radiation exposure. As a result there is an urgent need for an alternative AR treatment for STSS sensors that offers a significant increase in the quantum efficiency (QE) of silicon imagers combined with an improved lifetime when operating in space. The AR technology must also be compatible with the complex and demanding fabrication process employed to produce silicon-based sensors. A new type of high performance AR technology for silicon-based visible-NIR light sensors offering the potential for superior stray light suppression and increased lifetime in high radiation environments, has been demonstrated as a result of a Phase I project. Based on surface relief micro-structures built directly into the sensor window material, the new AR treatment completely eliminates the limitations due to stress, thermal mismatch, adhesion, radiation damage, complexity, narrow-band performance, and cost associated with conventional thin-film AR coatings. AR micro-structures fabricated in silicon windows showed extremely high suppression of reflected light, reducing the typical surface reflection of silicon from an average of 35% to a level below 0.3% over a broad wavelength range from 400 to 1000nm. This performance would result in a QE increase of more than 10% in the blue spectral region and an average QE increase of better than 20% in the red to NIR for a silicon imager incorporating the new AR treatment. Initial thermal cycling tests showed no reduction in performance or any micro-structure damage after repeated and rapid temperature excursions from 77K to 350K. The proposed Phase II project builds upon the success of the Phase I effort, and will demonstrate and deliver an AR treatment that can maintain a high level AR performance over an extended operational lifetime in the presence of proton and ionizing radiation. The potential for a near-term benefit to the STSS program through integration of the AR treatment fabrication process in an STSS sensor manufacturing line, will be realized and refined in Phase II. In parallel with another currently funded MDA SBIR project developing AR microstructures for the infrared detectors on STSS satellites, the proposed work will be afforded an increased efficiency through established working relationships with sensor prime contractors.