Degradation of a multilayer dielectric filter as a result of simulated space environmental exposure

Abstract

The exterior optical surfaces of satellites are directly exposed to the harsh space environment. Here, a multilayer dielectric solar rejection filter was deposited on a silicon substrate and then subjected to electron and proton irradiation, simulating an orbital environment. Following the exposure, damage was observed that was attributed to dielectric breakdown. Optical and scanning electron microscopy revealed extensive pitting as a result of this exposure. The typical size of discharge pits was 50-100 microns at the surface, extending to the substrate material, where a 10 micron diameter melt region was found. Pit damage occurred at pre-existing coating defects and was accelerated by preexposure to proton radiation. Pitting was not observed on similar samples that had also been overcoated with a conductive thin-film.

Space radiation testing of thin film and multilayer optical coatings

“…Studies suggest significant darkening (20% decrease in reflectance) and electrostatic charging are key problems resulting from the slow dose rates and charged electrons and protons that exist in the unshielded on-orbit environment [7]. However, the typical low temperatures of lenses on orbit will have the effect of reducing the number of lattice vacancy production and therefore also reducing darkening.…”

“…With heat, energy can excite an F center to an F+ center, which is mobile and either forms an aggregate or annihilates at a hole center, also called ripening. In the case of aggregation, the peak absorption wavelengths in the material will change [2,7].…”

“…Elastic collisions still produce damage but the probability of displacements through this mechanism is much lower than the radiolysis process [2,7].…”

“…The primary mechanisms for degradation due to ionizing radiation in optical materials can be put into three different categories (however for different materials, different mechanisms will be dominant) [2,7]:…”

“…These can capture -e's or +e's during irradiation to change valence state and can then have energy levels such that absorption of photons in the visible region occurs. Electronic processes produce a rapid growth of optical absorption, which saturates at a level dependent on the concentration of the impurities in the material [2,7].…”