The effect of deposition conditions on the atomic oxygen induced degradation of MgF2 anti-reflective coatings

Spacecraft orbiting below about 800 km are exposed to a high flux of atomic oxygen (AO) that can severely degrade surface materials. Of particular concern are the anti-reflective (AR) coating on solar array surfaces that not only protect the solar panel from the environment but help to boost the transmission of incoming light to the solar cells. Degradation of this AR coating can have serious effects on the performance of the solar panels, leading to insufficient power being available to perform any useful mission. A common space borne solar cell cover consists of a Ceria (CeO₂) doped borosilicate glass (CMG) substrate coated by a 90nm MgF₂ AR coating. Our previous work has shown that the transmission of commercially purchased MgF₂ anti-reflective coatings degrades significantly when exposed to an AO flux. In view of this MgF₂ films were deposited in-house where the deposition conditions were controlled. MgF₂ films were deposited on ceria micro sheet substrates via electron beam evaporation. The system base pressure 2.3 × 10 - 6 torr without the use of the liquid nitrogen (LN2) trap and <;5 × 10 - 7 torr when the LN2 trap is cooled. The thickness of the film was monitored by a quartz crystal microbalance and the substrate was mounted on a temperature controlled heater block. The source material was commercially purchased MgF₂ granules. The as-deposited and AO exposed films were analyzed via spectroscopic ellipsometry, x-ray diffractometry and auger electron spectroscopy. We will show that the transmission degradation can be reduced by an order of magnitude, without significantly changing the optical properties of the film, by controlling the oxygen content of the as-deposited film.