High Reflectance Coatings for the Extreme Ultra-violet

Abstract

Highly reflecting mirrors for wavelengths greater than 1200Å have been produced by evaporating MgF2 onto aluminium immediately after the deposition of the aluminium. The effect of MgF2 is two-fold; it prevents the growth of a layer of A12O3 which would lower the reflectance of the mirror, and it causes an increase in reflectance through constructive interference of the reflections from the MgF2-vacuum and AI-MgF2 interfaces. If the thickness of the MgF2 layer is 250Å;, the reflectance is approximately 80 per cent for 1200⩽λ⩽A2200, while for shorter wavelengths it drops rapidly, reaching 15 per cent at 1000Å. The use of LiF in place of MgF2 extends the high reflectance region to shorter wavelengths. Reflectances of 65 per cent at 1025Å, can be produced routinely while, on occasion, reflectances as high as 75 per cent are observed. For λ< 1025, the reflectance drops rapidly to about 8 per cent at 922Å. The reflectance averages about 60 per cent for 1100 ⩽λ⩽ 1800 and gradually increases to 85 per cent at 2200Å. These films show very little reflectance loss with age if kept in moderately dry air. When exposed to relative humidities greater than 50 per cent, the reflectance of the A1 + LiF combinations decreases by about 25 per cent in a few days while the A1 + MgF2 combinations are unaffected. Loss of reflectance of the AI + LiF mirrors can be avoided by overcoating the LiF with very thin layers of MgF2 or CaF2. For wavelengths shorter than 1000Å, Pt and Rh are the best reflectors, Pt having a reflectance of 23 per cent at 584Å. ZnS layers have reflectances of 34 per cent down to 1200Å, and have been used in combination with other dielectrics to make mirrors having less than 1 per cent reflectance in the visible while maintaining the reflectance characteristic of ZnS in the extreme ultra-violet.