VUV coatings of Al protected with MgF2 prepared both by ion-beam-sputtering and by evaporation

Mónica Fernández-Perea, Juan I. Larruquert, José A. Aznárez, Alicia Pons, and José A. Méndez

Abstract

Ion-beam sputtering (IBS) and evaporation are the two deposition techniques that have been used to deposit coatings of Al protected with MgF2 with high reflectance in the vacuum ultraviolet down to 115  nm. Evaporation deposited (ED) Al protected with IBS MgF2 resulted in a larger (smaller) reflectance below (above) 125  nm than the well-known all-evaporated coatings. A similar comparison is obtained when the Al film is deposited by IBS instead of evaporation. The lower reflectance of the coatings protected with IBS versus ED MgF2 above 125  nm is because of larger absorption of the former. Both nonprotected IBS Al, as well as IBS Al protected with ED MgF2, resulted in a band of reflectance loss that was peaked at 127 and 157  nm, respectively. This result was attributed to the excitation of surface plasmons due to the enhancement of surface roughness with large spatial wave vectors in the sputter deposition. This reflectance loss for IBS Al protected with MgF2 is small at the short (𝜆∼120  nm) and long (𝜆>350  nm) wavelengths investigated. IBS Al protected with ED MgF2 is thus a promising coating for these two spectral regions. Coatings protected with IBS MgF2 resulted in a reflectance as high as coatings protected with ED MgF2 at wavelengths longer than 550  nm, whereas the former had a lower reflectance below this wavelength.