Introduction

Magnesium fluoride is the low index coating material commonly used as a single layer antireflection coating on glass optics. The refractive index of approximately 1.38 in the visible [1] results in a reflection of less than 1% across the visible. These coatings have magenta color in reflection, as a quarter wave coating for 550 nm results in a preponderance of red and blue in reflection. Durable coatings require evaporation, either resistive or electron-beam, with a substrate temperature of 300°C [1]. The films have a very fine tetrahedal grain structure, with the crystallites only detectable by electron diffraction [2]. Magnesium fluoride is transparent from 0.11 nm in the vacuum ultraviolet to seven μm in the infrared [1]. The high tensile stress of evaporated MgF₂ coatings crazing for quarter-waves at wavelengths longer than 3 or 4 μm. MgF₂ dissociates when sputtered [3], leaving thermal evaporation as the deposition techniques of choice.