Magnesium fluoride is the most commonly used fluoride，After the substrate is heated to 250°C or higher, the film becomes hard and wear-resistant. The transparent area of the magnesium fluoride film is 0.21 μm to 10 μm, and the transparent area of the magnesium fluoride thick film is limited to less than 2 μm.Its refractive index is 1.32~1.39, and it is 1.38 at 550nm. The long-wave refractive index value is slightly lower, and it is 1.35 at a wavelength of 2μm. It is very suitable for single layer anti-reflection film, and also suitable for low refractive index materials with other materials to coat multi-layer film filters.
Magnesium fluoride film is the strongest film among all low refractive index materials,Especially when the substrate temperature is about 250℃, the film firmness can get very satisfactory results.The magnesium fluoride film deposited on the substrate at room temperature is porous, the refractive index measured in vacuum is 1.32~1.33, and its packing density is related to the temperature of the substrate. The packing density is 0.72 when the substrate temperature is 30℃, and at 300 It is 0.96 at ℃.After the prepared magnesium fluoride film is exposed to air, the thickness of the film can increase by 3 to 4 nm within 1 to 2 hours, and its refractive index rises to 1.37. This is because the pores of the film are filled with water vapor with a refractive index of 1.33.The refractive index of the magnesium fluoride film prepared on the substrate at 270℃~340℃ is close to that of bulk materials, and the filling density value is close to 1.The structure of magnesium fluoride thin films usually presents a crystalline state, and the grains of thinner films or films prepared on cold substrates are much finer than those on thick films or hot substrates. Magnesium fluoride thin films have a tendency to preferentially grow in the direction of the steam incident angle, and form a non-uniform thin film when the film thickness is greater than 100 nm.Magnesium fluoride film has a very high tensile stress, and its stress increases with the increase of film thickness. When the film thickness reaches about 100nm, its tensile stress value is about 3000~5000kg/cm2.Due to the low refractive index and high mechanical strength of the magnesium fluoride film, it occupies a particularly important position in the anti-reflection film. However, due to its high tensile stress, the thickness of the single-layer magnesium fluoride film cannot exceed 2 μm. Otherwise, the film will break in the vacuum.
Magnesium fluoride plating can be evaporated by a tantalum boat or electron gun. It will evaporate after being slightly melted, and occasionally splash during evaporation. The reason is that the material part (especially the surface or the part near the periphery in contact with the outside) is oxidized into oxidation Magnesium, the melting point of magnesium oxide is higher than that of magnesium fluoride, and magnesium oxide will be ejected when magnesium fluoride evaporates.The solution is to keep the crucible clean, do not fill with oxygen or air at high temperature, the purity of the magnesium fluoride material should be high, and the particles should not be too small. If there is white mist on the edge of the material after coating, scrape it off.