Coating related knowledge of optical coating machine: conventional coating process requires raising the substrate temperature (usually about 300 ℃); and more advanced technology, such as ion assisted deposition (IAD) can be carried out at room temperature. The IAD process not only produces films with better physical properties than conventional coating processes, but also can be applied to substrates made of plastic. The traditional method of thin film deposition has always been thermal evaporation, or the use of resistance heating evaporation sources or the use of electron beam evaporation sources. The characteristics of the film are mainly determined by the energy of the deposited atoms. The energy of the atoms in traditional evaporation is only about 0.1 eV. IAD deposition leads to the direct deposition of ionized vapor and adds activation energy to the growing film, usually on the order of 50 eV. The ion source directs the beam from the ion gun to the substrate surface and the growing film to improve the film characteristics of traditional electron beam evaporation.
The optical properties of the film, such as refractive index, absorption, and laser damage threshold, mainly depend on the microstructure of the film. The film material, residual air pressure, and substrate temperature may all affect the microstructure of the film. If the mobility of vapor deposited atoms on the surface of the substrate is low, the film will contain micropores. When the film is exposed to humid air, these pores are gradually filled with water vapor.
Packing density is defined as the ratio of the volume of the solid part of the film to the total volume of the film (including voids and micropores). For optical films, the packing density is usually 0.75 to 1.0, most of which are 0.85 to 0.95, and rarely reach 1.0. The packing density of less than 1 makes the refractive index of the evaporated material lower than the refractive index of the bulk material. During the deposition process, the thickness of each layer is monitored by optical or quartz crystal. These two technologies have their own advantages and disadvantages, so I won't discuss them here. The common point is that they are all used in vacuum when the material is evaporated. Therefore, the refractive index is the refractive index of the evaporated material in vacuum, not the refractive index of the material exposed to humid air. The moisture absorbed by the film replaces the micropores and voids, causing the refractive index of the film to increase. Since the physical thickness of the film remains unchanged, this increase in refractive index is accompanied by a corresponding increase in optical thickness, which in turn causes the spectral characteristics of the film to shift to the long-wave direction. In order to reduce the spectral drift caused by the volume and number of micropores in the film, high-energy ions are used to transfer their momentum to the material atoms being evaporated, thereby greatly increasing the mobility of the material atoms during condensation at the substrate surface .
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