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    Determinants of film refractive index
    Hits:   Date:2020-05-20  
    A good dielectric film must meet the following conditions: high transparency, low absorption; stable refractive index; high bulk density; low scattering; uniform material; good mechanical adhesion, hardness and stress; chemical stability, It is less affected by the environment and has a higher tolerance to radiant energy.
    The refractive index of the refractive index film depends on several factors, including the chemical elements that make up the film, the crystalline state that makes up the film, the size and bulk density of the crystal grains that are formed, and the chemical composition of the film.

    1. Related to the chemical elements that make up the film

    Refractive index is the performance of valence electron polarization in electric field. If the electrons in the outer layer of the element are easily polarized, its n must be high, so for single-element materials, the larger the atomic weight, the higher the refractive index. For example, at 4000nm, carbon is 2.38, silicon is 3.4, and germanium is 3.4. For compound materials, the covalent bond has a higher refractive index, because the ionicity of the covalent bond compound is small and easily polarized.

    2. Related to the crystalline state of the film

    Under different vapor deposition conditions, the crystalline state of the film will be different, resulting in different refractive index. For example, the refractive index of titanium dioxide changes from 1.9 to 2.3 (@ 500nm) between the substrate Ts = 20 ℃ -350 ℃. Another example is that the refractive index of zirconium dioxide on the substrate Ts = 20 ℃ -350 ℃ changes from 1.7 to 2.05 (@ 500nm).

    3. Related to the film-forming grain size and bulk density

    The grain size and packing density of the film layer will affect the refractive index of the film layer. The crystal grains that affect the film layer are larger or smaller, and the packing density is tight or loose, which is related to the substrate temperature Ts and the pressure P during vapor deposition. Generally speaking, the higher the temperature, the lower the gas pressure, the larger the crystal grains and the higher the packing density, so the larger the n, the common example is magnesium fluoride (MgF2) film.