JPH0211121B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an infrared antireflection coating on a germanium substrate used as an optical member in the infrared region. An example of the structure of a conventional achromatic antireflection film is:
Counting from the substrate side, the first to third layers are n ₁ , n ₂ ,
When n ₃ , the refractive indices of air and the substrate are n _p and n _s , it is known that a substance with a refractive index satisfying n ₁ n ₃ = n2 ² = n _p n _s is deposited to a thickness of λ/4. It is being However, all of the third layer materials that serve as the infrared antireflection film have the disadvantage of either low durability or high infrared absorption. It is not preferable to use a material with low durability as the top layer of an antireflection film, and a material with a high absorption rate needs to be as thin as possible. As an example of a conventional infrared antireflection coating, a three-layer antireflection coating described in U.S. Pat. No. 3,468,594 is known, in which the first layer counted from the substrate side is Si.
The second layer is made of ZnS and the third layer is made of BaF ₂ . By the way, since this first layer of Si is active, it is necessary to use an electron gun as an evaporation source during vapor deposition, and on the other hand, it is necessary to increase the vapor deposition rate to some extent in order to increase a certain production efficiency. However, when increasing the output of the electron gun to increase the deposition rate, Si is heated unevenly and non-gaseous Si scatters and adheres to the substrate, resulting in a decrease in the quality of the deposited film or an increase in production efficiency. There are problems such as not being able to do it. Furthermore, since BaF ₂ , which is the third material, has low durability against water and low Knoop hardness as shown in Table 1, this prior art also has the problem of poor durability.

[Table] The present invention was developed in view of the above-mentioned conventional problems, and has an excellent effect of preventing infrared reflection, enables high-speed vapor deposition, and easily obtains a high-quality vapor-deposited film. The purpose is to provide an infrared antireflection film that can Another object of the present invention is to provide an infrared antireflection film that absorbs little infrared rays and is highly durable. The present invention has the following structural features to achieve the above object. In other words, in the antireflection film on the germanium substrate, the first layer counting from the substrate side
The layer is a ZnS film with an intermediate refractive index, and the second layer is a high refractive index film.
Ge, the third layer is a ZnS film with the above-mentioned intermediate refractive index, the fourth layer is a low refractive index LaF ₃ , BaF ₂ or MgF ₂ , the fifth layer is
The reason is that the ZnS film with the above-mentioned intermediate refractive index is arranged as a layer. Further, in a preferred embodiment of the present invention, the thicknesses of the first to fifth layers are d ₁ , d ₂ , d ₃ ,
When the refractive indices of d ₄ and d ₅ are n ₁ , n ₂ , n ₃ , n ₄ , and n ₅ , n ₁ d ₁ : n ₂ d ₂ : n 3 d ₃ : n ₄ d ₄ : _{n 5 d 5} ≒ 1:2:
The ratio should be 4.2:2.5:0.7. Examples of the present invention will be described below. The first example is as follows: Substrate Ge 1st layer ZnS optical thickness (nd) 0.4μm 2nd layer Ge optical thickness 0.8μm 3rd layer ZnS optical thickness 1.7μm 4th layer LaF ₃ optical thickness 1.0μm 5-layer ZnS optical film thickness 0.3μm. With this spectral characteristic, as calculated values are shown in Figure 1 and experimental values are shown in Figure 2, high transmittance can be obtained in the atmospheric window of 8 μm to 12 μm in the infrared region. In FIGS. 1 to 4, the vertical axis represents wavelength (unit: μm), and the horizontal axis represents transmittance (unit: %). In the second example, substrate Ge 1st layer ZnS optical film thickness (nd) 0.4 μm 2nd layer Ge optical film thickness 0.8 μm 3rd layer ZnS optical film thickness 1.7 μm 4th layer BaF ₂ optical film thickness 1.0 μm 5-layer ZnS optical film thickness 0.3μm. With this spectral characteristic, as shown in FIG. 3, high transmittance can be obtained from 8 μm to 12 μm. The third example is as follows: Substrate Ge 1st layer ZnS Optical thickness 0.4 μm 2nd layer Ge Optical thickness 0.8 μm 3rd layer ZnS Optical thickness 1.7 μm 4th layer MgF ₂ Optical thickness 1.0 μm 5th layer ZnS The optical film thickness is 0.3 μm. With this spectral characteristic, as shown in FIG. 4, high transmittance can be obtained from 4 μm to 15 μm. The results of the moisture resistance test of the above example and the above conventional example in which the top layer was BaF ₂ are as follows, and it can be seen that part of the improvement in durability of the present invention can be obtained. This test was conducted at 50°C and 95% relative humidity.

【table】 [Brief explanation of drawings]

Figure 1 is a graph showing the spectral characteristics calculated in the first example, Figure 2 is a graph showing the spectral characteristics of experimental values in the first example, and Figure 3 is a graph showing the spectral characteristics calculated in the second example. The graph shown in FIG. 4 is a graph showing the spectral characteristics calculated in the third embodiment.

Claims (1)

[Claims] 1. In an antireflection film on a germanium substrate,
The first layer counting from the substrate side is a ZnS film with an intermediate refractive index.
The second layer is Ge with a high refractive index, the third layer is a ZnS film with the above intermediate refractive index, the fourth layer is LaF3, BaF ₂ or MgF ₂ with a low refractive index, and the fifth layer is a ZnS film with the above intermediate refractive index. An infrared antireflection coating made of germanium on a substrate. 2 The thickness of each of the first to fifth layers is d ₁ , d ₂ ,
When the refractive index of d ₃ , d ₄ , d ₅ is n ₁ , n ₂ , n ₃ , n ₄ , n ₅ , n ₁ d ₁ : n ₂ d ₂ : n ₃ d ₃ : n ₄ d ₄ : _{n5d5 ≒} 1:2:4.2:
An anti-infrared reflection film on a germanium substrate according to claim 1, wherein the ratio is 2.5:0.7.