JPS648801B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention provides glass in the visible and near-visible range,
It is a multilayer antireflection coating for an inorganic substrate such as a crystal, and the constituent films include a heterogeneous film and a base reflection adjustment film, and the strength of the film is increased by vacuum deposition at a base temperature of 200°C to 400°C. Concerning enhancing. Conventionally, in order to consistently form an efficient antireflection film on substrates having various refractive indices, at least one layer of material having an appropriate refractive index determined from the refractive index of the substrate has been required. However, in reality, there are limits to the materials that can be deposited, and the refractive index of a substrate that can prevent reflection is limited. Therefore, in order to compensate for the lack of membrane materials,
-29590 obtains the desired refractive index by uniformly mixing powders of two or more vapor deposited substances with different refractive indexes, and in Japanese Patent Publication No. 45-6193, it is divided into multiple layers and each layer is made of a material with a different refractive index. The refractive index is approximated by their composite refractive index. On the other hand, in actual deposition, even if a single substance is deposited, the crystal growth of the atoms and molecules that form the film differs depending on the thickness position of the film, so the refractive index differs in the thickness direction. ” appears, causing a problem in which properties cannot be obtained as calculated. This "heterogeneity" can be said to apply to deposited thin films in general, but is particularly noticeable in metal oxides having a high refractive index. It is also affected by the thickness; in the case of a film thickness of λ ₀ /4, there is little error even if it is a homogeneous film by using the average value of the upper and lower parts of the deposited film, but in the case of a film thickness of λ ₀ /2, the error is growing. In Japanese Patent Publication No. 49-15503, a layer with a thickness of λ ₀ /2 is divided into two layers with a thickness of λ ₀ /4 to reduce the influence of non-uniformity. Therefore, although the multilayer antireflection coating that has been put into practical use using the above technology has three or more layers and is complicated to work with, its performance is the same as that of a three-layer structure. On the other hand, as for the four-layer film structure, the logical values and the values of the examples shown in Table 1 are known, and their characteristics are as shown in FIG.

[Table] The refractive indices in Table 1 are the upper and lower refractive indices for the second layer of the examples, and the average refractive index for the others. It is deduced in FIG. 1 that the actual residual reflection is increased by the inhomogeneity of the second layer. Incidentally, the heterogeneity of the deposited film can be improved to some extent by lowering the substrate temperature during deposition, but at the same time the strength of the film is reduced, making it impossible to meet the practical requirements of the deposited film. In contrast to the above conventional techniques, the present invention has the following objects. (a) Improve the wavelength width of the antireflection band and residual reflection using four layers. (b) Even if the refractive index of the substrate changes, by adjusting the third and fourth layers, a virtual base consisting of the third layer, fourth layer, and substrate (the substrate, fourth layer, and third layer are collectively referred to as ) with the surface reflectance of the first and second layers kept constant.
Layers must always be the same. (c) It must be a strong film with a substrate temperature of 250°C or higher during deposition. Therefore, the second layer is a heterogeneous film and the first layer is
I chose _MgF2 . On the other hand, the third and fourth layers adjacent to the substrate are used as substrate reflection adjustment films, and the substrate and the substrate reflection adjustment film constitute a virtual base. When considering that the virtual base is anti-reflective by the first layer of MgF ₂ and the second layer of a heterogeneous film, the reflectance of the virtual base should be such that the first and second layers are effective. We investigated whether anti-reflection can be achieved in the first layer, and how the relationship between the refractive index of the underlying reflection adjusting film, the refractive index of the substrate, and the degree of non-uniformity of the second layer affects anti-reflection. The average refractive index of the third and fourth layers is
Where n ₃ , n ₄ and the refractive index of the substrate are n _s , the reflectance γ ₀ of the virtual base at the reference wavelength and its differential coefficient Δγ ₀ are as follows. γ ₀ = n ₄ ² −n _S n ₃ ² /n ₄ ² +n _S n ₃ ² Δγ ₀ = 2n ₃ (n ₃ + n ₄ ) (n _S ² n ₃ − n ₄ ³ ) / (n ₄ ² + n _S n ₃ ²
) ^2Here , if the virtual refractive index of the virtual base is N and the rate of change of the virtual refractive index with respect to wavelength is ΔN, then γ ₀ =1−N/1+N Δγ ₀ =2ΔN/(1+N) ² N=n _S n ₃ ² /n ₄ ² ΔN=n ₃ (n ₃ + n ₄ ) (n _S ² n ₃ − n ₄ ³ )/n ₄ ⁴ . By the way, assuming that the second layer is a homogeneous film, the reflectance γ in this case is determined by the following equation. γ=n ₁ ² −N/n ₁ ² +N Therefore, in this case, the condition for setting the reflection at wavelength λ ₀ to 0 is N=n ₁ ² . However, in reality, it is necessary to deviate from the condition of N=n ₁ ² because it is desired to prevent reflection in a wide wavelength range even if some residual reflection remains. Since this amount of deviation is a function of the refractive index of the second layer, if this function is f ₁ , then the virtual refractive index N is expressed as N=n ₁ ² +f ₁ (n ₂ ). Next, considering the case where the second layer is a non-uniform film, the virtual refractive index N is given as a first approximation: n ₂ = (n _H + n _L )/2 n _H : Refractive index on the third layer side _nL : This is the refractive index on the first layer side of the second layer of heterogeneous film, but the amount of deviation depends on the degree of heterogeneity, that is, the magnitude of the difference between _nH and _nL . Considering this, the virtual refractive index N is (n _H +n _L )/
2, and f ₁ , f ₂ which are the functions of (n _H −n _L ), respectively.
is expressed by the following formula. In addition, 4 shown here
The second layer of an anti-reflection coating is generally considered to have a high refractive index, and empirically the refractive index of the upper part of the heterogeneous layer that occurs when practical materials are laminated to a thickness of λ/2 is 2.0. 2.35, and Δn ₂ =n _H −n _L , which indicates the degree of heterogeneity, is about 0.05 to 0.35. N=n ₁ ² + f ₁ (n _H + n _L /2) + f ₂ (n _H − n _L ) The functions f ₁ and f ₂ in the above equation cannot be determined mathematically and must be found inductively. do not have. Therefore, in the present invention, the first layer is MgF ₂ with n ₁ ² ≒ 1.91, and the above-mentioned
Perform simulations by varying n _H and n _L ,
It has been confirmed that in order to reduce residual reflection in the entire visible range, the functions f ₁ and f ₂ can be approximated by the following formulas. f ₁ (n _H +n _L /2)≒−1.11+n _H +n _L /4 f ₂ (n _H −n _L )≒n _H −n _L /2 Therefore, N=1.91−1.11+n _H +n _L /4+n _H −n _L /2=0.8+3n _H −n _L /4. Similarly, for ΔN, n ₁ ; (n _H + n _L )/
2. It is possible that terms related to (n _H −n _L ) are also included,
A simulation was performed in the same manner as described above, and it was confirmed that the equation below can be approximated. ΔN=-6.7+7/4( _nH + _nL )-2( _nH + _nL )=-6.7- _nH - _15nL /4 Tolerable range of deviation from standard values of N and ΔN α, β
calculated the residual reflectance and determined −0.1≦α≦0.1 −0.4≦β≦0.4 as a practical range. Next, examples of the present invention will be shown. The first example is the second example.
These are shown in the table. The second layer is TiO ₂ deposited using standard methods. The third and fourth layers are successively deposited from two evaporation sources (simultaneous deposition method). FIG. 2 is a graph of residual reflection in the first embodiment. FIG. 3 is a graph of the trajectory of the amplitude reflectance of a virtual base, and it can be seen that the amplitude reflectance is equal in a fairly wide range centered around the reference wavelength λ ₀ =510 nm.

【table】

[Table] The second example is shown in Table 3, which also shows how the allowable range of the rate of change ΔN of the refractive index of the virtual base affects the residual reflection. Fourth
The figure is a graph showing residual reflection in the second example.

【table】

[Table] The third example is shown in Table 4. The degree of non-uniformity in the second layer was reduced by lowering the substrate temperature during vapor deposition, and conversely, the degree of non-uniformity in the second layer was reduced by increasing the substrate temperature. The degree of homogeneity is increased. FIG. 5 shows the residual reflection of the third embodiment.

【table】

[Table] The fourth embodiment is shown in Table 5, and the example indicated by reference numeral 42 is one in which the fourth layer indicated by reference numeral 41 is composed of an equivalent three-layer film (equivalent film method). .
One way to obtain a film with a constant refractive index is to construct it with several layers of a material with a higher refractive index and a material with a lower refractive index than that refractive index, and make the total thickness λ ₀ /4 or less. It has been known. Theoretically, a better approximation can be obtained by making each layer thinner and increasing the number of layers, but in consideration of work efficiency, a three-layer film is appropriate. FIG. 6 shows the residual reflection of the fourth embodiment, and FIG. 7 shows the trajectory of the amplitude reflectance in each layer of the fourth embodiment. In FIG. 7, the solid line arc ad shows the locus of the amplitude reflectance of the fourth layer made of a single layer, and the dotted line arcs ad, bc, and cd show the locus of the amplitude reflectance of the fourth layer made of three layers. Solid line arcs de, ef, and fg indicate the trajectories of the amplitude reflectances of the third layer, the second layer, and the first layer.

【table】

[Table] The fifth embodiment is shown in Table 6, and in the example indicated by 52, the third layer and fourth layer indicated by 51 are each composed of equivalent three-layer films. be. FIG. 8 shows the residual reflection of the fifth embodiment, and FIG. 9 shows the trajectory of the amplitude reflectance in each layer of the fourth embodiment. In Fig. 9, solid line arcs ad and dg indicate the trajectory of the amplitude reflectance of the fourth and third layers consisting of a single layer, and dotted line arcs ab, bc, c(d)e, ef, fg indicate the trajectory of the amplitude reflectance of the fourth and third layers consisting of a single layer. The trajectory of the amplitude reflectance of the fourth layer and the third layer is shown. Solid line arcs gh and hi indicate the trajectories of the amplitude reflectances of the second layer and the first layer.

【table】

[Table] The present invention shown in the above configuration and examples has the following effects. (a) The width of the antireflection band is wider than that of the conventional three-layer film structure, and the antireflection properties are almost the same regardless of the refractive index of the substrate. (b) Since a non-uniform layer can be used for the second layer, the vapor deposition process is easy, and the durability of the film is improved since the substrate can be heated to a high temperature for vapor deposition. (c) Even if the substrate is changed, only the third and fourth layers are changed, and the first and second layers can be deposited in the same manner regardless of the substrate. (d) The tolerance range for ΔN is wider than the tolerance range for the virtual refractive index N, so even if an error occurs in the refractive index of the film in the fourth layer, it can be corrected when depositing the third layer. . In addition, simultaneous vapor deposition, which has traditionally been avoided due to large errors in refractive index, becomes possible, improving work efficiency.

[Brief explanation of the drawing]

FIG. 1 shows the residual reflection of the known four-layer antireflection coating shown in Table 1. FIG. 2 shows the residual reflection of the four-layer reflective film of the first example. FIG. 3 shows the amplitude reflectance of the virtual base. FIG. 4 shows the residual reflection of the second embodiment. FIG. 5 shows the residual reflection of the third embodiment. 6th
The figure shows the residual reflection of the fourth embodiment. FIG. 7 shows the trajectory of amplitude reflectance in each layer of the present invention. 8th
The figure shows the residual reflection of the fifth embodiment. Figure 9 is the fourth
3 shows the locus of amplitude reflectance in each layer of the example. 1...Known theoretical value, 2...Known example, 1
1, 12, 13...first example, 21, 22, 2
3...Second Example, 31,32...Third Example,
41, 42...Fourth example, 51, 52...Fifth example
Example.

Claims (1)

[Claims] 1. The reference wavelength within the wavelength range in which reflection is to be prevented is λ ₀ , and the first to fourth layers are formed from the medium to the substrate, and the first layer has an optical thickness of λ ₀ /4, In a four-layer anti-reflection film in which the second layer has an optical thickness λ ₀ /2, and the third and fourth layers each have an optical thickness λ ₀ /4 and are combined to form a substrate reflection adjustment film, The first layer is MgF ₂ , the second layer is a heterogeneous film, and the refractive index of the third layer side of the second layer heterogeneous film is n _H , and the first layer is MgF 2 .
When the refractive index of the layer side is n _L , the average refractive index of the third and fourth layers is n ₃ , n ₄ , and the refractive index of the substrate is n _S , 2.0≦n _L ≦2.35 0.05≦Δn ₂ ≦0.35 Δn ₂ =n _H −n _L n _S n ₃ ² /n ₄ ² =0.8+3n _H −n _L /4+α −0.1≦α≦0.1 n ₃ (n ₃ +n ₄ ) (n _S ² n ₃ −n ₄ ³ )/n ₄ ⁴ = 15n _L −n _H /4−6.7+β −0.4≦β≦0.4 α: When considering a virtual base having a reflection equal to that due to the combination of the third layer, fourth layer, and substrate,
A multilayer anti-reflection film characterized in that the tolerance range β of the virtual refractive index at the reference wavelength λ ₀ of the virtual base is the permissible range β of the change rate of the virtual refractive index of the virtual base with respect to the wavelength. 2. The multilayer antireflection film according to claim 1, wherein one or both of the third layer and the fourth layer is formed of a layered film having an effect equivalent to that of the third layer and the fourth layer.