JPH0769484B2 - Multi-layer antireflection film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Use) The present invention is a glass, a crystal material, or a semiconductor that does not absorb in the visible region to the infrared region or the near infrared region to the infrared region and has a relatively high refractive index. , For example, ZnS, ZnSe, Si, G
E, TiO ₂ , SrTiO ₃ , As ₂ S ₃ , various laser rod materials, etc. are reflected at the same time in the visible and near infrared range, visible and infrared range, or near infrared and infrared range. The present invention relates to a multi-layer antireflection film for prevention.

(Prior Art) Conventionally, for the refractive index n _{sub of the} substrate, A thin film having a refractive index of the extent, single-layer antireflection film to be deposited in optical thickness of lambda _0/4 with respect to the wavelength used lambda ₀ is known. For example, for ZnSe (n _sub = 2.4), n _L =
If it is 1.55, there is no reflection at λ ₀ .

Usually, in order to obtain sufficient antireflection at n _L = 1.3 to 1.7, the metal oxide (Si
O ₂ , SiO, Al ₂ O ₃ , MgO, Y ₂ O _3, etc.), metal fluorides (MgF ₂ , C)
aF ₂ , Na ₃ AlF ₆ , YF ₃ , BuF, AlF, NaF, ThF ₄ , LaF ₃ , NbF ₃
Etc.) or a mixture of these, etc., a substance that does not absorb in the used wavelength range is selected and used.

For high refractive index substrates, n _sub > n ₁ > n ₂ > ...
Multilayer antireflection having a k-layer structure with n _k > n ₀ is used to widen the antireflection band.

(Problems to be Solved by the Invention) A conventional single-layer antireflection film and a conventional multilayer antireflection film having a wide antireflection band have an antireflection effect at a designed reference wavelength λ ₀ and its vicinity, and further λ ₀ / _{3, λ 0/5 ‥‥ λ} 0 / (2m
+1) (m = 0,1,2 ...) Has a high-order antireflection band, but the band is narrow. On the longer wavelength side than λ ₀ , the antireflection effect disappears, and the reflectance of the substrate approaches.

By the way, recently, there is a strong demand for high precision, high functionality, and compactness of optical devices, and it is necessary to transmit visible light and infrared light in the same optical system. For example, in an optical device that detects infrared light, visible light is used as alignment light, and a heat ray image is detected when it is dark and a visible image is detected when it is bright. In such an optical system, antireflection is required in the two wavelength ranges used. Particularly, in the case of a high refractive index substrate, the performance is remarkably deteriorated due to the decrease of the light quantity, the generation of ghost and the flare, unless the antireflection treatment is applied.

However, the conventional antireflection film is basically referred to as λ ₁ and λ ₂ (λ ₁ <λ ₂ ) in order to prevent reflection in a single wavelength range.
When antireflection is required in one wavelength range, the design reference wavelength of the film is λ ₁ or λ ₂ . However, for example, a ZnSe substrate (n
_sub = 2.4), in order to prevent reflection in the visible range (0.4 to 0.7 μm) and infrared range (8 to 12 μm), the wavelength λ ₁ =
When the design reference wavelength is made to coincide with λ ₁ at 0.55 μm and film configuration n _sub /1.5/air, as shown in graphs 10 of FIGS. 3 and 5, there is almost no antireflection effect at 8 to 12 μm. .

Further, when the design reference wavelength is made to coincide with λ ₂ , as shown in the graphs 12 of FIGS. 4 and 5, at λ ₁ , non-reflection portions and portions having no antireflection effect appear alternately, and the antireflection effect is This is not sufficient and coloring occurs in transmitted light and reflected light.

As described above, the conventional antireflection film that considers only the single wavelength region cannot meet the demand for the antireflection effect in the two wavelength regions.

The present invention has been made in view of such a problem relating to the conventional antireflection of two wavelengths, and provides a multilayer antireflection film having an effective antireflection effect with respect to light of two wavelengths including visible light and infrared light. The purpose is to

(Structure of the Invention) The present invention has a film structure of n _sub / n _M · (2n _L · 2n _M ) ⁱ · n _L / n ₀ (i = 1,2,3 ... n _sub is the refractive index of the substrate, n _M is the refractive index of the intermediate refractive index film, n _L is the refractive index of the low refractive index film, n ₀ is the refractive index of the medium, and n _sub > n _M > n _L > n _The multilayer antireflection film is characterized by performing antireflection for wavelength λ ₁ and wavelength λ ₂ which is approximately a positive integer multiple of wavelength λ ₁ .

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings. Membrane composition
n _sub / n _M · (2n _L · 2n _M ) ⁱ · n _L / n _{0 The} substrate refractive index is n _sub =
2.4, the refractive index of the intermediate refractive index thin film is n _M = 1.90, and the refractive index of the low refractive index thin film n _L is n _L = 1.35, i = 4. λ ₀ = 0.55μ
FIG. 1 shows the spectral transmittance of the multilayer antireflection film of this example when m. In this embodiment, the visible range is 0.4 to 0.7.
It has an antireflection effect and an effect in the infrared region of 8 to 12 μm. The expansion of the visible light range in FIG. 1 is shown by graph 1 in FIG.

Further, in the above-mentioned embodiment, the spectral transmittances when i is changed from 1 to 4 are graphs 101, 102, 10 in FIG.
As indicated by 3, 104, the visible region of 0.4 to 0.7 μm is always antireflection, and when i becomes large, the antireflection band on the long wavelength side of the infrared region moves to the long wavelength side.

(Effect of the Invention) In the formula of the film structure of the present invention, (2n _L , 2n _M ) ⁱ , where the central wavelength λ ₀ = λ ₁ and the antireflection bands are λ ₁ and λ _2.
Is equivalent to the state without a film at λ ₁ , and n _sub / n
_M · n _L / n ₀ acts as a two-layer antireflection film. Also,
At λ _2, it is regarded as a repetition of thin layers of n _M and n _L , and a single-layer film of about N = (n _M + n _L ) / 2 (center wavelength λ ₂ =
It is equivalent to (4i + 2) λ ₁ ). Therefore, it works as an antireflection film in two wavelength regions of λ ₁ and λ ₂ .

[Brief description of drawings]

FIG. 1 is a graph showing the spectral transmittance of an example of the present invention, and FIG.
FIG. 4 is a graph showing the spectral transmittance of the present invention when i is changed, FIG. 3 is a graph showing the spectral transmittance of a conventional single-layer antireflection film, and FIG. 4 is a graph showing the conventional single-layer antireflection film. FIG. 5 is a graph showing the spectral transmittance of the present invention and the conventional antireflection film in the visible region. 1 ... Spectral transmittance of multilayer antireflection film of the present invention 10, 12 ... Spectral transmittance of conventional single-layer antireflection film

Claims (3)

[Claims] 1. A substrate having a film structure of n _sub / n _M · (2n _L · 2n _M ) ⁱ · n _L / n ₀ , n _sub is the refractive index of the substrate, and n _M is an intermediate refractive index film. , N _L is the refractive index of the low refractive index film, n ₀ is the refractive index of the medium, and n _sub > n _M > n _L > n
₀ (i = 1,2,3 ‥‥) a A, a multilayer antireflection film which is characterized in that an anti-reflection for the wavelength lambda ₂ is positive integer multiple Abbreviation At the wavelength lambda _1. 2. The multilayer antireflection film according to claim ₁ , wherein the wavelengths λ ₁ and λ ₂ have a relationship of λ ₂ = (4i + 2) λ ₁ . 3. The multilayer antireflection film according to claim ₁ , wherein the refractive index n _sub of the substrate is relatively higher than the refractive index of the medium at the wavelengths λ ₁ and λ _{2 for} antireflection. A multilayer antireflection film characterized by the above.