JPH02228601A - Antireflection film for silicon - Google Patents

Abstract

PURPOSE:To prevent the reflection on the surface of silicon over a wide band by setting the optical thicknesses of a 1st layer consisting of calcium fluoride, a 2nd layer consisting of zinc sulfide and a 3rd layer consisting of antimony sulfide so as to resemble to a change in the refractive index of multilayered antireflection films of a Chebyshev type. CONSTITUTION:The 1st layer, counted from an oil side, which consists of the calcium fluoride and has the optical thickness of 0.24 times a reference wavelength lambda0, the 2nd layer which consists of the zinc sulfide and has 0.21 lambda0 optical thickness and the 3rd layer which consists of the antimony sulfide and has 0.09 lambda0 optical thickness are provided. The refractive indices and thicknesses are so set as to resemble the refractive indices and thicknesses of the multilayered antireflection film of the Chebyshev type. The reflected light rays of the respective boundary faces of the air and the 1st layer, the 1st layer and the 2nd layer, the 2nd layer and the 3rd layer, and the 3rd layer and silicon are negated by interference with each other and the reflections are decreased over the wide band. The reflections on the silicon surfaces are decreased in this way in a 3 to 5mum wavelength range.

Description

Detailed Description of the Invention (Field of Application) This invention prevents reflection on the surface of silicon used for optical elements such as lenses of infrared optical devices over a wide band, for example, over a wavelength range of 3 to 5 μm. This article concerns anti-reflective coatings for silicone.

[Conventional technology]

Silicon, which is used for optical elements such as lenses in infrared optical devices, has a high refractive index (because the reflection on its surface is large), the reduction in signal light is large, and it also causes flares and flares due to multiple reflections between optical elements. Since the ghost is large, the optical characteristics of the infrared optical system are significantly deteriorated. Therefore, an anti-reflection layer is provided on the silicon surface to prevent the above-mentioned reflection.

The optical refractive index is equal to the square root of the refractive index of the optical element.

In the case of a single layer whose optical thickness is M single wavelength "A6", the reflected light at the interface between the optical element and the optical element and the reflected light at the interface between the optical element and the optical element are The widths are equal and the phase is opposite. Therefore, the reflected light shape and interference cancel each other out, so the reflectance at the reference wavelength λ, 1/C becomes 0%. However, the wavelength range is wide from 3 to Sam. In infrared optical devices that use wavelength bands, a single layer is not sufficient to prevent reflection.

Multi-layer anti-reflection coating is required.

McGraw-Hilputta f) 7P is used as an anti-reflection solution for non-contact controllers consisting of multiple layers that prevent reflection in a wide band.
-(Mcgraw -hill fook aomps
Optics Handbook (Ha
ndbook of 0ptics) J's PI-49.

The optical thickness of each layer is K at the reference wavelength λ0, and the refractive index is 1.38 from the air side. 1.8g, 2.58
A three-layer anti-reflective layer is presented. but.

There is no material that has the above-mentioned refractive index and is transparent to infrared light, and in reality, it is necessary to use a material whose refractive index is close to the above-mentioned 1 line.

The $1 layer is sodium fluoride and cryolite.

Magnesium fluoride and calcium fluoride are used as the second layer, lead fluoride and zinc sulfide are used as the second layer, and antimony sulfide and zinc selenide are used as the third layer.

However, sodium fluoride, cryolite, and lead fluoride are hygroscopic and have reliability problems. However, zinc selenide has the problem of being toxic.

Furthermore, in the case of magnesium fluoride, it is necessary to vapor-deposit it at a high temperature in order to increase its strength, but magnesium nonafluoride produced in this way has a high tensile coefficient of 51, has a large stress, and is prone to peeling. There is a problem that cracks are likely to occur.

As shown in Figure 4, the optical thickness of the outer layer is equal to 8 of the reference wavelength λ0, as shown in Figure 4.
It is selected from the viewpoint of manufacturability, reliability and safety, and is composed of calcium nonafulfide, zinc sulfide and antimony sulfide.

In the figure, C5) is the first layer made of calcium fluoride, (6) is the second layer made of zinc sulfide, and (7) is the third layer made of antimony sulfide, and the optical thickness of each layer is different. Equal to λo/4. (4) is silicon used as an optical element.

[Invention tries to solve! l! Title]

As mentioned above, the optical thickness is composed of calcium fluoride, zinc sulfide, and antimony sulfide whose optical thickness is 4 at the reference wavelength λ0. The average residual reflectance in the wavelength range of 3 to 5 μm is 1.8%,
The problem is that the anti-reflection effect is not sufficient.

This invention was made to solve the above problems, and calcium fluoride and zinc sulfide are selected from the viewpoints of ease of production, reliability, and safety.

A layer consisting of antimony sulfide and a wavelength range of 3 to S.
The purpose is to reduce the reflection of the silicon surface in μm.

[Means to solve the original problem]

The anti-reflection layer for silicon according to the present invention has a first layer counted from the air side whose nine optical thickness is 0.24 times the reference wavelength λ0 made of calcium fluoride, and a first layer made of zinc sulfide whose optical thickness is 0.24 times the reference wavelength λ0. The second layer has a thickness of 0.21λ0, and the third layer is made of antimony sulfide and has an optical thickness of σ, 09λ0.

[Effect]

The silicon antireflection layer in this invention has a refractive index and a thickness similar to that of the Tievishev-type multilayer antireflection layer, so that the air and the first layer are similar.

The reflected light from each interface between the first layer and the second layer, the 1g2 layer and the third layer, and the third layer and silicon cancel each other out due to interference, resulting in a wide W! Coughing can reduce reflexes.

[Example of real power]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 91 is a configuration diagram showing an example of this real power.

In the figure, ('') is the first layer made of calcium fluoride, C2) is the second layer made of zinc sulfide, (3) is the third layer made of antimony sulfide, and (4) is used as an optical element. It's silicon. 1st layer (1), 2nd layer (2
), and the optical thickness of the third layer (3) is the reference wavelength λo
(For It, Q, 24λa, 0.21λG, a, respectively
, a9λ0.

The second color is a diagram showing the change in refractive index from the air side of the silicon antireflection layer to the silicon.

The horizontal axis is the optical thickness from the interface between the 22 Qi and the first layer (1), which is normalized to the thickness of the entire 0 anti-reflection layer4.
In Fig. 5, the nine-point axis is the refractive index, the solid line is the case of the anti-reflection film for silicon according to the present invention shown in Fig. 1, the broken line is the case of the conventional anti-reflection film for silicon, and the - point @ line is the case of the anti-reflection film for silicon according to the present invention shown in Fig. 1. In both cases, this is the case for a 3/i antireflection band for silicon of the Tievisiev type in which the antireflection band is 50% of the center wavelength.

The thickness from the interface between the air and the first layer to the center plane of each layer and the refractive index of that layer are plotted, and the change in refractive index of the conventional anti-reflection film for silicon is shown in Figure 5, which is shown as a straight line. In comparison, it can be seen that the change in the refractive index of the silicone anti-reflection layer according to the present invention is highly similar to the change in the refractive index of the Tievishiev-type anti-reflection layer. target thickness is λ
Just being equal at o/4 makes it 0g! The amplitude reflectance at the interface between the air and the first layer is equal to the amplitude reflectance at the interface between the third layer silicon, and.

Since the refractive index of each layer is related so that the amplitude reflectance at the interface between the first layer and the second layer is equal to the amplitude reflectance at the interface between the second layer and the third layer, the reference wavelength λ0 The light reflected at the interface between the air and the first layer and the light reflected at the interface between the third layer and silicon cancel each other out due to interference, and the light reflected at the interface between the first layer and the second layer and the light reflected at the interface between the first and second layers cancel each other out. 2199 - Not only does the reflected light at the interface between the third layers cancel each other out due to interference, resulting in a reflectance of 0% (but also for light in a wide wavelength range centered around the reference wavelength λ0, The reflected light can interfere and cancel each other out.For this reason, the Chiebishiev type anti-reflection hawk can reduce the residual reflectance sufficiently over a wide band.On the other hand,
In the antireflection layer for silicon according to the present invention, the amplitude reflectance of each boundary is unequal, but the optical thickness of each layer is λ
Since the change in refractive index is changed from Q/4 to be similar to the change in refractive index of a Tievishev-type antireflection filter, reflection can be prevented over a wide band.

Figure 3 shows the liquid length dependence of the residual reflectance of the silicon anti-reflection method according to the present invention when the reference wavelength λ0 is 115 μm. Recognize. In particular, the average residual reflectance in the wavelength box 12113~1gw is 0.1
Very busy at 8%.

〔Effect of the invention〕

As described above, according to the present invention, the first layer made of calcium fluoride is similar to the change in the refractive index of the Tievishev-type multilayer antireflection layer that can prevent reflection on the surface of silicon over a wide band. layer, a second layer consisting of zinc sulfide
The optical thickness of the layer and the third layer of antimony sulfide can be set to provide a broadband, e.g., wavelength range of 3 to 3
A thickness of 5 μm has the effect of preventing reflection on the silicon surface.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a leap diagram showing the refractive index of each layer constituting the anti-reflection panel in an embodiment of the invention, and Fig. 3 is an embodiment of the invention. Figure 4 is a diagram showing the wavelength dependence of the residual reflectance of the anti-reflection layer in the example, Figure 4 is a diagram showing the conventional anti-reflection layer for silicon, and Figure S is the wavelength dependence of the residual reflectance of the conventional anti-reflection layer for silicon. FIG. In the figure, (0 is the first layer made of calcium fluoride,
(2) is the 27i[F made of zinc sulfide, (3) is the third layer made of antimony sulfide, and (4) is silicon. Note that the same reference numerals in each figure indicate the same or corresponding parts. 1: A 21-year-old Japanese 1st layer made of calcium chloride! 4
Kuzu 2 eyebrows 3: E & Ihianti 7in fJ1 Naro No. 314: Silicon

Claims (1)

[Claims] Made of calcium fluoride, optical thickness is reference wavelength λ_0
The first layer counting from the air side is 0.24 times as large as
An antireflection film for silicon, comprising a third layer.