JPH01267501A - Antireflection film for germanium - Google Patents

Abstract

PURPOSE:To reduce residual reflectivity and to prevent the reflection on the surface of germanium over wide bands by determining the refractive indices and optical thicknesses of 2nd-4th layers at specific relations. CONSTITUTION:This antireflection film has the 1st layer 7 which is laminated on the surface of the germanium 6 and has the optical thickness of 1/4 the reference wavelength lambda0, the 2nd layer 2 which is laminated on the 1st layer and has refractive index Na and thickness da, the 3rd layer 3 which is laminated on the 3rd layer 3 and has refractive index Nb and thickness da, the 4th layer 4 which is laminated on the 3rd layer 4 and has the refractive index Na and thickness da, and the 5th layer 5 which is laminated on the 4th layer, has the optical thickness of 1/4 the reference wavelength lambda0 and consists of sodium fluoride. The refractive indices Na, Nb and optical thicknesses da, db of the 2nd-4th layers are in the relations expressed by the equation I. The residual reflectivity of the antireflection film 1 for germanium is thereby lowered and the reflection on the surface of the germanium 6 is prevented over wide bands.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an antireflection film for germanium that prevents reflection over a wide band on the surface of germanium used in infrared optical devices and the like.

[Conventional technology] In infrared optical devices using germanium.

Reflection on the surface of germanium reduces the signal light, and the reflected light reaches the detector, causing flares and ghosts, which deteriorate the optical characteristics of the infrared optical device. Therefore, in order to prevent the above-mentioned reflected light, an antireflection film for germanium has been provided using a vacuum deposition method or the like. Conventionally, there has been a type of antireflection film for germanium as shown in FIG. In the figure, 1 is an anti-reflection film for germanium, 6 is germanium (Ge)-7 is a first layer made of silicon (Si; refractive index 3.43) laminated on the germanium 6, and 8 is the first layer 7. Zinc sulfide (
A second layer 9 is made of ZnS (refractive index 2.35), and a third layer 9 is laminated on the second layer 8 and is made of sodium fluoride (NaF2; refractive index 1.30).

In addition, the optical thickness of each layer 7-9 is standard wavelength (lambda).

is 1/4 of the standard wavelength λ. is set to the reciprocal of the arithmetic mean of the reciprocals of the short wavelength limit wavelength λ1 and the long wavelength limit wavelength λ2 of the wavelength range of interest. FIG. 5 shows the wavelength dependence of the residual reflectance of the germanium antireflection film 1 shown in FIG. 4. In order to prevent reflections against the atmospheric window in the wavelength range 3-5 μm, which is important from a practical point of view, the reference wavelength λ. This is the characteristic when 3.75 μm.

A residual reflectance of 0.3% or less is obtained in the wavelength range of 3.6 to 3.9 μm.

The germanium antireflection film 1 configured as described above includes the germanium 6 and the first layer 7. 7-9 questions for each layer and 3rd
The light reflected at each boundary between the layer 9 and the air interferes with each other, and reflection at the surface of the germanium 6 can be prevented.

[Problems to be Solved by the Invention] Since the conventional antireflection film for germanium is configured as described above, it has a zero reflection value in the wavelength range of 3.6 to 3.9 μm.
．． A residual reflectance of 3% or less can be obtained, but in the wavelength range 3 to 5
It reaches 0.6% with respect to μm, and in the infrared region where contrast is small, prevention of reflection is not sufficient, and the residual reflectance must be further reduced.

In addition, as shown in FIG. 6, the refractive index of silicon as the first layer of the germanium antireflection film is 3.43°, the refractive index of sodium fluoride as the third layer is 1.3Q, and the reference wavelength λ
. is 3.75 μm, and when the material of the second layer is changed to change the refractive index, the maximum residual reflectance is when the refractive index of the second layer is 2.75 μm.
The optimum value is reached when the refractive index is 20, and the refractive index of the second layer must be within the range of 2.12 to 2.28 in order to reduce the residual reflectance to 0.3% or less over a wide wavelength range of 3 to 5 μm. . However, since there is no material having a refractive index of 2.12 to 2.28, there are problems in that the residual reflectance cannot be reduced to 0.3% or less. (However, in FIG. 5 above, the horizontal axis is the refractive index of the second layer, and the vertical axis is the maximum residual reflectance in the wavelength range of 3 to 5 μm.) This invention solves the above problems. The purpose of this invention is to obtain an anti-reflection film for germanium that can reduce the residual reflectance of the anti-reflection film for germanium and prevent reflection on the surface of germanium over a wide range.

[Means for Solving the Problems] The antireflection film for germanium according to the present invention is laminated on the surface of germanium and has an optical thickness of a reference wavelength λ. 1/ of
4, a second layer laminated on this first layer and having a refractive index of Na and a thickness of da, and a second layer laminated on this second layer and having a refractive index of Nb and a thickness of db. a fourth layer laminated on this third layer and having a refractive index of Na and a thickness of da; and a fourth layer laminated on this fourth layer and having an optical thickness of a reference wavelength λ. The fifth layer consists of sodium fluoride, which is 1/4 of the
The refractive index Na, Nb and the optical thickness da, db of the second to fourth layers have the following relationship.

cog-'[cos2δa--4(R+H4) 5l
n25asin5bko=jinδa=”Nada
5b = ”Nbdbλ0
λ.

[Function] The antireflection film for germanium in the present invention is obtained by appropriately selecting the refractive index and thickness of the second layer, third layer, and fourth layer,
The refractive index of the second to fourth layers is 2.12 to 2.28.
．． Optical thickness reference wavelength λ. The residual reflectance in germanium is reduced by forming a single-layer film that is 1/4 of that of germanium.

[Example] Hereinafter, an antireflection film 1 for germanium, which is an example of the present invention, will be described with reference to FIGS. 1 and 2. Components that are the same as those in FIG. 4 are designated by the same reference numerals, and the description thereof will be omitted.

In the figure, 2 is lead fluoride (Pb) laminated on the first layer 7.
F2: a second layer consisting of a refractive index of 1.75);
Zinc sulfide (ZnS; refractive index 2.35) laminated in layer 2
The third layer consisting of
A fourth layer 5 made of the same material and thickness as 2 is laminated on the fourth layer 4, has an optical thickness of M, and has a quasi-wavelength λ. 1/4 of
The fifth layer is made of sodium fluoride. - In general, the characteristics of a symmetrical three-layer film in which the outer layer has a refractive index of Na and a thickness of da, and the inner layer has a refractive index of Nb and a thickness of db, that is, the above-mentioned 2N2 to 4th ．． The refractive index and thickness of the shaft 4 can be calculated using the following formula.

= (, J+ i-”)
(1) 1m21m! 2 Boat = “-” Nbda (2) & = Go Nada λ.

Sacm2z=em2Sacm25b-+ (swij'r-
B) 5in2δas+na(3) mix ”s+n
2δ愼δb+l←%Sa −1Vas1nδa) si
r+5bNa Na Na Nb
(4) m,,=Nas+n2Saco
s&+Na (8txsa coffin sin δa) 5in5
b (5) On the other hand, the characteristic matrix of a single layer film with a refractive index of N and a thickness of D is Δ=←ND
It is expressed as (7)/bQ.

The following relationships hold from equations (1), (6), and (7).

N=iNs+nA/”s==νThree tiles 1-(8
)cz? ”ND=rJ+=mzz
(9) O By substituting equations (3), (4), and (5) into equations (8) and (9), the following equations are obtained.

ND=? l-'';, cas-' [cg 2δa
cm&-+ (female seconds s+n2δasinδb]
(11) In other words, the symmetrical three-layer film can be equivalently regarded as a single-layer film having the refractive index and optical thickness expressed by equations (10) and (11).

That is, the germanium antireflection film 1 is as follows.

Paying attention to the above relationship, the optical thickness is the reference wavelength λ. The first layer 7 is made of silicon and has an optical thickness of 1/4 of the nominal wavelength λ. ], /・1 The second to fourth layers sandwiched between the fifth layer 5 made of l-lium fluoride are symmetrical 3
A single-layer film with a layered structure was used, and its refractive index and optical thickness were set to have the following relationship.

As described above, when calculated from equations (121 and 13), the thickness da of the second layer 2t9 and the first layer 4 is 0.08 of the standard wavelength λ0, and the thickness db of the third layer M3 is 0.08 of the standard wavelength λ0. 0 of wavelength λ.
．． It becomes 0B.

The germanium antireflection film 1 constructed as described above has a symmetrical three-layer structure in which the second layer 2 to the fourth layer M4 have an isometric refractive index of 2.20 and an optical thickness as standard. Wavelength λ. It can be regarded as a single-layer film with a thickness of 1/4, and is composed of a first N7 made of silicon, a second layer 5 with a refractive index of 2.20, and a fifth layer 5 made of sodium fluoride. It can be regarded as the same as an anti-reflection film consisting of three layers with each layer having the same optical thickness. That is, as is clear from FIG. 5, germanium 6 and the first layer 7. The light reflected at each boundary between N2 to 5.7 and the fifth layer 5 and air interferes, and reflection on the surface of germanium 6 can be prevented over a wide band.

Reference wavelength λ. is 3.42 μm, and the residual reflectance of the germanium antireflection film 1 shown in FIG. 1 is shown in FIG. The residual reflectance is 0.3% or less over a wide wavelength range of 2.9 to 5.65 μm, and is 0.1% or less in the wavelength range of 3 to 5 μm.

In this example, the second layer 2 and the fourth layer 4 are made of Fuji and others with a refractive index of 1.75 and a thickness of 0.08λ0, and the third layer 3 is made of a material with a refractive index of 2.35 and a thickness of 0. 06λ0, the refractive index Na and thickness da of the second layer 2 and fourth layer 4, and the refractive index Nb and thickness d of the third layer 3.
A similar effect can be obtained by appropriately selecting b from the above equations (12) and (13).

FIG. 3 shows other combinations such as the second layer 2, the fourth layer 4, and the third layer 3. Furthermore, the combination shown in FIG. 3 not only has a symmetrical three-layer structure as in the above embodiment, but also has the reciprocal of the arithmetic mean of the reciprocals of the limit wavelength λ1 on the short wavelength side of the wavelength range and the limit wavelength λ2 on the long wavelength side. From the setting method, the standard wavelength λ. The method is to select appropriately for each combination shown in FIG. 3. In addition, although the antireflection film 1 for germanium in the wavelength range of 3 to S μm has been described, each layer has a wavelength range of 2 to 5 μm.
By multiplying the thickness of ,7 by about 2.6, the wavelength range is 8 to 13μ.
It goes without saying that it has a similar effect on the atmospheric window of m, and it also has a similar effect in other wavelength ranges, even if other fluorides that are transparent in that wavelength range are used. I can do it.

In addition, in this example, the second layer 2 to the fourth layer 4 have a symmetrical three-layer structure, and are equivalently a single layer film with a refractive index of 2.20, but as shown in FIG. 2.12-2.28
A similar effect can be obtained by constructing a typical single-layer film having a refractive index in the range of 0.3% or less, that is, a maximum residual reflectance of 0.3% or less.

[Effects of the Invention] As described above, according to the present invention, the germanium antireflection film has an optical thickness equal to the reference wavelength λ. a first layer made of silicon, which has a refractive index of 1/4, a second and fourth layer having a refractive index of Na and a thickness of da, and a third layer having a refractive index of Nb and a thickness of db; Optical thickness is reference wavelength λ. The second layer, the third layer and the fourth layer have a refractive index Na, Nb and a thickness da,
Select the db appropriately.

The refractive index of the second to fourth layers is equivalently 2.12 to 2.28.
゜Optical thickness reference wavelength λ. Since the single-layer film is 1/4 of the above, the residual reflectance of the anti-reflection film for germanium can be reduced, and reflection on the surface of germanium can be prevented over a wide range.

[Brief explanation of the drawing]

1 and 2 are diagrams showing the configuration of an antireflection film for germanium, which is an embodiment of the present invention, and a characteristic diagram of the wavelength dependence of the residual reflectance, and FIG. A diagram showing the combination of four layers. Figures 4 and 5 are a diagram of the configuration of a conventional antireflection film for germanium and a characteristic diagram of the wavelength dependence of residual reflectance. Figure 6 is a diagram showing the combination of three layers with equal optical thickness. FIG. 3 is a characteristic diagram showing the relationship between the refractive index of the second layer and the residual reflectance when the first layer is made of silicon and the third layer is made of sodium fluoride in the antireflection film of FIG. 1... Anti-reflection film for germanium, 2...
2nd layer, 3...3rd layer, 4...4th layer, 6...germanium. Agent Masuo Oiwa (and 2 others) Figure 1 Figure 2 Wavelength (μm) Figure 3 Figure 4 Figure 5 Wavelength (μm) Figure 6 Refractive index procedure correction Δ (spontaneous) 1. Incident display patent application No. 63-096108 2,
Name of the invention: Anti-reflection coating for germanium 3, Representative of the person making the amendment: Moriya Shiki 5 Scope of patent claims to be amended, column for detailed explanation of the invention 60 Contents of the amendment (1) The scope of claims is attached on a separate sheet. Correct accordingly. (2) Mei I, page 3, line 2, rNaFzJ
Correct with NaFJ. (3) In the same book, page 4, line 12, the phrase ``becomes the optimum value'' is corrected to ``becomes the minimum value.'' (4) Same book, page 5, No. 1. s'71st to 19th line,
On page 9, line 10, "and optical thickness" is corrected to "and thickness." (5) Same book, page 6, line 1, page 8, line 12, page 9, line 11 NaNb5ln2δacos&+(Na2cos22δ
a−Nb2sln2δa) Sinδb” is replaced with NaNb5ln2δacosEh+ (Na2cos2
Correct as δa-Nb"s+n"&)sln&J. (6) The same book, page 6, lines 7 to 8, page 12 is 2
．． 12-2.28. The optical thickness is corrected as the reference wavelength. (7) In the same book, page 6, lines 9 and 10, the phrase ``on germanium'' is corrected to ``on the surface of germanium.'' (8) In the same book, page 7, lines 8 to 9, the statement "Matrix, . . . thickness is based on the following formula" is corrected to "Matrix is based on the following formula". (9) Same book, page 7, lines 14 to 16' m l
l = m z 2 :・・・・・・・・・・・・・・・
・(5)" is replaced by 'm++"m2*=cos2δ
a! δb l (”+’:) 5ln2δaslnδb
(3) mB-Nasln2δbodyδb4 scoldingcas”δa
4sIn”δa)sinδb(4)”. (10) In the same book, page 9, line 9, the phrase ``A single layer film with a symmetrical 3N structure,'' has been corrected to ``A symmetrical 3M structure.'' (11) Ibid., page 9, line 14, "The thickness da of the fourth layer 4 is" is corrected to "The optical thickness da of the fourth layer 4 is J." (12) Ibid., page 9. In the 15th line, "the thickness db of the third layer 3 is" is corrected to "the optical thickness db of the third layer 3 is". (13) In the same book, page 10, line 19, "Refractive index 1.75, thickness 0.08" is corrected to "Refractive index 1.75, optical thickness 0.08λ." (14) Ibid., page 10, line 20 [Refractive index 2.35, thickness 0.06λ. " is corrected to "Refractive index 2.35 and optical thickness 0.06λ." (15) In the same book, page 11, lines 11 to 12, the phrase ``It is what I did.'' is amended to ``It is what I did.'' The above two claims are laminated on the surface of germanium and have an optical thickness of reference wavelength λ.
. a first layer made of silicon, which is 1/4 of the
A second layer is laminated on the second layer and has a refractive index of Na and a thickness of da; a third layer is laminated on this second layer and has a refractive index of Nb and a thickness of db; A fourth layer having a refractive index of Na and a thickness of da, and a fourth layer laminated on this fourth layer and having an optical thickness of a reference wavelength λ. The refractive index of the second to fourth layers is Na,
An antireflection film for germanium, characterized in that Nb and history da and db have the following relationship. ays-'[ct1s25acos&-+ (R+R
) 5ln2δaslnδb]=JinSa=””N
ada Eh = ”' Nbdbλ0
λ0

Claims (1)

[Claims] Laminated on the surface of germanium, the optical thickness is equal to the standard wavelength λ.
A first layer made of silicon which is 1/4 of _0, a second layer laminated on this first layer and having a refractive index of Na and a thickness of da, and a second layer laminated on this second layer and having a refractive index of Nb. A third layer having a thickness of db, a fourth layer laminated on this third layer and having a refractive index of Na and a thickness of da, and a fourth layer laminated on this fourth layer and having an optical thickness of a reference wavelength λ_0. The refractive index N of the second to fourth layers is 1/4 of sodium fluoride.
An antireflection film for germanium, characterized in that a, Nb and optical thickness da, db have the following relationship. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ ▲There are mathematical formulas, chemical formulas, tables, etc.▼ δa = (2π/λ_0) Nada, δb = (2π/λ_
0) Nbdb