JPS6161642B2 - - Google Patents

Description

[Detailed description of the invention]

INDUSTRIAL APPLICATION FIELD The present invention relates to an antireflection coating for potassium chloride, which is a material for optical parts (windows, lenses, beam splitters, etc.) used in infrared optical equipment such as carbon dioxide lasers. Conventional Structure and Problems Conventionally, ZnSe, GaAs, etc. have been used as substrate materials for transparent optical components such as windows, lenses, and beam spiriters for high-power carbon dioxide lasers. ZnSe has the advantage that it is transparent at wavelengths of 10.6 μm and 0.6328 μm, has excellent water resistance, and can obtain large crystals, but has the disadvantages that it is expensive, has large optical distortion caused by high power irradiation, and For safety reasons, this method is difficult to handle, as toxic H ₂ Se gas is used during crystal growth, and toxic selenium-based gas is generated during the crystal surface polishing process. Wavelength for GaAs
It is transparent at 10.6μm and its thermal conductivity is
Although it has the advantage of being about 3 times better than ZnSe and has excellent water resistance, it is not possible to obtain a He-Ne laser beam with a wavelength of 0.6328 μm for visible light superimposition, which is difficult to obtain with a diameter of about 8 cm or more. It has a number of drawbacks: it is not transparent, it is expensive, it has large optical distortion, and it also contains a harmful element called arsenic (As), so safety measures are required during growth and processing. There is. On the other hand, the biggest reason why KCl is not used as a substrate for practical transparent optical components is that it is transparent at wavelengths of 10.6 μm and 0.6328 μm, has low optical distortion, is nontoxic, and is inexpensive. This means that it cannot withstand long-term use in humid environments. in this case,
If the anti-reflection coating added to the surface of the KCl substrate not only has optical properties such as a reflectance close to zero and is transparent to a wavelength of 0.6328 μm, but also water resistance, KCl Since the drawback of KCl being weak against water can be overcome, it is possible to realize KCl windows and lenses that have many of the above-mentioned advantages. However, there is currently no long-life antireflection film that satisfies optical properties, water resistance, and abrasion resistance at the same time. The simplest anti-reflection film for KCl windows and lenses is known to be a single-layer NaF film. The refractive index of NaF at a wavelength of 10.6 μm is 1.23.
Since it is very close to the square root of the refractive index of KCl (√1.45=1.20 ₄ ), if an optical film thickness of nd=λ/4=2.65 μm is deposited, a reflectance of 0.05% can be expected, which is close to the ideal value. . Actually 1 inch in diameter and 5 mm thick.
An anti-reflection film made of NaF was formed on both sides of the KCl Kenmaro substrate, and a KCl window was prototyped.The reflectance was measured at a wavelength of 10.6μm, and the reflectance was approximately 0.4% and the absorption rate was approximately 0.3%, which is sufficient for practical use in terms of optical properties. However, in an environmental test at a temperature of 45°C and a relative humidity of 95%, peeling of the NaF film was clearly observed after 6 hours. Therefore, we have to judge that the water resistance is not satisfactory and is not practical. NaF is a single layer that satisfies the requirements for an anti-reflection film.
Since there are no other methods available, antireflection coatings with a two-layer or three-layer structure are being considered. The conditions that must be met as a material for an anti-reflection film include: being difficult to dissolve in water, being transparent at wavelengths of 10.6 μm and 0.6328 μm, having good adhesion to the substrate, and being difficult to form pinholes when formed into a thin film. A substance exhibiting an amorphous state must be selected. Promising materials include chalcogenide glasses such as arsenic trisulfide (As ₂ S ₃ ) and selenium trisulfide (As ₂ Se ₃ ), and thorium tetrafluoride (ThF ₄ ). Regarding the structure of the two-layer antireflection film, the following can be considered. (a) KClGATS/As ₂ S ₃ (b) KClAs ₂ S ₃ /ThF ₍₄ ) All structures of (a) are made of chalcogenide glass, so they are water resistant and have low heat generation (low absorption rate at a wavelength of 10.6 μm). However, GATS has the disadvantage that it does not transmit light with a wavelength of 0.6328 μm. B) Both exhibit an amorphous state and have the advantage that ThF ₄ has strong mechanical strength, but ThF ₄ is radioactive and difficult to use, and it is difficult to obtain high-purity materials with low absorption. It has the disadvantage of being In order to solve the above drawbacks, an antireflection film with a three-layer structure was investigated. Since ThF ₄ , a low refractive index material, cannot be used, the following three-layer antireflection coating using NaF, KCI, and PbF ₂ , which have low absorption, was investigated. A KClAs ₂ Se ₃ /NaF/As ₂ Se _3B KClAs ₂ S ₃ /KCl/As ₂ S 3C KClAs _{2 S 3 /} PbF ₂ /As ₂ S _3These common advantages are due to the three-layer structure. Since the absorption rate can be lower than that of single-layer or double-layer antireflection films, it can be used for high power applications. These are NaF, KCl, and
The basic idea is to protect PbF ₂ by sandwiching it with As ₂ S ₃ , which does not easily form pinholes, and to further protect KCl with the three-layer film. The effect is that As ₂ S ₃ and As ₂ S, which are chalcogenide glasses with low light absorption, have good adhesion to the KCl substrate and act as a protective film against humidity, and on top of that, NaF, which has low light absorption, KCl,
By adding PbF ₂ , the disadvantage of water resistance is further protected by As ₂ Se ₃ and As ₂ S ₃ , and at the same time, it satisfies the anti-reflection coating condition of zero reflectance, and at the same time has a wavelength of 0.6328μ.
We have realized a high-power anti-reflection film that is transparent even to m-light. However, when the three examples a), b), and c) above are compared in detail, the structure c) is considered to be the best overall. In other words, from the viewpoint that substances with low absorption coefficients are good because they can be used for high power, KCl (absorption coefficient 1 cm ^-1 or less), PbF ₂ (absorption coefficient 1 to 2 cm ^-1 ), and NaF (absorption coefficient 6 cm ^-1 ) are recommended. The structure shown in (b) using KCl is the best. However, from the viewpoint of moisture resistance, substances with low solubility in water are better, such as PbF ₂ (amount dissolved in 100 g of water: 0.064 g), NaF
(Amount dissolved in 100g of water: 4.22g), KCl (Amount dissolved in 100g of water: 34.7g), and the structure C) using PbF ₂ is the best, and the structure B) using KCl is the best. is the worst. In addition, from the viewpoint of transparency of He-Ne light, As ₂ S ₃ has better transparency than As ₂ S ₃ , so structure a) is the worst. Table 1 summarizes the types of conventional antireflection coatings for KCl and their characteristics.

【table】

[Table] As can be seen from this table, the three-layer structure of As ₂ S ₃ /PbF ₂ /As ₂ S ₃ is the most well-balanced, and is actually used in 20KW KCl windows and has a high light resistance. In addition to having a proven track record in terms of moisture resistance, it has also been found to be sufficiently practical as long as there is no condensation on the surface. As mentioned above, the three-layer anti-reflection film has many advantages, but it has the disadvantage that the As ₂ S ₃ film on the surface oxidizes and increases absorption when exposed to high temperatures in the air for a long time. Since it is mechanically soft, it has the disadvantage that the surface is easily scratched during cleaning, and there are still problems in terms of extending its life. Purpose of the invention The present invention replaces the conventional As ₂ S ₃ /PbF ₂ /As ₂ S ₃
The three-layer anti-reflection coating for KCl has many advantages, including low absorption and moisture resistance against 10.6μm wavelength carbon dioxide laser light, and transparency against He-Ne laser light. without any damage,
The object is to solve the disadvantages of oxidation and mechanical weakness of the surface and to provide an antireflection film that is comprehensively excellent and has a long life. Structure of the Invention The antireflection film for KCl of the present invention is formed by forming a chalcogenide glass layer represented by As ₂ S ₃ on a KCl substrate, forming _two PbF layers on top of the chalcogenide glass layer, and further forming zinc selenide (Zinc Selenide) as the outermost layer. This is a three-layer anti-reflection coating for KCl with ZnSe). DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a structural diagram of a two-layer antireflection coating according to the present invention on a KCl substrate. 4 in the figure is a KCl substrate with a refractive index of 1.45, which has been optically polished on both sides with ultra-precision. 1 is an As ₂ S ₃ film with a refractive index n ₁ of 2.31, and an optical thickness n ₁ d ₁ =2.650 μm. 2 is PbF ₂ with refractive index n ₂ of 1.67 and optical thickness n ₂ d ₂ = 1.3023μ
It is m. 3 is a ZnSe film having a refractive index n ₃ of 2.4 and an optical thickness n ₃ d ₃ =0.9110 μm. Typical deposition conditions are described below. The vapor deposition crucible for As ₂ S ₃ uses a molybdenum (MO) lid with holes to prevent popping, and the substrate temperature is 70.
°C, working pressure 1.5×10 ^-6 Torr, deposition rate 12Å/see
It was deposited with PbF ₂ was deposited on a platinum board (Pt) at a substrate temperature of 118° C., a working pressure of 3×10 ⁻⁶ 10 Torr, and a deposition rate of 12 Å/sec. ZnSe uses polycrystals made by CVD method as a deposition source,
It was deposited by electron beam evaporation at a substrate temperature of 118° C., a working pressure of 2×10 ⁻⁶ Torr, and a deposition rate of 2 Å/sec. During deposition, the substrate moved in a self-propelled manner to ensure uniform film thickness. The deposition rate is controlled using a crystal oscillator.
The deposition film pressure was controlled by a transmission type optical film thickness control device using infrared light with a wavelength of 1.505 μm. The transmittance spectrum of the KCl window formed by forming anti-reflection films on both sides of the KCl flat substrate in this way is
The figure shows that the reflectance becomes zero at a wavelength of 10.6 μm. The total absorption of the antireflection film is estimated as follows, taking into account the absorption coefficient of each deposited film. In the case of a three-layer anti-reflection film, considering the electric field strength within the anti-reflection film, the total absorption (βd) can be approximated to about one-half of the sum of the absorptions (βidi) of each film.
Here, βi is the absorption coefficient of each film, and di is the thickness of each film. In the case of this example, the absorption β ₁ d ₁ of the first layer As ₂ S ₃ film is 1 cm ⁻¹ ×1.15×10 ⁻⁴ cm≈1.2×10 ⁻⁴ , that is, about 0.012%. Absorption β of second layer PbF ₂ film
₂ d ₂ is 1 cm ⁻¹ ×0.78×10 ⁻⁴ cm≈0.8×10 ⁻⁴ , or 0.008%. The absorption β ₃ d ₃ of the third layer ZnSe film is
0.5cm ^-1 ×0.38× ^10-4 cm=0.19× ^10-4 , or approx.
It is 0.0019%. Therefore, the total absorption of the three-layer film is approximately 0.01
%. When a carbon dioxide laser beam with a high power of 20KW is incident on this anti-reflection film, the output power is 20KW.
0.01% of the incident power is generated as heat. In other words, it acts as a heat generation source of 2W, but this level of heat generation can be adequately dealt with with practical cooling methods and does not cause damage to optical components. Note that this anti-reflection film has a transmittance of about 40% or more for He--Ne laser light, and beam alignment is easy. In addition, PbF ₂ , which is relatively weak against humidity and mechanical strength, is protected by ZnSe, which is chemically stable, including water resistance, and has strong mechanical strength, so it is resistant to scratches during cleaning and has a long life. be. In the ideal case, the film thickness of each layer shown in the above embodiments is such that the reflectance becomes zero at a wavelength of 10.6 μm. In reality, it is difficult to completely reduce the reflectance to zero due to problems in film thickness control ability. On the other hand, from the point of view of actual use, there is no practical problem if the reflectance per surface of the antireflection film is 0.5% or less. Figure 3 shows the reflectance spectra when only the optical thickness n ₁ d ₁ of the first layer As ₂ S ₃ film is increased or decreased by 3.7% from the set value of 2.650 μm. In either case, the reflectance is 0.1 at a wavelength of 10.6 μm. % or less, indicating that it is practical. It is possible to achieve an accuracy of ±3% during actual film thickness control. Figure 4 shows the reflectance spectrum when only the optical thickness n ₂ d ₂ of the second layer PbF ₂ film is increased or decreased by 3.7% from the set value of 1.3023 μm, and in both cases the reflectance is 0.1%.
The following shows how practical it is. Figure 5 shows the reflectance spectra when only the optical thickness n ₃ d ₃ of the third layer ZnSe film is increased or decreased by 3.7% from the set value of 0.9110 μm. Show that something is true. Figure 6 shows the first layer of As ₂ S ₃ film, the second layer of PbF ₂ film,
This is a reflectance spectrum when the optical thickness of each of the third layer ZnSe films is simultaneously increased or decreased by 3.7% of each setting value, showing that the reflectance can be reduced to 0.5% or less in all cases, which is practical. Effects of the Invention As shown in the above embodiments, the present invention uses chalcogenide glass made of As ₂ S ₃ , which has good adhesion and does not easily form pinholes and is insoluble in water, as the first layer in contact with the KCl substrate, with an optical film thickness of n. 2.552μm ~ 2.748μ as ₁ d ₁
PbF _2, which is a low refractive index material with little absorption, is formed as a second layer with an optical thickness of n ₂ d _2.
Formed in the range of 1.2541 μm to 1.3505 μm,
Furthermore, the third layer is ZnSe, which is insoluble in water, chemically stable, and has excellent mechanical strength.
By forming n ₃ d ₃ in the range of 0.8773 μm to 0.9447 μm, an anti-reflection film with a low reflectance and low absorption with a reflectance of 0.5% or less and an absorption rate of 0.01% for carbon dioxide laser light with a wavelength of 10.6 μm is created. can be realized, He
- Good transparency for Ne laser light, excellent moisture resistance, hard ZnSe film on the surface that prevents scratches during cleaning, and excellent environmental resistance such as abrasion resistance and chemical resistance. In order to withstand high power of 20KW and exhibit long life characteristics,
This makes it possible to create high-power windows, lenses, beam splitters, etc. using KCl as a substrate.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing an embodiment of the antireflection coating for potassium chloride according to the present invention, and FIG. 2 is a cross-sectional view showing an example of the antireflection coating for potassium chloride according to the present invention formed on both sides.
The transmittance spectrum characteristic diagrams of the KCl window and FIGS. 3 to 6 are reflectance characteristic diagrams of examples of the antireflection film for potassium chloride according to the present invention, respectively. 1... As ₂ S ₃ film, 2... PbF ₂ film, 3... ZnSe
Film, 4...KCl substrate.

Claims (1)

[Claims] 1. On at least one surface of the potassium chloride substrate,
An antireflection film for potassium chloride, characterized in that an arsenic trisulfide film is formed as a first layer in contact with a substrate, a lead fluoride film is formed as a second layer, and a zinc selenide film is formed as a third layer. 2 The optical thickness values of the first layer arsenic trisulfide film, the second layer lead fluoride film, and the third layer zinc selenide film are 2.552 μm to 2.748 μm and 1.2541 μm to
The antireflection film for potassium chloride according to claim 1, which has a thickness of 1.3505 μm and 0.8773 μm to 0.9447 μm.