JPH05188203A - Antireflection film for caf2 substrate - Google Patents

Abstract

PURPOSE:To provide the antireflection film for a CaF2-substrate which is usable for optical parts for large-output high-repetition excimer lasers, has the sufficient heat resistance to enable the use of the optical parts even in a high-temp. environment, is mild in the film thickness-controllability at the time of production and is small in the number of layers. CONSTITUTION:The antireflection film is formed of the two-layered structure constituted by successively forming an MgO film as a 1st layer and an SiO2 film as a 2nd layer on an optically polished CaF2 substrate and selecting the optical film thicknesses of the time respectively at 0.33 to 0.38lambda and 0.19 to 0.21lambda ranges (where lambda is a central wavelength). The antireflection film is otherwise formed of the three-layered structure constituted by successively forming an SiO2 film as a low-refractive index material for the 1st layer, an MgO film as a high-refractive index material for the 2nd layer and an SiO2 film as a low-refractive index material for the 3rd layer and setting the respective optical film thicknesses at 0.23 to O.27lambda, 0.35 to 0.40lambda and 0.18 to 0.20lambda ranges.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CaF ₂ substrate which is a material for optical components (windows, lenses, beam splitters, etc.) used in ultraviolet optical equipment such as high-power / high-repetition excimer lasers. The present invention relates to an antireflection film.

[0002]

2. Description of the Related Art Dielectric multilayer films are used in various infrared, visible, and ultraviolet laser optical components (total reflection mirrors, beam splitters, lenses, window materials, etc.). Alternatively, it is known as a general technique to stack a dielectric substance on the surface of an optical substrate by a vacuum deposition method or the like to obtain a reflection characteristic and form an antireflection film, a partial reflection film or a high reflection film. (For example, Kubota et al. “Optical Technology Handbook”), conventional antireflection films used for visible camera lenses and the like have absorption in the ultraviolet region in most cases, and these dielectric multilayers for antireflection films are used. The film material cannot be directly applied to the ultraviolet region such as excimer laser. It is necessary to select and use a dielectric multilayer film material having good optical characteristics in the wavelength range to be used.

[0003]

However, many oxides used as high and low refractive index substances undergo thermal decomposition by conventional vapor deposition methods and conditions such as resistance heating and electron beam vapor deposition, and are lower oxides. , Which results in an increase in absorption, and further changes in optical and physical constants such as refractive index and density. Magnesium fluoride (hereinafter referred to as MgF ₂ ) which is optically transparent and easy to manufacture in a wide range from the ultraviolet region to the infrared region is often used as a low refractive index material for an antireflection film.
Due to the hygroscopicity of the MgF ₂ film, the absorbed heat increases with the irradiation of the laser beam, so that the MgF ₂ film at the irradiated portion is deteriorated, and the transmittance is greatly reduced.

As described above, the optical parts for laser produced by the conventional method of forming a multilayer film and the conventional multilayer film material have low light resistance, and the usable power level is about 100 W at most. Further, there is a problem that the heat generated by the absorbed light changes the refractive index and the density of the film and the spectrum largely changes even before the destruction.

Furthermore, the output of a typical excimer laser that generates output light in the ultraviolet region is 2 KW and the number of repetitions is 1 KH.
When it comes to z, the excimer laser light, which should be a pulse laser for the optical components used, is also subject to the same thermal damage as continuous light. In addition to this, the irradiation of the laser beam, which is higher than that of the conventional one by one digit or more, causes a problem that the heat generated by even a slight absorption is generated in the optical component.

In particular, the temperature of the beam-irradiated portion increases sharply and local heating occurs. As a result, in the first part of the optical component, the multilayer film part, which is the most vulnerable to heat, is destroyed, then the substrate is thermally deformed, and finally the optical component itself is destroyed. It also had challenges. Specifically, the multilayer film produced by the conventional method had a problem that the spectrum was significantly changed by heating at 300 ° C.

The present invention was devised in order to solve the problems of the prior art as described above, and can be used for an optical component for a high output / high repetition excimer laser, and the optical component is exposed to a high temperature. It is an object of the present invention to provide an antireflection film for a CaF ₂ substrate which has sufficient heat resistance so that it can be used even under such an environment.

[0008]

In order to achieve this object, the present invention provides an optically polished calcium fluoride (Ca).
On the F ₂ ) substrate, silicon dioxide (SiO ₂ ) is used as a low refractive index substance.
₂ ) Film, magnesium oxide (Mg
O) The respective films are sequentially formed so as to have a two-layer or three-layer structure.

[0009]

According to the present invention, an oxygen gas is injected into an inert gas for sputtering only when a silicon dioxide (SiO ₂ ) film, which is a low-refractive substance that easily migrates to a lower oxide, is formed by the above-mentioned formation method. In order to achieve a low absorptivity by suppressing the formation of a low-grade oxide, a non-hygroscopic oxide (high: MgO, low: SiO ₂ ) is used as a high / low refractive index substance, and By stabilizing the physical constants, a CaF ₂ substrate that can be used as an optical component for a high-power, high-repetition excimer laser and has sufficient heat resistance that can be used even in an environment where the optical component is exposed to high temperatures The antireflection film for use can be obtained.

[0010]

【Example】

(Embodiment 1) A first embodiment of the antireflection film for a CaF ₂ substrate of the present invention will be described with reference to the drawings. FIG. 1 is a sectional view showing a case where a three-layer antireflection film is formed on only one surface of a CaF ₂ substrate in one embodiment of the present invention.

As shown in FIG. 1, optically polished CaF ₂
On one surface of the substrate 1, a silicon dioxide (SiO ₂ ) film 2 as a first low-refractive index material which is in contact with the substrate 1 is provided, and an optical film thickness (nd) is 0.25λ (where λ is KrF of 248 nm). Then 6
2 nm), and a magnesium oxide (MgO) film 3 as a second layer having a high refractive index material is formed thereon.
With an optical film thickness (nd) of 0.375λ (where λ is Kr of 248 nm).
In the case of F, it is formed to have a thickness of 93 nm, and silicon dioxide (S
The iO ₂₎ film 4, an optical film thickness (nd) is 0.19λ (λ 248
In the case of KrF of nm, it is formed to be 47 nm).

FIG. 3 shows the case where λ is 248 nm (for KrF) only for the silicon dioxide (SiO ₂ ) film 2 as the low refractive index material of the first layer of the antireflection film shown in FIG. The calculation result of the spectral reflectance characteristic when only the optical film thickness (nd) of 62 nm is changed by ± 7% is shown.

FIG. 4 is a graph showing magnesium oxide (Mg) as a high refractive index material for the second layer of the antireflection film shown in FIG.
O) Only the film 3 has an optical film thickness (nd) of 93 nm when λ is 248 nm (for KrF) ± 7
The calculation result of the spectral reflectance characteristic when the percentage is changed is shown.

FIG. 5 shows the case where λ is 248 nm (for KrF) only for the silicon dioxide (SiO ₂ ) film 4 as the low refractive index material of the third layer of the antireflection film shown in FIG. The calculation result of the spectral reflectance characteristic when the optical film thickness (nd) 47 nm is changed by ± 7% is shown.

FIG. 6 shows the case where the optical thickness (nd) of each layer of the antireflection film shown in FIG. 1 is changed by ± 7% when λ is 248 nm (for KrF). The calculation result of the spectral reflectance characteristic of is shown.

Using the magnetron sputtering method,
Only when forming the SiO ₂ film which is a low refractive index material, oxygen gas is injected into argon gas at a rate of 10%, and as the high refractive index material, a non-hygroscopic magnesium oxide (MgO) film is used. As can be seen from the characteristic curves of FIGS. 3 to 6, even when the value is changed by ± 7%, an antireflection film having an antireflection property with a reflectance of 0.5% or less can be obtained, and the absorption is small and the heat resistance is high. Can be expected.

(Embodiment 2) A second embodiment of the antireflection film for a CaF ₂ substrate of the present invention will be described with reference to the drawings.

FIG. 2 is a view showing a cross section of a double-layer antireflection film 1 for a CaF ₂ substrate when the film is formed on only one side of the CaF ₂ substrate. The optical film thickness (nd) of the magnesium oxide (MgO) film 5 as the high refractive index material of the first layer is 0.355λ (where λ is
In the case of KrF of 248 nm, it is formed to have a thickness of 88 nm), and a silicon dioxide (SiO ₂ ) film 6 as a _second low-refractive index material is formed thereon to have an optical film thickness (nd) of 0.20λ.
(50 nm when λ is KrF of 248 nm).

FIG. 7 shows magnesium oxide (M) as the high refractive index material of the first layer of the two-layer antireflection film shown in FIG.
gO) Only for film 5 λ is 248 nm (for KrF)
If the optical thickness (nd) is 88 nm,
The calculation result of the spectral reflectance characteristic when changing by 7% is shown.

FIG. 8 shows a case where λ is 248 nm (for KrF) only for the silicon dioxide (SiO ₂ ) film 6 as the low refractive index material of the second layer of the antireflection film shown in FIG. The calculation result of the spectral reflectance characteristic when only the optical film thickness (nd) of 50 nm is changed by ± 7% is shown.

FIG. 9 shows the optical film thickness (nd) of ± 7 for all layers 5 and 6 of the two-layer antireflection film shown in FIG. 2 when λ is 248 nm (for KrF).
The calculation result of the spectral reflectance characteristic when the percentage is changed is shown.

As is clear from the characteristic curves shown in FIGS. 7 to 9, even if the optical film thickness is changed by ± 7%, the same antireflection characteristics as those described in Example 1 and less absorption and high heat resistance are obtained. Needless to say, it has sex.

[0023]

As it is apparent from the based on the above embodiments description, according to the present invention, the present invention is, by only the inert injects oxygen gas to the scan in the case of forming the SiO ₂ film, a lower oxide of SiO ₂ film Controls the optical absorption within ± 7% by using MgO film, which is a high melting point oxide with no hygroscopicity, as a high-refractive index material, while suppressing the migration to substances The reflectance of 0.5% or less with respect to the desired center wavelength can be obtained by simply doing it, and the number of layers is small, and the optical and physical constants such as the refractive index, density, and stress of the film can be stabilized. An antireflection film for a CaF ₂ substrate that has improved light resistance and can be used for high-power, high-repetition excimer laser optical components, and has sufficient heat resistance to be used even in an environment where the optical components are exposed to high temperatures. As it can be obtained, its effect is great.

[Brief description of drawings]

FIG. 1 is a three-layer structure of CaF _{2 according to a} first embodiment of the present invention.
Sectional view showing an antireflection film for a substrate,

FIG. 2 is a two-layer structure of CaF _{2 according to a} second embodiment of the present invention.
Sectional view showing an antireflection film for a substrate,

FIG. 3 is a first layer of SiO _{2 according to} the first embodiment of the present invention.
Showing the calculation result of the reflectance wavelength-dependent characteristics when the optical film thickness of is changed by 7%,

FIG. 4 is a second layer of MgO according to the first embodiment of the present invention.
Showing the calculation result of the reflectance wavelength-dependent characteristics when the optical film thickness of is changed by 7%,

FIG. 5 is a third layer of SiO _{2 according to} the first embodiment of the present invention.
FIG. 6 is a diagram showing a calculation result of reflectance wavelength dependence characteristics when the optical film thickness of is changed by 7%.

FIG. 6 is a diagram showing calculation results of reflectance wavelength dependence characteristics when the optical film thickness of all three layers in the first embodiment of the present invention is increased or decreased by 7% from the set value at the same time;

FIG. 7: MgO of the first layer in the second embodiment of the present invention
Showing the calculation result of the reflectance wavelength-dependent characteristics when the optical film thickness of is changed by 7%,

FIG. 8 is a second layer of SiO ₂ in the second embodiment of the present invention.
Showing the calculation result of the reflectance wavelength-dependent characteristics when the optical film thickness of is changed by 7%,

FIG. 9 is a diagram showing calculation results of reflectance wavelength dependence characteristics when the optical film thickness of the two layers in the second embodiment of the present invention is increased or decreased by 7% from the set value at the same time.

[Explanation of symbols]

1 CaF ₂ substrate 3, 5 MgO film 2, 4, 6 SiO ₂ film

Claims (6)

[Claims] 1. Optically polished calcium fluoride (Ca)
F ₂ ) On one or both sides of the substrate, the first contact with the substrate
Oxide layer (MgOx)
O) film and the second layer include silicon dioxide (S
A two-layer structure C characterized by sequentially forming an iO ₂ ) film
Anti-reflection film for aF ₂ substrate. 2. The first layer of magnesium oxide (Mg
O) film and the second layer of silicon dioxide (SiO ₂ ) film have optical thicknesses (nd) of 0.33λ to 0.38λ and 0.19λ to 0.21 respectively.
The antireflection film for a CaF ₂ substrate according to claim 1, wherein λ (where λ is a design center wavelength) is selected. 3. Optically polished calcium fluoride (Ca)
F ₂ ) On one or both sides of the substrate, the first contact with the substrate
Layer of silicon dioxide (SiO ₂ ) as a low refractive index material, and the second layer of high refractive index material of magnesium oxide (Mg)
O) film and the third layer include silicon dioxide (S
C of a three-layer structure characterized by sequentially forming an iO ₂ ) film
Anti-reflection film for aF2 substrate. 4. An optical film thickness (nd) of a silicon dioxide (SiO ₂ ) film of a first layer, a magnesium oxide (MgO) film of a second layer, and a silicon dioxide (SiO ₂ ) film of a third layer. Values of 0.23λ to 0.27λ, 0.35λ to 0.40λ and 0.
The antireflection film for a CaF ₂ substrate according to claim 3, wherein the thickness is selected from 18λ to 0.20λ. 5. The antireflection film for a CaF ₂ substrate according to claim 1, wherein the film forming method is a magnetron sputtering method. 6. The cleaning of a substrate surface before film formation is performed by ion cleaning using an inert gas under a vacuum degree higher than that during sputtering during film formation. An antireflection film for a CaF ₂ substrate as described above.