JP2002037697A - Method for producing optical material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a production method for producing optical materials which suppresses a decrease in transmittance in ultraviolet and visible regions due to optical damage of a fluoride crystal and hence improves durability for laser radiation. SOLUTION: This method for producing optical materials comprises heat- treating the fluoride crystal, is characterized in that a direct current is applied to the fluoride crystal during heat treatment, in addition, a fluorine pressed powder body is disposed at a cathode side of the fluoride crystal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an optical material, and more particularly, to a method for producing an optical system such as an excimer laser, which is used as an optical element such as a lens or a prism, and having a transmittance in an ultraviolet or visible region due to optical damage. The present invention relates to a method for producing an optical material in which a decrease in the optical material is reduced, and as a result, the durability against laser irradiation is improved.

[0002]

2. Description of the Related Art In order to improve the resolution of a semiconductor exposure apparatus, the wavelength of a laser beam as a light source has been shortened and the diameter of a projection lens has been increased. Ie, calcium fluoride (Ca
F ₂ ) single crystal or the like is used.

Conventionally, this fluorite single crystal has been produced by the Bridgman method. Fluorite single crystals used in the deep ultraviolet or vacuum ultraviolet region do not use natural fluorite as a raw material, but generally use a high-purity raw material produced by a chemical synthesis method. Since the volume of the raw material greatly decreases when it is melted, a semi-molten product or a pulverized product is generally used.

According to this method, a crucible filled with the above-mentioned raw material is placed in a growing apparatus, and the inside of the growing apparatus is 10 ^-3 to 10 ^-4 P
The temperature in the growing apparatus is raised to the melting point of fluorite or higher (1370 to 1450 ° C.) and the raw material is melted while maintaining the vacuum atmosphere of a. After melting, the crucible is pulled down and solidified (crystallized).

[0005] When the crystal is crystallized to the uppermost part of the melt, the crystal growth is terminated, and the grown crystal (ingot) is gradually cooled so as not to be broken. Open to take out the ingot. Since the taken out ingot has a very large residual stress and strain, heat treatment is performed as a post-treatment.

[0006] The fluorite single crystal thus obtained is cut into an appropriate size for each target product.

However, when the fluorite single crystal thus synthesized is used as an optical element such as a lens or a prism, light damage due to ultraviolet light irradiation, specifically, transmission of ultraviolet light and visible light by a color center. The drop in rates was a problem. That is, as the defect density of fluorine increases, a color center seen by laser damage is formed. This has caused a decrease in the durability of laser irradiation, particularly excimer laser irradiation. This problem could not be solved by heat treatment under various conditions.

Accordingly, an object of the present invention is to provide a method for producing an optical material in which a decrease in transmittance in an ultraviolet region and a visible region due to optical damage of a fluoride crystal is reduced, and as a result, durability against laser irradiation is improved. To provide.

[0009]

Means for Solving the Problems As a result of the examination, the present inventors have found that, during heat treatment of a fluoride crystal, a current is supplied and a fluorine-based green compact is disposed on the cathode side of the fluoride crystal. It has been found that the above object is achieved.

The present invention has been made based on the above findings, and is a method for producing an optical material for heat-treating a fluoride crystal, wherein a direct current is applied to the fluoride crystal during the heat treatment, and the fluoride crystal is heat-treated. And a method for producing an optical material, characterized in that a fluorine-based green compact is disposed on the cathode side.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the method for producing an optical material according to the present invention will be described below. According to the present invention, when heat treating the fluoride crystal, a direct current is applied to the fluoride crystal.

The fluoride crystal includes a fluoride-containing single crystal and a fluoride-containing polycrystal, and specifically, a calcium fluoride (CaF ₂ ) single crystal, a magnesium fluoride single crystal (MgF ₂ ) and the like. Especially calcium fluoride (C
aF ₂ ) single crystals are preferred.

The heat treatment (annealing) is performed at 600 to 1300 ° C. in a vacuum or an inert gas atmosphere, for example, an argon gas atmosphere in an atmosphere control electric furnace.
The energization is performed, for example, by applying a maximum DC voltage of 18 V between the electrodes.

In the present invention, when a direct current is applied to the fluoride crystal, a fluorine-based green compact is disposed on the cathode side of the fluoride crystal. This fluorine-based green compact is a source of fluoride ions to the fluoride crystal, and the fluorine ions in the fluorine-based green compact are transferred to the fluoride crystal by energization,
Spread. That is, utilizing the ion conductivity of fluorine, fluorine ions of the fluoride crystal are supplied to compensate for the lattice defect of fluorine of the fluoride crystal. Further, by this energization, oxygen as an impurity in the fluoride crystal moves to the anode side due to the potential gradient and is removed from the fluoride crystal, so that defects due to oxygen are also eliminated.

As such a fluorine-based green compact, a green compact such as calcium fluoride, lead fluoride or a mixture thereof which can supply fluorine ions to the fluoride crystal is preferably used. However, it is not preferable that the cation becomes an impurity of the fluoride crystal.

When direct current is applied to the fluoride crystal, it is desirable to arrange a fluorine-based green compact not only on the cathode side but also on the anode side of the fluoride crystal. By arranging the fluorine-based green compact on the anode side in this manner, a direct current can be sufficiently and uniformly passed between the electrode, for example, a platinum or molybdenum electrode and the fluoride crystal.

The present invention controls fluorine ion defects in a fluoride crystal, and supplies fluorine ions into a crystal lattice by utilizing the ionic conductivity of fluorine in the crystal. Further, by forming a potential gradient in the crystal, fluorine ions and fluorine ion vacancies are diffused.

[0018]

EXAMPLES Hereinafter, the present invention will be specifically described based on examples and the like.

REFERENCE EXAMPLE In an electric furnace, when heat treatment is performed at a maximum of 860 ° C. in an argon gas atmosphere, platinum electrodes 2 are formed on both end surfaces of a calcium fluoride single crystal 1 by sputtering, as shown in FIG. The membrane was applied and a DC voltage of maximum 18 V was applied between the electrodes.

After optically polishing both end faces of the calcium fluoride single crystal, the transmittance of the colored portion was measured with a spectrophotometer.

FIG. 2 shows the transmittance. From the graph of the transmittance, it can be seen whether or not the color center is formed.

Example 1 In an electric furnace, when a heat treatment was performed at a maximum of 734 ° C. in an argon gas atmosphere, as shown in FIG. 3), and a platinum electrode 2
And a maximum DC voltage of 18 V was applied between the electrodes.

Both the unheated calcium fluoride single crystal and the heat-treated calcium fluoride single crystal not subjected to the above-mentioned heat treatment were optically polished at both end surfaces, and then irradiated with an excimer laser as in the reference example. FIG. 4 shows the transmittance of the unheated calcium fluoride single crystal, and FIG. 5 shows the absorption coefficient. FIG. 6 shows the transmittance of the heat-treated calcium fluoride single crystal, and FIG. 7 shows the absorption coefficient.

The transmission spectrum was compared between the transmission spectrum immediately before laser irradiation (initial transmission spectrum) and the transmission spectrum after laser irradiation. The part where the transmittance has decreased from the initial value is the induction of the color center by laser irradiation. Note that the peaks near 660 nm and 340 nm in FIGS. 4 and 6 are derived from a spectrophotometer.

The absorption coefficient was determined as follows. That is,
To make it easier to see the spectral structure of the induced color center, the absorption spectrum was calculated as a change from the initial transmission spectrum. The absorption coefficient was calculated according to Lambert's law. I = I ₀ exp (αt) α = −ln (I / I ₀ )
/ T I: transmittance after laser irradiation, I ₀ : transmittance before laser irradiation, t: sample thickness (cm), α: absorption coefficient (c
m ^-1 )

Example 2 In an electric furnace, when a heat treatment was performed at a maximum of 800 ° C. in an argon gas atmosphere, as shown in FIG. A green compact 3 made of a mixture of calcium fluoride and lead fluoride is arranged and connected to the platinum electrode 2, and a fluorine-based green compact (lead fluoride green compact) 4 is It was connected to the electrode 2 and a maximum DC voltage of 18 V was applied between the electrodes.

Both end faces of the unheated calcium fluoride single crystal and the heat-treated calcium fluoride single crystal not subjected to the heat treatment were optically polished, and then irradiated with an excimer laser as in the Reference Example. FIG. 9 shows the transmittance of the unheated calcium fluoride single crystal, and FIG. 10 shows the absorption coefficient. FIG. 11 shows the transmittance of the heat-treated calcium fluoride single crystal, and FIG. 12 shows the absorption coefficient. The method for measuring the transmittance and the absorptance is the same as in Example 1.
Note that the peaks near 660 nm in FIG. 9 and the peaks near 660 nm and 340 nm in FIG. 6 are derived from a spectrophotometer.

[Example 3] Heat treatment and energization were performed in the same manner as in Example 2 except that calcium fluoride compact was used instead of lead fluoride compact as the fluorine-based compact 4 on the anode side. Was done.

Both end faces of the unheated calcium fluoride single crystal and the heat-treated calcium fluoride single crystal not subjected to the heat treatment were optically polished, and then irradiated with an excimer laser in the same manner as in the reference example. FIG. 13 shows the transmittance of the unheated calcium fluoride single crystal, and FIG. 14 shows the absorption coefficient. FIG. 15 shows the transmittance of the heat-treated calcium fluoride single crystal, and FIG. 16 shows the absorption coefficient. The method for measuring the transmittance and the absorptance is the same as in Example 1. The peak near 660 nm in FIGS. 13 and 15 is derived from a spectrophotometer.

[0030]

According to the method for producing an optical material of the present invention,
A decrease in transmittance in the ultraviolet region and the visible region due to optical damage of the fluoride crystal is reduced, and the performance as an optical element is improved. As a result, durability against laser irradiation, particularly excimer laser irradiation, is improved.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory diagram of a reference example.

FIG. 2 is a graph showing a relationship between transmittance and wavelength of a calcium fluoride single crystal in a reference example.

FIG. 3 is a schematic explanatory diagram of a first embodiment;

FIG. 4 is a graph showing the relationship between the transmittance and the wavelength of the unheated calcium fluoride single crystal in Example 1.

FIG. 5 is a graph showing the relationship between the absorption coefficient and the wavelength of the unheated calcium fluoride single crystal in Example 1.

FIG. 6 is a graph showing the relationship between the transmittance and the wavelength of the heat-treated calcium fluoride single crystal in Example 1.

FIG. 7 is a graph showing the relationship between the absorption coefficient and the wavelength of the heat-treated calcium fluoride single crystal in Example 1.

FIG. 8 is a schematic explanatory diagram of a second embodiment.

FIG. 9 is a graph showing the relationship between the transmittance and the wavelength of the unheated calcium fluoride single crystal in Example 2.

FIG. 10 is a graph showing the relationship between the absorption coefficient and the wavelength of the unheated calcium fluoride single crystal in Example 2.

FIG. 11 is a graph showing the relationship between the transmittance and the wavelength of the heat-treated calcium fluoride single crystal in Example 2.

FIG. 12 is a graph showing the relationship between the absorption coefficient and the wavelength of the heat-treated calcium fluoride single crystal in Example 3.

FIG. 13 is a graph showing the relationship between the transmittance and the wavelength of an unheated calcium fluoride single crystal in Example 3.

FIG. 14 is a graph showing the relationship between the absorption coefficient and the wavelength of the unheated calcium fluoride single crystal in Example 3.

FIG. 15 is a graph showing the relationship between the transmittance and the wavelength of the heat-treated calcium fluoride single crystal in Example 3.

FIG. 16 is a graph showing the relationship between the absorption coefficient and the wavelength of the heat-treated calcium fluoride single crystal in Example 3.

[Explanation of symbols]

 1: Single crystal of calcium fluoride 2: Platinum electrode 3: Fluorine compact on the cathode side 3: Fluorine compact on the anode side

Claims (4)

[Claims] 1. A method for producing an optical material for heat-treating a fluoride crystal, comprising applying a direct current to the fluoride crystal during the heat treatment, and arranging a fluorine-based green compact on the cathode side of the fluoride crystal. A method for producing an optical material. 2. The method for producing an optical material according to claim 1, wherein a fluorine-based green compact is disposed on the anode side of the fluoride crystal. 3. The method of manufacturing an optical material according to claim 1, wherein the fluorine-based green compact on the cathode side is a green compact of calcium fluoride, lead fluoride, or a mixture thereof. 4. The method according to claim 1, wherein the fluoride crystal is a calcium fluoride single crystal.