JP2000211920A - Calcium fluoride crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a calcium fluoride crystal excellent in durability and to provide a method for testing the calcium fluoride crystal having a high accuracy in sorting out a non-defective. SOLUTION: A calcium fluoride crystal having <=8% decreasing rate of internal transmittance based on 10 mm thickness at 260-280 nm wavelength after irradiation with γ-rays at 1×105 R/h radiation dose is sorted out and used to prepare an optical part which is an optical part for ultraviolet rays and vacuum ultaviolet rays. An exposure device is capable of irradiating a material to be exposed with the ultraviolet rays by using the part in an optical system. The internal transmittance can be increased by carrying out a heat treatment in a vacuum or an inert gas.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to KrF, ArF,
Also, the present invention relates to a calcium fluoride crystal, an optical component, and an exposure apparatus used for an optical component of an optical system using ultraviolet light as a light source, such as an F ₂ excimer laser, and particularly to a method for inspecting a calcium fluoride crystal using gamma ray (γ ray) irradiation. The present invention relates to a method for manufacturing an optical component.

[0002]

2. Description of the Related Art Fluoride crystals such as calcium fluoride,
It has a high transmittance in a wide wavelength range from the vacuum ultraviolet region to the far infrared region, and is widely used for optical components such as lenses, window materials, and prisms. In addition, fluorite, which has excellent transmission characteristics at short wavelengths, is useful as an optical member for excimer lasers, has excellent durability against KrF excimer lasers, Ar excimer lasers, and F ₂ excimer lasers. Also hardly deteriorates its transmission characteristics.

[0003]

However, not all calcium fluoride crystals for ultraviolet optics are excellent in the above-mentioned ultraviolet resistance. In order to obtain a calcium fluoride crystal which does not deteriorate its transmittance even by long-time irradiation, conventionally, much change of the transmittance and the transmittance with the irradiation time has been inspected with much labor and time. That is, conventionally, the same ultraviolet light as the light of the light source actually used is continuously irradiated on the calcium fluoride crystal, and the ultraviolet light resistance is inspected by measuring the transmittance at that time. For example, Japanese Unexamined Patent Publication No. 7-281001 discloses that the transmittance change depends on the energy density of the irradiated light and is independent of the number of shots. It is shown that inspection can be completed in a short time by irradiation.

[0004] However, for example, when the ultraviolet ray is an ArF excimer laser, the energy density is 100 mJ / cm.
^A device that can oscillate ^two or more high-power lasers is very expensive and has a considerable running cost. Also,
Calcium fluoride crystals subjected to high-energy irradiation have a small decrease in transmittance, and even if it can be judged that they can be used practically, the irradiation for this inspection greatly reduces the transmittance, making them unusable. It is very likely that

[0005] In this case, a sample dedicated to the inspection must be prepared as appropriate, which increases the material cost of the crystal.

In particular, in the case of an optical calcium fluoride crystal using vacuum ultraviolet light, such as an ArF excimer laser or an F ₂ excimer laser, a high transmittance and high purity crystal is used. Rate (eg 193n
Even if m) is measured, it is difficult to select good products.

[0007]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a calcium fluoride crystal, an optical component and an exposure apparatus having excellent durability.

Another object of the present invention is to provide a method for inspecting a calcium fluoride crystal and a method for manufacturing an optical component which can achieve high accuracy in selecting good products and achieve low manufacturing cost.

According to the present invention, the reduction of the internal transmittance per 10 mm thickness at a wavelength of 260 to 280 nm after irradiating a gamma ray with a dose of 1 × 10 ⁵ R / hour for 1 hour is 8% or less of that before irradiation. It is characterized by being. Calcium fluoride crystals or optical components.

And an optical system having the optical component;
And a support means for supporting the object to be exposed, and irradiating the object on the support means with ultraviolet light from a light source via the optical system.

Further, the present invention is a method for inspecting a calcium fluoride crystal, which comprises a step of irradiating the calcium fluoride crystal with gamma rays and measuring optical characteristics at 200 nm to 400 nm.

Further, the present invention irradiates a gamma ray at a dose of 1 × 10 ⁵ R / hour for one hour, and emits a wavelength of 260 to 280 n.
This is a method for manufacturing an optical component by selecting a calcium fluoride crystal in which the amount of decrease in internal transmittance per 10 mm thickness in m is 8% or less of that before irradiation and processing the calcium fluoride crystal. .

Then, in vacuum or in an inert gas, 20
A step of performing a heat treatment at 0 ° C. to 1000 ° C. to increase the internal transmittance is included.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors irradiate a calcium fluoride crystal with an ArF excimer laser to observe a change in internal transmittance, and irradiate the calcium fluoride crystal with gamma rays to change the internal transmittance. Was observed.

While repeating such experiments (observations), the change in internal transmittance (dTa) at a wavelength of 193 nm in the case of an ArF excimer laser and the internal transmittance at a wavelength of 200 to 400 nm in the case of gamma rays, particularly 260 to 280 nm, were observed. The change (dTγ) was found to be correlated.

Specifically, as dTγ decreases, d
It was found that Ta also decreased.

DTγ and dTa are defined as in the following equation 1. (Equation 1) dTγ = T0γ−T1γ, dTa = T0a−T1a where T0γ is the internal transmittance of a certain wavelength within 200 to 400 nm before γ-ray irradiation, and T1γ is 200 to 4 after γ-ray irradiation.
The internal transmittance at a certain wavelength within 00 nm, T0a is the internal transmittance at 193 nm before γ-ray irradiation, T1a is the internal transmittance at 193 nm after γ-ray irradiation,

As a result, a gamma ray irradiating step is provided as an inspection method for calcium fluoride crystals.
By measuring the optical characteristics at 00 nm, for example, the internal transmittance, the durability under actual use conditions (for example, ArF excimer laser irradiation conditions) can be measured.

Separately from this, an acceleration test in which gamma rays are irradiated at a dose of 1 × 10 ⁴ R (X-ray) is performed as a durability test, and the internal transmittance at a wavelength used (for example, 193 nm) or a shorter wavelength is measured. There are ways.
However, as the optical properties of calcium fluoride crystals have improved, it has become difficult to distinguish very excellent calcium fluoride crystals from excellent calcium fluoride crystals even with this method. Therefore, the above-described inspection method of the present invention has increased in importance.

[0020] Particularly, in the optical system of the exposure apparatus for photolithography using ArF excimer laser, since the uniformity of the light intensity, durability and high transmittance is desired, a dose 1 × 10 ⁵
Wavelength 260 before and after irradiating R / hour gamma ray for 1 hour
Calcium fluoride having an amount of decrease in internal transmittance (percentage per 10 mm thickness) at 280 nm, that is, dTγ of 8 (%) or less is more preferable.

That is, a large number of calcium fluoride crystals are irradiated with the above-mentioned dose of gamma rays, and after the irradiation, dTγ ≦ 8 (%)
Is selected and ground and polished, and processed into the shape of an optical component such as a convex lens or a concave lens.

Thereafter, if necessary, at least one kind of a reflection-enhancing or anti-reflection film selected from silicon oxide, aluminum oxide, aluminum fluoride, magnesium fluoride and the like is formed.

The optical component thus obtained is used as a window material of an excimer laser device, or a plurality of lenses as optical components are combined to constitute an optical system, and a stage for supporting an object to be exposed such as a wafer coated with a photoresist. And an exposure apparatus for photolithography is completed.

The calcium fluoride irradiated with the above dose of gamma rays has an internal transmittance of 0.1% or less at the wavelength used.
After the inspection, the heat treatment is preferably performed at 200 ° C. to 1000 ° C. in a vacuum or in an inert gas since the reduction is 0.4%.

The heat treatment time may be sufficient for increasing the internal transmittance and depends on the size of the calcium fluoride crystal, and is therefore at least 10 minutes to 20 hours.
There is no upper limit if costs are ignored.

FIG. 1 shows a flowchart of a process for manufacturing an optical component using the inspection method described above.

In step S1, a raw material of calcium fluoride is prepared, and the raw material is melted in step S2 and then gradually cooled to obtain calcium fluoride crystals.

In step S3, gamma rays are irradiated,
In No. 4, an inspection for measuring the internal transmittance at 200 nm to 400 nm is performed.

The calcium fluoride crystal that has passed (selected) the inspection is subjected to a heat treatment in step S5. Then grinding,
It is polished and processed into a desired surface shape.

A coating is formed on the calcium fluoride optical component which has been processed in step S6 in step S7, and a high quality optical component is obtained.

In particular, the reduction amount ΔT of the internal transmittance at a wavelength of 260 to 280 nm due to the above-mentioned gamma ray irradiation.
The present inventor tried a method for producing a calcium fluoride crystal described below to reduce γ to 8% or less. As a result, a crystal having desired characteristics was obtained at a relatively high yield.

The present inventor has noticed that the atmosphere of a chamber (for example, a crucible) for storing a fluoride raw material in the purification step is very important. That is, during the heating step, if the atmosphere in the room containing the fluoride raw material is changed to an atmosphere in which the gas in the room is more easily released to the outside of the room than the atmosphere, the scavenger stays in the crucible during the heating of the raw material, and the reaction proceeds. After the melting, all scavengers and reaction products not used in the reaction go out of the crucible to maximize the effect of the scavenger. Was obtained. Then, it was found that a crystal having excellent optical performance could be obtained by using it.

FIGS. 2 and 3 show a flowchart and a timing chart of a method for purifying fluoride.

In step S11, heating is started to melt a fluoride raw material such as calcium fluoride to which a solid scavenger such as zinc fluoride is added. Thereafter, as shown in step S12, at a predetermined timing, the atmosphere of the room (for example, a crucible) in which the fluoride is stored is changed to an atmosphere in which the gas in the room is relatively easily discharged outside the room.

Heating is continued even after the atmosphere in the room is changed to an atmosphere in which gas is easily released to the outside so that reaction products such as carbon dioxide and vaporized scavengers are not taken into the raw material.

In step S13, cooling is started to solidify the melted raw material. Incidentally, in this step, since it is sufficient that the temperature of the raw material is lower than the melting point, it is not necessary to completely stop the external heating.

The temperature reached in the heating step may be equal to or higher than the melting point of the fluoride material to be purified. In the process of raising the temperature to the melting point, the temperature may be continuously increased as indicated by reference numeral 1 in FIG. Alternatively, the temperature may be increased intermittently as shown by reference numeral 2 in FIG.

The timing of the atmosphere change is determined by the temperature T at which the scavenger starts the impurity removal reaction (scavenge reaction).
1 and before the start of cooling. That is, it is between time t1 and time t2 in FIG.

Preferably, the scavenge reaction start temperature T1
, After a while, start to change atmosphere,
It is desirable to end the atmosphere change at a point sufficiently before the start of cooling. That is, it is preferable to start changing the atmosphere between times t3 and t4 in FIG. Note that t3
Is the time when the impurity removal reaction by the scavenger is completed (t3), and the temperature of the fluoride raw material is lower than the melting point.

T4 is an arbitrary time point after the melting of the fluoride raw material and before the start of cooling, and the temperature of the fluoride raw material is equal to or higher than the melting point. Changing the atmosphere is preferably performed at a temperature equal to or higher than the melting point from the viewpoint of obtaining a fluoride having higher purity. Note that t4
The period from the time t2 to the time t2 is the state of the atmosphere that is easily released to the outside of the gas chamber.

For example, when zinc fluoride is used as a solid scavenger and calcium fluoride is used as a fluoride raw material, the scavenge reaction start temperature is about 872 ° C. at which zinc fluoride melts.

The time required for the scavenging reaction depends on the amount of the raw material to be purified. Therefore, in the actual work, for example, the relationship between the required time and the amount of the raw material may be determined in advance by an experiment or the like, and the time may be determined based on the determined time.

To change the atmosphere means to change from a first atmosphere in which gas is hardly released to the outside to a second atmosphere in which gas is easily released to the outside in a room such as a crucible containing a fluoride raw material. It is.

In the first atmosphere, since the gas is hardly released to the outside of the room, the solid scavenger is vaporized and hardly escapes to the outside of the room. Therefore, the scavenging reaction proceeds efficiently.

In the second atmosphere, gas is easily released to the outside of the room, so that reaction products and residual scavengers easily escape to the outside of the room. Therefore, it is difficult for reaction products and residual scavengers to be mixed into the purified raw material.

Specific examples of the atmosphere change include reducing the pressure of the second atmosphere from the pressure of the first atmosphere, or connecting the inside and outside of the chamber from a state in which the inside and outside of the chamber are closed (closed state) ( (Open state).

The former can be achieved by increasing or decreasing the amount of exhaust in the room or the amount of inert gas supplied into the room.

The latter is achieved by providing an opening in a chamber such as a crucible and opening and closing the opening, and by opening and closing a gas supply pipe or a gas exhaust pipe communicating with the chamber.

The pressure of the first atmosphere is not particularly limited, but is preferably 1.3 Pa or more, more preferably 1 atmosphere (about 10 atmospheres).
1.325 kPa) or more. By adjusting the pressure to 1 atm or more, an effect that the impurities in the raw material and the scavenger react efficiently can be obtained.

The pressure of the second atmosphere is not particularly limited, but is preferably 1 atm or less, more preferably 10 ^-3 Pa or less. By setting the pressure to 10 ⁻³ Pa or less, an effect that a reaction product and a residual scavenger are easily removed from the raw material can be obtained. In addition, the pressure of the second atmosphere may be set so that a part of the raw material itself does not vaporize in large quantities.

As a gas for forming the first atmosphere, an inert gas such as nitrogen, helium, argon, neon, krypton, or xenon is preferably used.

As the gas for forming the second atmosphere, the above-mentioned inert gas is used. However, it is also possible to perform vacuum only by exhausting without supplying the gas.

After a while after the transfer to the second atmosphere, cooling is started to solidify the fluoride raw material.

The cooling rate is not particularly limited, but is preferably 300 ° C./h or less, more preferably about 100 ° C./h.

When the crystal is grown at the same time as the purification, the temperature drop rate is preferably 3 to 4 ° C./h.

The fluoride thus obtained has an oxygen content of 50 ppm by weight or less and a metal element constituting the scavenger of 10 ppm by weight or less.

As described above, according to the present invention, substances that deteriorate light transmittance, such as oxygen and metal elements constituting a scavenger, can be sufficiently removed in a purification step. That is, in a crystal growth step that requires strict control for crystal growth, the operation of removing the substance can be simplified.

Further, if necessary, it is also preferable to subject the room to a dehydration treatment by purging the room with an inert gas or evacuating the room before the scavenging reaction.

The solid scavenger used in the present invention is, for example, lead fluoride, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, lithium fluoride and the like. The addition amount of the solid scavenger is 0.1 mol% or more and 1 m of the fluoride raw material.
ol% or less is preferable.

FIG. 4 shows a flowchart of a method for manufacturing a fluoride crystal using the above-described fluoride purification method.

(Raw Material Mixing Step S41) A solid scavenger is added to the fluoride raw material and mixed well. The amount of the solid scavenger added should be 0.1 mol% or more of the raw material,
mol% or less. Desirably, the fluoride raw material is calcium fluoride, barium fluoride, or magnesium fluoride. The fluoride used as the solid scavenger is desirably lead fluoride, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, and lithium fluoride.

(Purification Step S42) The fluoride raw material to which the solid scavenger is added and mixed is put into a crucible of a purification furnace shown in FIG. In FIG. 5, reference numeral 201 denotes a chamber of a refining furnace, which is connected to a vacuum exhaust system. 2
02 is a heat insulating material, 203 is a heater, 204 is a crucible as a chamber for accommodating raw materials, and 205 is a fluoride raw material.
206 is connected to a mechanism for raising and lowering the crucible. 2
Reference numeral 07 is fixed to 202, and opens and closes the upper portion of the crucible by moving the crucible up and down, and controls the size of the opening. Reference numeral 208 denotes an inert gas introduction device.

When using an inert gas: The furnace was evacuated with the opening opened (S21).
Thereafter, the furnace is filled with an inert gas at a pressure of about 1 atm or more (S22). The heater is energized to heat the crucible (S23), and the temperature is continuously raised to a temperature at which the impurity removal reaction by the scavenger is performed. The reaction is represented by the following formula, for example, when PbF ₂ is used as a scavenger, CaO + PbF ₂ → CaF ₂ + PbO PbO + C → Pb + CO. Here, C is a carbon material such as a crucible. Since the reaction involves the production of carbon monoxide, the reaction temperature can be easily known by performing gas analysis and detecting carbon monoxide. In this temperature range, the rate of heating the raw materials must be reduced in order to sufficiently promote the reaction. The furnace is evacuated to a vacuum at the temperature at which the reaction is completed, and then the raw materials are completely melted (S2).
4). Alternatively, after completely melting the raw material, the inside of the furnace is evacuated to a vacuum (S25). After the degree of vacuum is sufficiently stabilized, the melted fluoride is gradually cooled (S26) and solidified (melting / growth).

In the case of opening and closing the crucible opening, evacuation is performed with the crucible opening closed (S31).
The heater is energized to heat the crucible (S32). In order to sufficiently promote the impurity removal reaction by the scavenger, the rate of heating the raw material must be slow in the temperature range where the reaction proceeds. When the temperature reaches the temperature at which the reaction is completed, the opening of the crucible is opened, and heating is further continued to completely melt the raw materials. Alternatively, the raw material is completely melted (S
33) After that, the opening of the crucible is opened (S34). After the degree of vacuum is sufficiently stabilized, the melted fluoride is gradually cooled (S35) and solidified (melting / growth).

Since the fluoride obtained in this step may be either a polycrystal or a single crystal having grain boundaries, precise temperature control is not required. In addition, it is preferable to lower the crucible at the time of slow cooling. By lowering, the removal of impurities is further improved.

In particular, the upper part of the thus obtained crystal, that is, the part that has crystallized last with time, is removed. Since impurities tend to collect in this portion, impurities that adversely affect the characteristics are removed by this removing operation.

(Single Crystal Growth Step S43) A single crystal is grown from the purified crystal as a raw material. An appropriate growth method is selected according to the size of the crystal and the purpose of use. As an example, a growth step by the Bridgman method will be described.

The purified crystal is put in a crucible of a growth furnace shown in FIG. In FIG. 6, reference numeral 301 denotes a chamber of a growth furnace, which is connected to a vacuum exhaust system. 302
Is a heat insulating material, 303 is a heater, 304 is a crucible, 305
Is a crucible lowering mechanism, and 306 is a fluoride crystal.
The crucible of the growth furnace has high hermeticity against the external atmosphere.

After evacuating the furnace, the heater is energized to heat the crucible and completely melt the crystal as a raw material.
Thereafter, the crucible is gradually lowered and cooled to grow a single crystal. Crucible descending speed is 0.1 ~ per hour
5.0 mm is preferred.

The fluoride crystal thus obtained has a very low content of oxygen and the constituent elements of the scavenger. This is because oxygen and constituent elements of the scavenger can be sufficiently removed in the purification step.

(Annealing Step S44) Subsequently, the fluoride single crystal that has grown is subjected to a heat treatment in an annealing furnace shown in FIG. In FIG. 7, reference numeral 401 denotes an annealing furnace chamber, 402 denotes a heat insulating material, 403 denotes a heater, 404 denotes a crucible, and 405 denotes a fluoride crystal.

In this annealing step, the crucible is heated to a temperature of 400 to 500 ° C. or less, which is the melting point of the fluoride crystal. The heating time is 20 hours or more, more preferably 20 to 30 hours.

(Forming Step S45) Thereafter, the required shape of the optical article (convex lens, concave lens, disk shape,
(Plate shape etc.). Further, if necessary, an antireflection film may be provided on the surface of the fluoride crystal optical article. As the antireflection film, magnesium fluoride, aluminum oxide, or tantalum oxide is preferably used, and these can be formed by vapor deposition by resistance heating, electron beam vapor deposition, sputtering, or the like. Since the optical article obtained by the present invention hardly contains water, the adhesion of the antireflection film is excellent.

An optical system suitable for an excimer laser, in particular, an ArF excimer laser can be constructed by variously combining the lenses thus obtained. In particular, when the fluoride crystal is calcium fluoride, an exposure system is provided by combining an optical system having an excimer laser light source and a lens made of the calcium fluoride crystal with a stage capable of moving a substrate with a photoresist as an object to be exposed. Can be configured.

(Exposure Apparatus) Hereinafter, an exposure apparatus using the optical article of the present invention will be described.

Examples of the exposure apparatus include a reduction projection exposure apparatus using a lens optical system and a lens-type equal-magnification projection exposure apparatus.

In particular, in order to expose the entire surface of the wafer, one small section (field) of the wafer is exposed, the wafer is moved by one step, and the next one field is exposed.
A stepper employing a step-and-repeat method is desirable. Of course, the present invention is also suitably used for a scanning type exposure apparatus.

FIG. 8 shows the structure of the exposure apparatus of the present invention. In the figure, reference numeral 21 denotes an illumination light source unit, 22 denotes an exposure mechanism unit, and 21 and 22 are configured separately and independently. That is, both are physically separated. Reference numeral 23 denotes an illumination light source, which is a high-output large light source such as an excimer laser.
24 is a mirror, 25 is a concave lens, 26 is a convex lens, and 25 and 26 have a role as a beam expander, and expand the laser beam diameter to approximately the size of an optical integrator. 27
Is a mirror, and 28 is an optical integrator for uniformly illuminating the reticle. Illumination light source 2
Reference numeral 1 denotes a portion from the laser 23 to the optical integrator 28. Reference numeral 29 denotes a mirror, and reference numeral 30 denotes a condenser lens that collimates a light beam emitted from the optical integrator 28. Reference numeral 31 denotes a reticle on which a circuit pattern is drawn, 31a denotes a reticle holder that holds the reticle by suction, 32 denotes a projection optical system that projects a reticle pattern, and 33 denotes a wafer on which a pattern of the reticle 31 is printed by a projection lens 32. Numeral 34 denotes an XY stage which holds the wafer 33 by suction and moves in the XY directions when printing is performed in a step-and-repeat manner. 3
Reference numeral 5 denotes a surface plate of the exposure apparatus.

The exposure mechanism section 22 is composed of a mirror 29 which is a part of the illumination optical system and a platen 35. 36
Is an alignment means used for TTL alignment. The normal exposure apparatus includes an autofocus mechanism, a wafer transfer mechanism, and the like, and these are also included in the exposure mechanism section 22.

The fluorite optical article of the present invention is used as a lens or a mirror shown in FIG. 8 or, although not shown, as a mirror or a lens of a mirror type exposure apparatus. More preferably, an antireflection film or a reflection-enhancing film may be provided on the surface of the lens or the mirror.

By using this exposure apparatus to irradiate a photosensitizing resist on a substrate with an excimer laser beam via a reticle pattern, a latent image corresponding to the pattern to be formed can be formed.

[0082]

The present invention will be described in more detail with reference to the following examples.

Example 1 To a general synthetic calcium fluoride raw material having a purity of 99%, zinc fluoride as a scavenger was added at about 0.5 mol% with respect to calcium fluoride and mixed. Next, the mixture was put into a crucible of a refining furnace shown in FIG. 5, and the inside of the furnace and the crucible were evacuated. Then, Ar was charged until the pressure reached 10 ⁵ Pa. At this time, the crucible was opened so that the degassing property was enhanced. Next, the opening of the crucible was closed and the crucible was heated to 1360 ° C. to melt the raw materials. Then, the opening was opened again and the inside of the furnace was evacuated to a vacuum degree of 6.66 × 10 ⁻⁴ Pa. Thereafter, the crucible was lowered and gradually cooled to crystallize the raw material. The upper part of the crystallized calcium fluoride corresponding to the upper part of the crucible was removed by 1 mm in thickness.

Next, the crystal block was placed in a crucible of a single crystal growth furnace shown in FIG. The furnace was evacuated and the degree of vacuum was 2.66 × 10 ⁻⁴ Pa and the temperature was 1360 ° C.
After holding for one hour, the growth crucible was lowered at a speed of 2 mm / h.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of zinc fluoride were charged. Evacuate the furnace and raise the temperature of the crucible from room temperature to 900 ° C
At a rate of 100 ° C./h for 20 hours at 900 ° C.
Held. Then, the temperature was lowered at a rate of 6 ° C./h and cooled to room temperature.

Thus, a large number of calcium fluoride crystals were prepared, cut and polished to obtain a disk having a thickness of 10 mm.
0 ⁵ 800 from wavelength 200nm after irradiation with γ-rays of R
The transmittance in nm was measured. Among them, five samples having different transmittances were selected, and the energy density was 5 mJ / c.
After irradiating 10 ⁸ shots of m ² ArF excimer laser light, the transmittance at 193 nm was measured. FIG. 9 shows a transmission spectrum after γ-ray irradiation. In addition, 26
FIG. 10 shows the relationship between the transmittance decrease amount dTγ near 0 to 280 nm and the transmittance decrease amount dTa after ArF laser irradiation.

Based on the relationship shown in FIG. 10, crystals were selected according to the application. First, for optical parts that require a high degree of ultraviolet light resistance such that the transmittance decrease dTa by laser irradiation is 0.2% or less, the transmittance decrease dTγ after γ-ray irradiation is used.
Was selected to be 4% or less (samples 1 and 2). Then
Optical parts that permit a decrease in transmittance of up to about 0.4% due to laser irradiation have a transmittance decrease of 8% after γ-ray irradiation.
% Of crystals (samples 3 and 4) were selected.

Sample 5 was used for ordinary ultraviolet light such as KrF excimer laser instead of being used for vacuum ultraviolet light such as ArF excimer laser because the reduction amount dTa was about 0.5%.

FIG. 11 shows the internal transmittance of Samples 1 to 5 in the wavelength range of 300 nm or less before gamma ray irradiation.

Thus, even if it is difficult to distinguish the characteristics before gamma ray irradiation, as shown in FIG. 9, if the optical characteristics of 200 nm to 400 nm after gamma ray irradiation are measured,
Excellent or good durability can be judged.

In particular, those having a transmittance reduction of 8% or less after gamma ray irradiation in the wavelength range of 260 to 280 nm are not suitable for high energy vacuum ultraviolet laser light.
Shows excellent durability.

It is a highly reliable calcium fluoride crystal.

Samples 1 to 3 after γ-ray irradiation were heated to about 200 ° C. in a vacuum atmosphere, and then gradually cooled to room temperature. Thereafter, when the transmittance at a wavelength of 200 to 800 nm was measured, it was possible to return to the transmittance before γ-ray irradiation shown in FIG.

The calcium fluoride crystal of this example emits a laser beam having an energy density of 5 to 50 mJ / cm ² from 10 ⁷ to 10
The internal transmittance upon irradiation with ¹¹ shots was 99.5% or more per 10 mm thickness.

(Example 2) Except that the energy density of the irradiated ArF excimer laser was set to 50 mJ / cm ² ,
The crystals were evaluated in the same manner as in Example 1. As a result, the same relationship as in FIG. 10 was derived.

[0096] (Example 3) the energy density of the ArF excimer laser irradiation and 50 mJ / cm ^2, except that the number of shots was 10 ³ shots evaluated the crystals in the same manner as in Example 1. As a result, the same relationship as in FIG. 10 was derived.

[0097]

According to the present invention, a calcium fluoride crystal having excellent durability against ultraviolet rays can be selected by measuring the transmittance after γ-ray irradiation, and a calcium fluoride crystal suitable for the intended use can be provided. As a result, it is possible to provide a stable and reliable optical component for ultraviolet light and an exposure apparatus at low cost.

[Brief description of the drawings]

FIG. 1 is a view showing a flowchart of a manufacturing process of an optical component according to the present invention.

FIG. 2 is a view showing a flowchart of a purification process of a fluoride used in the present invention.

FIG. 3 is a diagram showing a timing chart of a purification step of a fluoride used in the present invention.

FIG. 4 is a view showing a flowchart of a manufacturing process of a fluoride crystal used in the present invention.

FIG. 5 is a schematic sectional view of a refining furnace.

FIG. 6 is a schematic sectional view of a growth furnace.

FIG. 7 is a schematic sectional view of an annealing furnace.

FIG. 8 is a view showing an exposure apparatus according to the present invention.

FIG. 9 is a diagram showing a transmission spectrum of fluoride after gamma ray irradiation.

FIG. 10 is a graph showing the amount of decrease in the transmittance of fluoride.

FIG. 11 is a diagram showing a transmission spectrum of fluoride before gamma ray irradiation.

Continued on the front page F-term (reference) 2H097 BA02 BA10 CA13 LA10 4G076 AA05 AB04 BA17 BB05 BC02 BD03 BE02 BE12 BF05 BF10 BG04 CA08 CA11 CA23 CA33 DA11 4G077 AA02 BE02 CD02 EC08 EG01 EG20 FE17 FG11 F043 CA04 HA04 CB02 CB12 CB22 CB23 CB25 DA01

Claims (8)

[Claims] 1. A gamma ray having a dose of 1 × 10 ⁵ R / hour is applied to one gamma ray.
A calcium fluoride crystal in which the amount of decrease in internal transmittance per 10 mm thickness at a wavelength of 260 to 280 nm after irradiation for 8 hours is 8% or less of that before irradiation. 2. An ultraviolet optical component comprising the calcium fluoride crystal according to claim 1. 3. An optical component for vacuum ultraviolet light comprising the calcium fluoride crystal according to claim 1. 4. An optical system having the optical component according to claim 3,
An exposure apparatus comprising: a support unit configured to support an object to be exposed, and irradiating the object on the support unit with ultraviolet light from a light source via the optical system. 5. A method for inspecting a calcium fluoride crystal, comprising the step of irradiating the calcium fluoride crystal with gamma rays and measuring optical characteristics at 200 nm to 400 nm. 6. A gamma ray having a dose of 1 × 10 ⁵ R / hour is applied to one gamma ray.
Irradiation for a period of time, a calcium fluoride crystal in which the amount of decrease in internal transmittance per 10 mm thickness at a wavelength of 260 to 280 nm after irradiation is 8% or less of that before irradiation is selected. Manufacturing method of optical parts. 7. 200 ° C. or more in a vacuum or an inert gas
7. The method for manufacturing an optical component according to claim 6, further comprising a step of performing a heat treatment at 1000 [deg.] C. to increase the internal transmittance. 8. The method for manufacturing an optical component according to claim 7, wherein after the step, processing is performed to form a film.