JPH11228292A - Raw material for producing fluoride crystal and its purification, fluoride crystal and its production, and optical component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a raw material for producing a cheap fluoride crystal obtained by using a readily treatable carbon fluoride-based gas, capable of avoiding the decrease of transmission or the like by the gas taken in the crystal, and excellent in optical properties, and further to provide a method for producing the raw material, to provide a fluoride crystal and its production method, and further to obtain an optical component having hardly deteriorated transmission characteristics even after repeating irradiation of a short wave high power light for a long period. SOLUTION: This method for producing a fluoride crystal comprises inserting fluoride raw material 305 to a crucible 304 in a purifying furnace, introducing reactive gas 306 to the purifying furnace, heating the fluoride raw material 305 in a reactive gas atmosphere to the temperature below the melting point, changing the atmosphere in the purifying furnace to a vacuum atmosphere or an inert gas atmosphere, melting the fluoride raw material 305, and cooling the melted fluoride raw material 305 to solidify the fluoride raw material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a raw material for producing a fluoride crystal, a method for purifying the same, a fluoride crystal, a method for producing the same, and an optical component, and more particularly, to a wide wavelength range from vacuum ultraviolet to far infrared. Fluoride crystal suitable for various optical elements, lenses, window materials, prisms and the like to be used, particularly a fluoride crystal raw material suitable for manufacturing calcium fluoride crystal crystal as an optical component for excimer laser, and a purification method thereof , A fluoride crystal, a method for producing the same, and an optical component.

[0002]

2. Description of the Related Art Fluoride crystals such as calcium fluoride have high transmittance in a wide wavelength range from the vacuum ultraviolet region to the far infrared region, and are widely used for various optical elements, lenses, window materials, prisms, and the like. I have.

[0003] Fluorite having excellent transmission characteristics at short wavelengths is an optical member for excimer lasers, and particularly has a diameter of 200 mm.
It is useful as a lens having a large diameter of not less than mm. In particular, a calcium fluoride crystal having an internal transmittance of 70% or more for light having a wavelength of 135 nm is excellent in durability against an ArF excimer laser, and its transmission characteristics are hardly deteriorated even by repeated irradiation of a high-power laser.

[0004] When such a fluoride crystal is crystallized by melting only a crystal raw material and moving the crucible in a furnace having a temperature gradient, it tends to become cloudy and devitrified.

Therefore, in order to obtain a crystal having good transmission characteristics, it is necessary to add a scavenger for preventing oxidation of the crystal raw material and removing impurities.

Accordingly, as a scavenger, lead fluoride (Stockbarger, J. Opt. Soc. Am. 39, (1949) 731-740) is used.
Page) and cadmium fluoride (Radzhabov and Figur
a, Phys. Stat, Sol. (b) 136, (1986) K55-K59)
Technologies using such methods have been developed.

In addition, instead of using a scavenger, an attempt has been made to use a reactive gas to melt a raw material in the atmosphere and then gradually cool the crystal to purify and grow the crystal.
As the reactive gas, hydrogen fluoride gas diluted with helium (Guggenheim, J. Appl. Phys. 34, pp. 2482-2485 (1963), Robinson and Cripe, J. Appl. Phys. 37.
2072-2074 (1966)), and a mixture of hydrogen fluoride gas with methane tetrafluoride, sulfur tetrafluoride, and boron trifluoride (Pastor, Robinson and Braunstain, M
at. Res. Bull. 15, pp. 469-475 (1980)), Teflon decomposition product gas (Chernevskaya and Korneva, Opt. T
ech. 39, pages 213 to 215 (1972)).

[0008]

However, when a scavenger is used, the metal element constituting the scavenger remains in the crystal. If the residual amount is large, there is a great possibility that the transmission characteristics will be adversely affected. Therefore, the residual amount in the crystal must be reduced by reducing the added amount. On the other hand, if the addition amount is too small, the effect as a scavenger is reduced, and the transmission characteristics are significantly deteriorated due to oxidation of the crystal raw material and the like. Therefore, it is important to determine the optimal amount of the scavenger to be added, but the value can vary depending on the water concentration and the impurity content in the crystal raw material, so the operation becomes very complicated, and the production cost is increased. I will.

When a reactive gas is used, the reactive gas often dissolves in the melt and is taken into the crystal as bubbles (Guggenheim, J. appl. Phy. 3).
4, pages 2482 to 2485 (1963)), and in most cases, satisfactory transmission characteristics cannot be obtained.

The present invention has been made in view of the above technical problems, and it is difficult to incorporate a component of a reactive gas into a crystal by using a reactive gas as a scavenger and appropriately setting a reaction temperature. It is an object of the present invention to provide a raw material for producing a fluoride crystal which is inexpensive and has excellent optical properties, a method for producing the same, and a fluoride crystal and a method for producing the same.

Another object of the present invention is to provide an optical component whose transmittance characteristics are hardly degraded even when light of short wavelength and high output is repeatedly irradiated for a long period of time.

[0012]

According to the present invention, there is provided a method for producing a fluoride crystal, comprising: heating a fluoride raw material in a reactive gas atmosphere at a temperature lower than the melting point of the fluoride raw material; Wherein the fluoride raw material is melted and then cooled to crystallize.

[0013] The fluoride crystal of the present invention is characterized by being produced by the above-mentioned method for producing a fluoride crystal.

An optical component according to the present invention is characterized in that the above fluoride crystal is processed and formed.

In the method for purifying a raw material for producing a fluoride crystal according to the present invention, the fluoride raw material is heated at a temperature lower than the melting point in a reactive gas atmosphere, and then the fluoride raw material is heated in a vacuum atmosphere or an inert gas atmosphere. Melting, and then cooling and solidifying the fluoride raw material.

The raw material for producing a fluoride crystal according to the present invention is characterized in that it is purified by the above-mentioned method for purifying a raw material for producing a fluoride crystal.

In the method for producing a fluoride crystal according to the present invention, the raw material for producing a fluoride crystal purified by the above-described purification method is placed in a growth furnace, and the growth furnace is placed in an inert gas atmosphere or a vacuum atmosphere to form the fluorine crystal. The method is characterized in that a raw material for producing a fluoride crystal is dissolved, and then the crucible is moved to cool the raw material for producing a fluoride crystal to grow a crystal.

[0018] The fluoride crystal of the present invention is characterized by being produced by the above-mentioned method for producing a fluoride crystal.

An optical component according to the present invention is characterized in that the above fluoride crystal is processed and formed.

An exposure apparatus according to the present invention includes the above-described optical component and a stage for mounting a support having a photosensitive material for exposing light transmitted through the optical component.

In the method of purifying a raw material for producing a fluoride crystal according to the present invention, the temperature at which the fluoride raw material is exposed to a reactive gas atmosphere during the process of raising the temperature of the fluoride raw material is set to a temperature lower than the melting point of the crystal. The purification is performed in an atmosphere or an inert gas atmosphere.

[0022]

FIG. 1 shows the steps of a method for purifying a raw material for producing a fluoride crystal or a method for producing a fluoride crystal according to an embodiment of the present invention.

In the case of purification, first, a powdery or granular fluoride raw material is placed in a purification furnace as in step S1, and the fluoride raw material is heated. The atmosphere at this time is a reactive gas atmosphere.

The fluoride raw material used in the present invention includes, for example, calcium fluoride, barium fluoride, magnesium fluoride and the like. And it is good to use the raw material whose total content of the mass of at least one or more rare earth elements contained in this raw material is less than 10 ppm.

Here, the reactive gas used is preferably a fluorinated compound which reacts with an oxide, particularly a fluorocarbon-based gas. Specifically, methane tetrafluoride (CF ₄ ), methane trifluoride (CHF ₃ ), methane difluoride (CH
₂ F _2), ethane hexafluoride (C ₂ F _6), 1 or more kinds is preferred was chosen et-eight fluoride propane (C ₃ F _8). In addition, HF, NF ₃ , SF ₆ , XeF ₂ , B
F ₃ or the like can be used. These gases are A
It may be used after being diluted with a fluorinated gas such as r, He, Ne, or Xe. In this case, the reactive gas is easily decomposed, and the reaction proceeds quickly.

Such a fluorocarbon-based gas is preferable because it has a particularly excellent effect of removing oxides in the solid phase as compared with other fluoride compound gases. In addition, the fluorocarbon-based gas is
Because of its low corrosiveness and easy handling, it is more preferable to use other reactive gases from this point.

The temperature of the fluoride raw material in the heating step S1 is lower than the melting point of the fluorinated compound. In particular, the melting point is 50 to
A temperature 200 ° C. lower is preferred, 75 to 175 below the melting point.
Lower temperature is more preferable, 100-150 ° C than melting point
Lower temperatures are most preferred. At temperatures closer to the melting point,
The reactive gas tends to remain in the raw material, which may reduce the light transmission of the crystal. When the temperature is lower than the melting point by 200 ° C., the reaction with the oxide may be insufficient at the temperature.

The heating time is preferably 4 to 30 hours, more preferably 8 to 25 hours, even more preferably 10 to 20 hours. If it is less than 4 hours, the reaction with the oxide may be insufficient, and if it is more than 30 hours, the furnace is easily damaged by hydrogen fluoride generated by the reaction. This heating time does not need to be a holding time at a constant temperature,
The temperature may change continuously or intermittently. For example, when the temperature is raised from a temperature 200 ° C. lower than the melting point to a temperature 50 ° C. lower than the melting point, the heating time may be considered as the heating time.

The pressure in step S1 is at least 0.01 atm, more preferably in the range of 0.1 to 1 atm. Further, by setting the pressure on the high pressure side in the above pressure range, the pressure set in the purification step for removing a large amount of impurities in the raw material is set higher than the pressure set in the single crystal growth step. Is preferred.

After performing the heat treatment step S1 by introducing a reactive gas, the fluoride raw material is dissolved in a vacuum atmosphere or an inert gas atmosphere as in step S2. In the case of a vacuum atmosphere, the pressure (degree of vacuum) is preferably less than 1 × 10 ⁻⁵ Torr. By setting the degree of vacuum to less than 1 × 10 ⁻⁵ Torr, a fluoride single crystal having more excellent permeability can be finally produced. In the present invention, one atmospheric pressure is 760 Torr, and one atmospheric pressure is about 101.32.
5 kPa

The inert gas atmosphere used in step S2 is preferably at least one kind of atmosphere selected from rare gases such as He, Ne, Ar, and Xe. Also, in the melting step, in the atmosphere, the reactive gas hardly remains, and even if the reactive gas slightly remains in the atmosphere, the amount thereof is such that the internal transmittance of the fluoride crystal decreases. Not the amount. Further, the pressure in the inert gas atmosphere may be appropriately selected in the range of 1 × 10 ⁻⁵ Torr to 1 atm or more.

After heating and melting the fluoride raw material at or above its melting point, the raw material is cooled as shown in step S3.

When the raw material is purified by a series of steps shown in FIG. 1, since the crystal obtained by cooling does not need to be a single crystal, it can be rapidly cooled. The preferred cooling temperature is 300 ° C./h or less.

When the process of FIG. 1 is employed in the method of producing a fluoride crystal for obtaining a single crystal, the cooling temperature is preferably about 3 to 4 ° C./h. As a method of crystal growth, the Bridgman method is preferably used.

FIG. 2 is a flow chart of a method for purifying a raw material for producing a fluoride crystal or a method for producing a fluoride crystal according to another embodiment of the present invention. Dehydration treatment is being performed. From a different point of view, the heat treatment S12 may be regarded as including a dehydration step.

In step S11, after the fluoride raw material is charged into the furnace, the furnace atmosphere is changed to a vacuum atmosphere or an inert gas atmosphere before introducing a reactive gas, and the furnace interior is heated to 100 to 100%.
Dehydration is performed by heating to 300 ° C. At that time, it is preferable to perform dehydration until the degree of vacuum once becomes less than 1 × 10 ⁻⁵ Torr.

Dehydration is physical desorption by heat.

If the temperature is lower than 100 ° C., the moisture attached to the fluoride raw material or the moisture attached to the inner wall of the refining furnace cannot be completely removed. When the temperature exceeds 300 ° C., the reaction of the formula 1 proceeds, and a large amount of oxide is generated. Even in the range of 100 to 300 ° C, 150 to 200 ° C is particularly preferable.

CaF ₂ + H ₂ O → CaO + 2HF (Equation 1) By evacuating until the pressure becomes less than 1 × 10 ⁻⁵ Torr, the desorption of the water from the fluoride raw material or the inner wall of the refining furnace is completed. The separated water can be considered to be exhausted from the inside of the refining furnace to the outside, and the purpose of dehydration is achieved.

When dehydrating in an inert gas atmosphere, the pressure is 0.1 atm or less, preferably 1 × 10 ⁻³ To.
rr or less, more preferably about 1 × 10 ⁻⁴ Torr or less, and the furnace may be purged with an impermeable gas.

The time of the dehydration step S11 is about several hours, preferably 24 hours or more, and more preferably 4 hours.
It is about 0 hours.

In the heating step S12 after the dehydration step S11, the melting step S13 and the cooling step S14 can be performed in the same manner as steps S1, S2 and S3 in FIG. 1, respectively.

FIG. 3 shows the change over time in temperature. T0
Indicates the melting point of the fluoride. T1 as described above
Is in the range of 100 ° C to 300 ° C, and T2 is 2
It is preferable to select from the range of a temperature lower than 00 ° C. to a temperature lower by 50 ° C. than the melting point. It is also preferable that the temperature rise rate be changed at least once as shown by the dashed line in FIG. Further, in the melting step, the temperature may be higher than the melting point.

In the method for producing a fluoride crystal of the present invention, the raw material for producing a fluoride crystal purified by the above-described purification method is charged into a crucible in a growth furnace, and the inside of the growth furnace is filled with an inert gas atmosphere or a vacuum atmosphere. The raw material for producing a fluoride crystal is then melted, but it is preferable to perform purification before the melting. That is, it is preferable to perform the same refining as in the refining method of the present invention again before melting in the growth furnace from the viewpoint of obtaining better permeability. However, it is not always necessary when the oxide is sufficiently removed in the purification of the raw material for producing a fluoride crystal and the moisture is sufficiently removed from the atmosphere in the growth furnace.

FIG. 4 is a flowchart showing an example of a crystal manufacturing process according to still another embodiment of the present invention.

The fluoride raw material varies depending on the transmission characteristics required for the crystal. However, in order to obtain a crystal having excellent transmission characteristics, a high-purity powder raw material must be used.

When the raw material is already high in purity, the following purification step may be omitted.

When manufacturing calcium fluoride crystals for excimer laser, it is preferable to prepare a raw material having a total content of at least one or more rare earth elements of less than 10 ppm.

(Purification Step) The fluoride raw material is put into the crucible 304 of the purification furnace shown in FIG. In FIG. 5, reference numeral 301 denotes a chamber of the refining furnace, which is connected to a vacuum exhaust system. 302 is a heat insulating material, 303 is a heater, 30
4 is a crucible and 305 is a fluoride raw material. Reference numeral 306 denotes a reactive gas source, which is connected to a refining furnace via a pipe 311 via a valve 308. Reference numeral 307 denotes an inert gas source.
1 connected. By opening the valves 308 and 309, a reactive gas diluted with an inert gas can be introduced into the chamber 301 of the purification furnace. Note that a mass flow controller may be provided on the pipes 311 and 312 to control the flow rates of the reactive gas and the inert gas.

The interior of the refining furnace is evacuated, and the heater is energized while continuing the evacuation to heat the fluoride raw material 305. The temperature in the refining furnace is maintained at about 100 to 300 ° C. to remove most of the adsorbed moisture contained in the fluoride raw material 305 and the moisture adhering to the inner wall of the refining furnace (dehydration).

When the degree of vacuum in the refining furnace reaches less than 1 × 10 ⁻⁵ Torr, the valve 308 is opened and a reactive gas (eg, a fluorocarbon-based gas) is supplied from the reactive gas source 306 into the chamber 301 of the refining furnace. ) (Gas filling).
If the degree of vacuum is 1 × 10 ⁻⁵ Torr or more, the dehydration may not be performed sufficiently. Therefore, it is preferable to introduce the reactive gas after the pressure reaches less than 1 × 10 ⁻⁵ Torr.

It is desirable to open the valves 308 and 309 and introduce an inert gas such as Ar or He from the inert gas source 307 to dilute the reactive gas.

It is preferable that the pressure in the chamber 301 of the refining furnace is sealed at less than 1 atm. When the pressure exceeds 1 atm, the reactive gas tends to remain in the raw material.

The fluoride raw material 305 is gradually heated and maintained at a temperature before melting for a predetermined time.

In this process, the fluorocarbon-based gas converts oxides in the fluoride raw material into fluoride. For example,
As shown in the following formula, methane tetrafluoride converts calcium oxide to calcium fluoride.

2CaO + CF ₄ → 2CaF ₂ + CO ₂ (1000 ° C. or higher) (Equation 2) Thereafter, the inside of the chamber 301 of the refining furnace is evacuated.
The degree of vacuum is preferably less than 1 × 10 ⁻⁵ Torr. Alternatively, the valve 308 is closed and evacuated to 1 × 10 ^−5.
After reducing the pressure to less than Torr, the valve 309 may be opened to replace the inside of the chamber 301 of the refining furnace with an inert gas such as Ar or He. Thereafter, the material is further heated to completely melt the fluoride raw material 305. Subsequently, the melted fluoride raw material is gradually cooled (cooled) to grow a crystal (melting / growing).

In this step, a block-like fluoride raw material is obtained, and since the crystal of the block-like fluoride raw material may have a grain boundary, a precision is required in the cooling step as in a single crystal growth step described later. No temperature control is required. In addition, at the time of slow cooling, it is preferable to pull down the crucible 304 with the pull-down bar 310. By lowering, the effect of removing impurities is further improved.

In particular, the upper portion of the block-like fluoride raw material thus obtained, that is, the portion 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.

If necessary, the block-form fluoride raw material is again put into a crucible of a refining furnace, and the above-described series of steps of gas filling, melting, growth, and removal of the upper portion are repeated a plurality of times.

(Single Crystal Growth Step) Next, the purified block-like fluoride raw material is put in a crucible, and this is attached to a growth furnace shown in FIG. In FIG. 6, reference numeral 401 denotes a chamber of a growth furnace, which is connected to a vacuum exhaust system. 402
Is a heat insulating material, 403 is a heater, 404 is a crucible, 405
Is a block-like fluoride raw material. Reference numeral 410 denotes a crucible pulling bar. 406 is a reactive gas source, 407 is an inert gas source, and 408 and 409 are valves. 311,3
Reference numeral 12 denotes a pipe, on which a reactive gas can be diluted with an inert gas. These points are the same as in the case of the refining furnace shown in FIG.

Dehydration, gas filling,
Melt and gradually crucible 4 with crucible withdrawal rod 410
04 is lowered and cooled to grow a fluoride single crystal (Bridgeman method). Here, in the gas filling step, the oxide (formula 1) generated by the reaction between the moisture in the structure of the growth furnace such as the surface of the block-like fluoride raw material 405 and the heat insulating material and the block-like fluoride raw material 405 is used. The purpose is to return to the compound (formula 2), and this step can be omitted if the dehydration before the gas filling is sufficiently performed.

In this slow cooling, 0.1 hour per hour
It is preferable to lower the crucible at a speed of up to 5.0 mm and gradually cool the crucible because good crystals with few crystal defects (vacancies, dislocations) can be obtained.

(Annealing Step) 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 501 denotes a chamber of an annealing furnace,
502 is a heat insulating material, 503 is a heater, 504 is a crucible,
505 is a fluoride crystal.

In this annealing step, crucible 504 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. It should be noted that a crystal having strength against thermal shock such as magnesium fluoride may be omitted from the annealing step.

(Processing and Assembling Process) Thereafter, the optical component is formed into a required shape (convex lens, concave lens, disk shape, plate shape, etc.). Such an optical component, for example, in the case of a calcium fluoride crystal, has an excellent characteristic that the internal transmittance for light having a wavelength of 135 nm is 70% or more.

Further, if necessary, an anti-reflection film may be provided on the surface of the fluoride crystal optical article. As the antireflection film, magnesium fluoride, aluminum oxide, or aluminum fluoride 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, particularly an ArF excimer laser, can be constructed by combining the lenses thus obtained in various ways. In particular, when the fluoride crystal is calcium fluoride, a stepper 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 on which a layer of photosensitive material is formed is used. And an exposure device such as a scanner.

(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 one adjacent field is exposed.
A stepper employing a step-and-repeat method is desirable. Of course, the present invention is also suitably used for a microscan type exposure apparatus.

FIG. 9 is a schematic view showing the structure of an exposure apparatus according to 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 illuminating the reticle uniformly. The illumination light source unit 21 includes a part from a laser 23 to an optical integrator 28. 29 is a mirror, 3
0 is a condenser lens and optical integrator 2
The luminous flux that emitted 8 is collimated. 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. Reference numeral 35 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 surface plate 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.

FIG. 10 shows an example of an optical article used in the exposure apparatus of the present invention, which is a lens used in the projection optical system of the exposure apparatus shown in FIG. This lens assembly is constituted by combining eleven lenses L1 to L11 without bonding them to each other. The optical article made of the fluorite of the present invention is used as a lens or a mirror shown in FIGS. 9 and 10 or, although not shown, as a mirror or a lens of a mirror type exposure apparatus. More preferably,
It is preferable to provide an antireflection film or a reflection-enhancing film on the surface of the lens or the mirror.

The optical component made of the fluoride crystal of the present invention can be used as a prism or an etalon.

FIGS. 11 (a) and 11 (b) are diagrams schematically showing the configuration of an excimer laser oscillator using an optical crystal composed of a fluoride crystal according to the present invention.

The excimer laser oscillator shown in FIG. 11A has a resonator 83 for emitting an excimer laser to resonate, an aperture hole 82 for narrowing the excimer laser emitted from the resonator 83, and a wavelength of the excimer laser. It is composed of a prism 84 for making a single wavelength and a reflecting mirror 81 for reflecting an excimer laser.

The excimer laser oscillator shown in FIG. 11B has a resonator 83 for emitting an excimer laser to resonate, an aperture hole 82 for narrowing the excimer laser emitted from the resonator 83, and an excimer laser oscillator. The etalon 85 includes an etalon 85 for reducing the wavelength to a single wavelength, and a reflecting mirror 81 for reflecting excimer laser light.

The excimer laser light oscillator of the present invention, in which the optical crystal composed of a fluoride crystal is provided in a device as a prism or an etalon, can further narrow the wavelength of the excimer laser through the prism or the etalon. The excimer laser can be made to have a single wavelength.

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

[0080]

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

(Example 1) The content of the rare earth element was 10 p
pm or less high-purity synthetic calcium fluoride raw material (melting point 13
(60 ° C.) was placed in the crucible 304 of the refining furnace shown in FIG. 5 and evacuated, and then heated to 300 ° C. to perform dehydration from the calcium fluoride raw material.

The degree of vacuum in the chamber 301 of the purification furnace is 1
At the time when the pressure becomes × 10 ⁻⁶ Torr or less, methane tetrafluoride diluted to about 30% with Ar is supplied to the chamber 301 of the purification furnace.
And the pressure in the chamber 301 of the refining furnace is set to 0.
The pressure was increased to 5 atm.

Then, the temperature was raised to a temperature lower than the melting point of calcium fluoride, that is, from 300 to 1250 ° C. at a rate of about 50 ° C./h, and then the inside of the chamber 301 of the refining furnace was evacuated.

The degree of vacuum in the chamber 301 of the refining furnace is 1
After the temperature has become less than × 10 ⁻⁵ Torr, the material is further heated to 1380 ° C. to completely melt the fluoride raw material.
4 was lowered by the pull-down bar 310 and gradually cooled to obtain a polycrystalline block-shaped fluoride raw material.

An upper portion corresponding to the upper portion of the crucible of the block-like calcium fluoride raw material was removed by 1 mm in thickness.

Next, the block-like calcium fluoride raw material was put in a crucible 404 for growing a single crystal in a growth furnace shown in FIG. After evacuating the inside of the chamber 401 of the growth furnace, dehydration, gas filling, and melting were performed in the same manner as in the above-described purification step.

The degree of vacuum was 2 × 10 ⁻⁶ Torr and the temperature was 13
After keeping the temperature at 60 ° C. for 11 hours, a crucible 304 for growth was used.
At a speed of 2 mm / h. The temperature drop rate at this time corresponds to about 3 to 4 ° C / h.

Next, the crucible 50 of the annealing furnace shown in FIG.
4 and a 0.1 wt% zinc fluoride single crystal grown calcium fluoride. Annealing furnace chamber 50
4 is evacuated, and the temperature of the crucible 504 is raised 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.

The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmission spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance were measured. Some of the results are shown in Table 1 and FIG.

The internal transmittance was measured with a vacuum ultraviolet spectrophotometer. The degradation rate was determined by irradiating 1 × 10 ³ pulses of a laser with an output of 30 mJ / cm ² and gamma rays of 1 × 10 ⁴ R / H for 1 hour.
It was expressed as the decrease rate of the transmittance in nm.

Example 2 In Example 1, methane tetrafluoride was used as a reactive gas in the purification step. In this example, methane trifluoride (CH 3) was used as the reactive gas.
F ₃ ), methane difluoride (CH ₂ F ₂ ), ethane hexafluoride (C ₂ F ₆ ), and propane octafluoride (C ₃ F ₈ ) were used.

Otherwise, the procedure of Example 1 was repeated to prepare a calcium fluoride single crystal. Table 1 and FIG. 8 show the results in the case of methane trifluoride.

As a result, a crystal having the same transmittance as in the case shown in Example 1 was obtained.

(Example 3) After gas filling in the purification step, 3
After the temperature was raised from 00 ° C. to 1250 ° C. at a rate of about 50 ° C./h, the inside of the chamber 301 of the refining furnace was replaced with Ar gas (purging of the remaining reactive gas). To about 1 atm. 1380
C. to completely melt the fluoride raw material. Otherwise, the procedure of Example 1 was repeated to prepare a calcium fluoride single crystal. As a result, a crystal having the same transmittance as that shown in Example 1 was obtained (Table 1, FIG. 8).

(Example 4) In the purification step, when the degree of vacuum in the chamber 301 of the purification furnace became 1 × 10 −6 Torr or less, methane tetrafluoride diluted to about 10% with Ar was supplied to the chamber of the purification furnace. The pressure in the chamber 301 of the refining furnace was set to 0.1 atm. After the temperature was raised from 300 ° C. to 1250 ° C. at a rate of about 50 ° C./h, Ar gas was charged into the chamber 301 of the purification furnace, and the pressure in the chamber 301 of the purification furnace was set to 1 atm. The power supply to the heater was controlled so that the change over time in temperature was similar to the solid line in FIG. This reduced the concentration of tetrafluoromethane to about 1%. Further heating to 1380 ° C. completely melted the fluoride raw material. As a result, crystals having the same transmittance as in Example 1 were obtained. (Table 1, FIG. 5).

Comparative Example 1 In Example 1, dehydration was performed at 300 ° C., and when the degree of vacuum in the chamber 301 of the refining furnace reached 1 × 10 ⁻⁶ Torr, the reactive gas was sealed at 300 ° C. In this example, the degree of vacuum was 1 × 1
When the pressure reaches 0 ^-6 Torr, the temperature is reduced from 300 ° C. to 1380 ° C. at a rate of about 50 ° C./h without filling the reactive gas.
Heating and heating were performed until the fluoride raw material was dissolved.

When the temperature reached 1380 ° C., a reactive gas was sealed in the chamber 301 of the refining furnace. After sealing the reactive gas, the mixture was maintained for 15 hours, and then the inside of the chamber 301 of the purification furnace was replaced with Ar gas, and cooled as in Example 1, to obtain a calcium fluoride single crystal.

The other points were the same as in the first embodiment.

[0099]

[Table 1]

[0100]

According to the present invention, it is possible to provide a fluoride crystal whose transmittance characteristics are hardly deteriorated even when a high-output light having a short wavelength is repeatedly irradiated for a long time. As a result, it is possible to provide an optical component for an excimer laser with high stability and reliability, and furthermore, an optical system of an exposure apparatus.

[Brief description of the drawings]

FIG. 1 is a flowchart showing a process from a heating process to a cooling process.

FIG. 2 is a flowchart illustrating a process from a dehydration process to a cooling process.

FIG. 3 is a diagram showing a change indicating an atmosphere in a purification step.

FIG. 4 is a flowchart illustrating steps from synthesis of raw materials to assembly of the apparatus.

FIG. 5 is a sectional view of a purification device.

FIG. 6 is a sectional view of a growth apparatus.

FIG. 7 is a sectional view of an annealing apparatus.

FIG. 8 is a graph showing experimental results in the example.

FIG. 9 is a schematic configuration diagram of an exposure apparatus using the fluoride crystal of the present invention as an optical article.

FIG. 10 is a schematic diagram showing a projection optical system of an exposure apparatus using the fluoride crystal of the present invention as an optical article.

FIG. 11 is a schematic view showing an optical system of an excimer laser oscillator using the fluoride crystal of the present invention.

[Explanation of symbols]

 301 Refining furnace chamber, 302 Adiabatic furnace, 303 heater, 304 crucible, 305 Fluoride raw material, 306 Reactive gas source, 307 Inert gas source, 308,309 valve, 310 Crucible withdrawal rod, 311,312 piping, 401 growth Furnace chamber, 402 insulated furnace, 403 heater, 404 crucible, 405 block fluoride raw material, 406 reactive gas source, 407 inert gas source, 408,409 valve, 410 crucible pull down rod, 501 annealing furnace chamber, 502 heat insulation Furnace, 503 heater, 504 crucible, 505 fluoride single crystal, 21 illumination light source section, 22 exposure mechanism section, 23 illumination light source, 24 mirror, 25 concave lens, 26 convex lens, 27 optical integrator, 29 mirror 30 condenser lens, 31 Reticle, 31a reticle holder, 32 projection optical system, 33 wafer, 34 XY stage, 35 surface plate, 36 alignment means, L1 to L11 lens, 81 reflecting mirror, 82 aperture hole, 83 resonator, 84 prism, 85 etalon.

──────────────────────────────────────────────────続 き Continued on front page (51) Int.Cl. ⁶ Identification code FI G02B 1/02 G02B 1/02

Claims (32)

[Claims] 1. A method for heating a fluoride raw material in a reactive gas atmosphere at a temperature lower than the melting point of the fluoride raw material,
A method for producing a fluoride crystal, comprising: melting a fluoride raw material in a vacuum atmosphere or an inert gas atmosphere; and thereafter cooling and crystallizing the fluoride raw material. 2. The method according to claim 1, wherein the reactive gas is a fluorocarbon-based gas. 3. The method according to claim 2, wherein the fluorocarbon-based gas is at least one selected from methane tetrafluoride, methane trifluoride, methane difluoride, ethane hexafluoride, and propane octafluoride. The method for producing a fluoride crystal according to claim 2. 4. The method according to claim 1, wherein the temperature lower than the melting point is a temperature lower by 50 to 200 ° C. than the melting point of the fluoride raw material. . 5. A heating time at a temperature lower than the melting point is 4 to 5.
The method for producing a fluoride crystal according to any one of claims 1 to 4, wherein the time is 30 hours. 6. After disposing the fluoride raw material in a furnace,
Before introducing the reactive gas, the atmosphere in the furnace is changed to a vacuum atmosphere or an inert gas atmosphere, and the inside of the furnace is set to 100 to 300.
The method for producing a fluoride crystal according to any one of claims 1 to 5, wherein dehydration is performed by heating to a temperature of ° C. 7. The method according to claim 6, wherein the reactive gas is introduced into the furnace after the pressure in the furnace is reduced to less than 1 × 10 ⁻⁵ Torr. 8. The method for producing a fluoride crystal according to claim 1, wherein the pressure in the furnace at the time of dissolving the fluoride raw material is set to less than 1 × 10 ⁻⁵ Torr. 9. The method according to claim 1, wherein the reactive gas atmosphere is an atmosphere containing a reactive gas and an inert gas. 10. The fluoride crystal production method according to claim 1, wherein the fluoride raw material contains at least one or more rare earth elements in a total content of 10 ppm. 11. The method according to claim 1, wherein the fluoride raw material is calcium fluoride, barium fluoride or magnesium fluoride. 12. A fluoride crystal produced by the method for producing a fluoride crystal according to claim 1. Description: 13. An optical component obtained by processing and forming the fluoride crystal according to claim 12. 14. Heating the fluoride raw material in a reactive gas atmosphere at a temperature below the melting point, then melting the fluoride raw material in a vacuum atmosphere or an inert gas atmosphere, and then cooling the fluoride raw material A method for purifying a raw material for producing a fluoride crystal, comprising solidifying. 15. The method according to claim 14, wherein the reactive gas is a fluorocarbon-based gas. 16. The carbon-based gas is at least one selected from methane tetrafluoride, methane trifluoride, methane difluoride, ethane hexafluoride, and propane octafluoride. The method for purifying a raw material for producing a fluoride crystal according to claim 15, wherein 17. The temperature lower than the melting point is 50 degrees below the melting point.
The temperature is lower by ~ 200 ° C.
17. The method for purifying a raw material for producing a fluoride crystal according to any one of claims 16 to 16. 18. A heating time at a temperature lower than the melting point is 4 hours.
14. The method according to claim 14, wherein the time is up to 30 hours.
7. The method for purifying a raw material for producing a fluoride crystal according to any one of 6. 19. After the fluoride raw material is placed in the refining furnace, before introducing a reactive gas, the atmosphere in the refining furnace is set to a vacuum atmosphere or an inert gas atmosphere, and the inside of the refining furnace is heated to 100 to 300 ° C. The method for purifying a raw material for producing a fluoride crystal according to any one of claims 14 to 18, wherein the dehydration is carried out by heating the mixture. 20. The pressure in the refining furnace is 1 × 10 ^−5.
20. The method for purifying a raw material for producing a fluoride crystal according to claim 19, wherein the reactive gas is introduced into the purification furnace after the pressure is reduced to less than Torr. 21. The fluoride crystal production according to claim 14, wherein the pressure in the refining furnace at the time of dissolving the fluoride raw material is set to less than 1 × 10 ⁻⁵ Torr. Purification method for raw materials. 22. The purification of a raw material for producing a fluoride crystal according to claim 14, wherein the reactive gas atmosphere is an atmosphere containing a reactive gas and an inert gas. Method. 23. The raw material for producing a fluoride crystal according to claim 14, wherein the fluoride raw material contains at least one or more kinds of rare earth elements in a total content of 10 ppm. Purification method. 24. The method for purifying a raw material for producing a fluoride crystal according to claim 14, wherein the fluoride raw material is calcium fluoride, barium fluoride or magnesium fluoride. . 25. A raw material for producing a fluoride crystal, which is purified by the method for purifying a raw material for producing a fluoride crystal according to any one of claims 14 to 24. 26. A fluoride crystal-producing raw material purified by the purification method according to claim 14 is placed in a growth furnace, and the inside of the growth furnace is set to an inert gas atmosphere or a vacuum atmosphere. Melting the raw material for producing a fluoride crystal,
Next, a method for producing a fluoride crystal, comprising cooling the raw material for producing a fluoride crystal by moving the crucible to grow the crystal. 27. Before melting the raw material for producing a fluoride crystal, the inside of the growth furnace is made a reactive gas atmosphere, and the raw material for producing a fluoride crystal is heated at a temperature lower than the melting point in the reactive gas atmosphere. The method for producing a fluoride crystal according to claim 26, wherein: 28. The method for producing a fluoride crystal according to claim 27, wherein the reactive gas used for setting the growth furnace to a reactive gas atmosphere is a fluorocarbon-based gas. 29. A fluoride crystal produced by the production method according to claim 26. 30. An optical component obtained by processing and molding the fluoride crystal according to claim 29. 31. The optical component according to claim 30, comprising a calcium fluoride crystal having an internal transmittance of 70% or more for light having a wavelength of 135 nm. 32. An exposure apparatus comprising: the optical component according to claim 13; and a stage for mounting a support having a photosensitive material that exposes light transmitted through the optical component.