JP2004262754A - Method for refining fluoride, method for manufacturing fluoride crystal, optical part, and exposure device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for refining a fluoride and a method for manufacturing a fluoride crystal, which are highly versatile and by which the manufacturing cost can be suppressed to a low level; and to inexpensively provide a fluoride crystal whose transmission characteristic is hardly degraded even when it is repeatedly irradiated with a high-output light of a short wavelength for a long term, an optical part and an exposure device. <P>SOLUTION: In the method for refining the fluoride comprising a heating step of heating a solid scavenger-added fluoride raw material to melt the raw material, and a cooling step of cooling the molten fluoride raw material to solidify, the environment of a chamber housing the fluoride raw material is changed to such an environment that a gas in the chamber is discharged to the outside of the chamber more easily than the environment before the change, during the heating step. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention provides a method for purifying a fluoride suitable for various optical elements, lenses, window materials, prisms, and the like used for light having a predetermined wavelength selected from a wide wavelength range from the vacuum ultraviolet region to the far infrared region. And a method for producing a fluoride crystal, furthermore, an optical component, and an exposure apparatus using the same.

  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. Among them, fluorite (calcium fluoride) having excellent transmission characteristics at short wavelengths is useful as an optical component for an excimer laser. 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.

In such a fluoride crystal, in order to increase the bulk density of the raw material and remove impurities of the raw material, a step of melting and purifying the raw material is required. In this purification step, a scavenger, which is a metal fluoride, must be added to the raw material in order to remove oxides generated by the raw material reacting with moisture or the like and impurities in the raw material. For example, when the fluoride crystal is calcium fluoride and the scavenger is solid ZnF ₂ , CaO produced by reacting the raw material with moisture reacts with ZnF ₂ to become CaF ₂ , and the scavenger becomes ZnO and melts the raw material. It evaporates.

  In order to obtain a high-quality fluoride crystal having excellent optical performance, the above-described purification step must be repeated a plurality of times. In addition, a high-purity raw material with few impurities must be used as the raw material.

  When a crystal is manufactured using the fluoride crystal block thus obtained as a raw material, a fluoride crystal having very excellent optical performance such as transmission characteristics can be obtained.

  On the other hand, there is a method using a gas scavenger instead of a solid scavenger. As a reactive gas that can be used as a gas scavenger, a hydrogen fluoride gas or a fluorocarbon-based gas is used.

However, since the solid scavenger contains a metal element, it may remain in the crystal and adversely affect the transmission characteristics. Therefore, if the amount of the solid scavenger added is reduced or if the degassing property is improved by making a hole in the crucible lid, the metal element from the scavenger will not easily remain in the crystal. However, if the amount is too small, the effect as a scavenger decreases. Also, if a large hole is opened in the crucible lid to increase the degassing property too much, the scavenger will be released outside the crucible before exerting its effect. In this way, the improvement of the transmission characteristics is hindered by mixing of the oxide into the fluoride. Therefore, it is important to determine the optimal amount of scavenger to be added, or to design a crucible shape that optimizes degassing properties.However, these methods vary depending on the water concentration and impurity content in the raw material, and the method is generally used. Lack of sex.

  On the other hand, among the gas scavengers, hydrogen fluoride gas is corrosive, and sulfur tetrafluoride and boron trifluoride are toxic. Therefore, the management cost increases. Also, the gas often dissolves in the melt and is taken in as bubbles in the crystal (Guggenheim, J. Appl. Phys. 34, P.2482-2485 (1963)), and it is difficult to obtain satisfactory transmission characteristics. .

Also, if a fluorocarbon-based gas such as methane tetrafluoride (CF ₄ ) is used as a reactive gas, crystals having excellent permeation characteristics can be obtained, but these gases are very stable in the atmosphere. . In addition, since these gases are greenhouse gases that cause global warming like carbon dioxide, they must be decomposed by plasma to take measures such as harm, which also increases the management cost.

  An object of the present invention is to provide a fluoride purification method and a fluoride crystal production method which are versatile and can keep production costs low.

  Another object of the present invention is to provide an inexpensive fluoride crystal, an optical component, and an exposure apparatus whose transmittance characteristics are hardly degraded even when repeatedly irradiated with short-wavelength, high-output light for a long period of time. is there.

The method for purifying fluoride of the present invention is a heating step of heating the molten raw material to which the solid scavenger is added, loaded in a crucible installed in a refining furnace, and maintaining a dissolved state,
A cooling step of cooling to solidify the dissolved fluoride raw material, and a fluoride purification method comprising:
In the middle of the heating step, a second environment in which the crucible communicates with the purification furnace from a first environment in which an opening connecting the inside of the crucible provided in the crucible and the inside of the purification furnace is closed. The environment is changed.

  The present inventor seeks to reduce the enormous costs as described above, and seeks to increase the optical performance by using inexpensive and general raw materials and solid scavengers, while storing the fluoride raw materials in the purification process. I noticed that the environment of the room (eg, crucible) was very important. That is, during the heating step, if the environment of the room containing the fluoride raw material is changed to an environment in which the gas in the room is more easily released to the outside of the room than the environment, the scavenger stays in the crucible during the heating of the raw material, and the After melting, all scavengers and reaction products not used in the reaction after melting go out of the crucible, maximizing the effect of the scavenger, and achieving extremely high purity. Was obtained. Then, it was found that a crystal having excellent optical performance could be obtained by using it.

Note that the expression “atmosphere” is sometimes used as an expression having the same meaning as the expression “environment”, but the expression “atmosphere” is used hereinafter in the text.
Changing the environment, that is, changing the atmosphere, means changing the gas in the crucible into a state in which the gas in the room is easily released to the outside.

More specifically, the first meaning is to reduce the pressure outside the crucible chamber to change the gas in the room to a state where it is easily released to the outside, and the second meaning is to provide the crucible to the crucible. This means that the opening / closing means is changed to the open state, or, in the third sense, the inert gas supplied to the outside of the room is exhausted and the pressure is reduced, so that the gas in the room is taken out of the room. This means that the state is changed to a state that is easily released.
Further, a combination of at least any two of the above three meanings is also included in the meaning of changing the environment in the present invention.

  According to the present invention, the cost can be kept low because an inexpensive solid scavenger is used.

  Further, since the purification ability is improved, even if a raw material having a large amount of impurities is used, a fluoride having desired characteristics can be obtained, so that the cost can be kept low.

  Furthermore, the cost can be kept low since the number of purifications does not need to be increased.

  In addition, a fluoride crystal, an optical component, and an exposure apparatus having desired high durability and excellent transmittance characteristics can be provided at low cost.

    FIG. 1 shows a flowchart of a method for purifying fluoride according to one embodiment of the present invention.

  In step S1, 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 S2, at a predetermined timing, the environment of the room (for example, a crucible) in which the fluoride is stored is changed to an environment in which the gas in the room is relatively easily discharged outside the room.

  Heating is continued even after changing to an environment in which indoor gas is likely to be released outside, so that reaction products such as carbon dioxide and vaporized scavengers are not taken into the raw material.

  In step S3, 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 raw 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. The temperature may be increased intermittently as shown by reference numeral 2 in FIG.

  The timing of the environment change is after the temperature reaches the start temperature T1 of the impurity removal reaction (scavenge reaction) by the scavenger and before the start of cooling. That is, it is between time t1 and time t2 in FIG.

  Preferably, after reaching the scavenge reaction start temperature T1, it is desirable to start the environment change some time later, and to end the environment change sufficiently before the start of cooling. That is, it is preferable to start changing the environment between times t3 and t4 in FIG. Note that t3 is the time (t3) at which the impurity removal reaction by the scavenger is completed, 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 environment at a temperature equal to or higher than the melting point is preferable from the viewpoint of obtaining a fluoride having higher purity. It is to be noted that the period from t4 to t2 is an environment state in which the gas is likely to be released outside 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 scavenge reaction depends on the amount of 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 experiments or the like, and the time may be determined based on the determined time.

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

  In the first environment, it is difficult for the gas to be released outside the room, so that it is difficult for the solid scavenger to vaporize and escape outside the room. Therefore, the scavenging reaction proceeds efficiently.

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

  A specific example of the environment change is that the pressure of the second environment is lower than the pressure of the first environment, or the state where the inside and outside of the room are closed (closed state) to the state where the inside and outside of the room are communicated (open state). And so on.

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

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

  The pressure of the first environment is not particularly limited, but is preferably 1.3 Pa or more, more preferably 1 atm (about 101.325 kPa) or more. In the present invention, one atmosphere is 760 Torr. 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 environment 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 environment may be set so that a part of the raw material itself does not vaporize in large quantities.

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

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

  After a while after moving to the second environment, 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 cooling rate is preferably 3 to 4 ° C./h.

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

  As described above, the present invention can sufficiently remove substances that deteriorate light transmittance, such as oxygen and constituent metal elements of a scavenger, in the purification step, so that in the crystal manufacturing step subsequent to the purification step, In a crystal growth step that requires strict control for growth, the operation of removing the substance can be simplified.

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

  The fluoride raw material used in the present invention is, for example, calcium fluoride, barium fluoride, magnesium fluoride and the like.

  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 preferably from 0.1 mol% to 1 mol% of the fluoride raw material.

  FIG. 3 shows a flowchart of a method for purifying fluoride and manufacturing a fluoride crystal according to another embodiment of the present invention.

(Raw material preparation step S11)
Add solid scavenger to fluoride raw material and mix well. The amount of the solid scavenger added is from 0.1 mol% to 1 mol% of the raw material. 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 S12)
The fluoride raw material to which the solid scavenger has been added and mixed is put into the crucible of the refining furnace shown in FIG. In FIG. 4, reference numeral 201 denotes a chamber of a refining furnace, which is connected to a vacuum exhaust system. 202 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. Reference numeral 206 denotes a mechanism for moving the crucible up and down. The crucible opening / closing mechanism 207 is fixed to the heat insulating material 202, and opens and closes the upper crucible by moving the crucible up and down, and controls the size of the opening. Reference numeral 208 denotes an inert gas introduction device.

(1) When using an inert gas After the inside of the furnace is evacuated while the opening is opened (S21), the inside of 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 increased to a temperature at which the impurity removal reaction by the scavenger is performed. The reaction is, for example, when PbF2 is used for the scavenger,
CaO + PbF ₂ → CaF ₂ + PbO
PbO + C → Pb + CO
It is expressed by the formula. 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 inside of the furnace is evacuated to a vacuum at the temperature at which the reaction is completed, and then the raw materials are completely melted (S24). 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).

(2) Opening and Closing the Crucible Opening The crucible is evacuated with the opening closed (S31), and the heater is energized to heat the crucible (S32). In order to sufficiently promote the impurity removal reaction by the scavenger, the rate at which the raw material is heated must be reduced 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 material. Alternatively, after the raw material is completely melted (S33), 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 a grain boundary, precise temperature control is not required. In addition, it is preferable to lower the crucible during the slow cooling. By lowering, the removal of impurities is further improved.

  Of the crystals thus obtained, particularly the upper part, 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 S13)
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 placed in the crucible of the growth furnace shown in FIG. The crucible of the growth furnace is different from the crucible of the refining furnace. In FIG. 5, 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 to the external environment. The fluoride raw material obtained by cooling in the refining furnace is taken out of the crucible of the refining furnace, moved to a clean crucible of the growth furnace, and stored therein. As a result, the impurities removed from the fluoride raw material in the refining furnace and adhered to the wall of the refining furnace crucible do not re-enter the fluoride raw material that melts during crystal growth.

  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. The lowering speed of the crucible is preferably 0.1 to 5.0 mm per hour.

  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. In the growth step, the operation of changing the atmosphere outside the crucible as described in the purification step may be performed. In this case, by putting the solid scavenger in the growth crucible, the operation of removing impurities may be further performed in the growth step, or the operation of removing impurities may be performed without using the solid scavenger. Further, the operation of changing the atmosphere may be performed only in the growth step.

(Annealing step S14)
Subsequently, the crystal-grown fluoride single crystal is heat-treated in an annealing furnace shown in FIG.
6, 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.

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

(Formability S15)
Thereafter, it is formed into a required optical article shape (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 also excellent.

By combining the lenses thus obtained in various ways, an optical system having high durability suitable for high energy laser light such as an excimer laser, particularly an ArF excimer laser or an F ₂ excimer laser can be constituted. In particular, when the fluoride crystal is calcium fluoride, an exposure apparatus can be configured by combining an optical system having an excimer laser light source and a lens made of a calcium fluoride crystal, and a stage capable of moving a substrate as an object to be exposed. .

(Exposure equipment)
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 1: 1 projection exposure apparatus.

  In particular, in order to expose the entire surface of the wafer, a stepper adopting a step-and-repeat method that exposes one small section (field) of the wafer, moves the wafer by one step, and exposes one adjacent field is desirable. . Of course, the present invention is also suitably used for a microscan type exposure apparatus.

FIG. 7 shows a schematic diagram of the configuration 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. The illumination light source unit 21 includes a laser 23 to an optical integrator 28. Reference numeral 29 denotes a mirror, and reference numeral 30 denotes a condenser lens for collimating a light beam emitted from the optical integrator 28.
Reference numeral 31 denotes a reticle on which a circuit pattern is drawn; 31a, a reticle holder for attracting and holding the reticle; 32, a projection optical system for projecting a reticle pattern; and 33, a projection lens 32 on which a pattern of the reticle 31 is printed. Reference numeral 34 denotes an XY stage which holds the wafer 33 by suction and moves in the XY directions when performing printing by step-and-repeat. Reference numeral 35 denotes a surface plate of the exposure apparatus.

  The exposure mechanism 22 includes a part from the mirror 29 which is a part of the illumination optical system to 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. 8 is an example of an optical article used in the exposure apparatus of the present invention, and is a lens used in the projection optical system of the exposure apparatus shown in FIG. This lens assembly is configured 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. 7 and 8 or, although not shown, as a mirror or a lens of a mirror type exposure apparatus. More preferably, an antireflection film or an enhanced reflection film is provided on the surface of the lens or the mirror.

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

  FIGS. 9A and 9B are diagrams schematically showing a configuration of an excimer laser oscillator using an optical part crystal made of a fluoride crystal of the present invention.

  The excimer laser oscillator shown in FIG. 9A includes 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 single wavelength excimer laser. And a reflecting mirror 81 for reflecting the excimer laser.

  The excimer laser oscillator shown in FIG. 9B has a resonator 83 for emitting and resonating the excimer laser, a diaphragm hole 82 for narrowing the excimer laser emitted from the resonator 83, and a single wavelength excimer laser. And a reflecting mirror 81 for reflecting excimer laser light.

  The excimer laser light oscillator provided in the apparatus as an optical crystal composed of a fluoride crystal of the present invention as a prism or an etalon can narrow the wavelength of the excimer laser via the prism or the etalon. A single wavelength can be obtained.

By irradiating the photosensitizing resist on the substrate with the excimer laser light via the reticle pattern using this exposure apparatus, a latent image corresponding to the pattern to be formed can be formed.
In the present invention, the crucible used for purification is a space for accommodating the above-mentioned fluoride raw material, in other words, in addition to a crucible having one chamber, for example, a form having at least two chambers. You may.

A crucible having at least two or more chambers is shown in FIGS. 10A and 10B.
In FIGS. 10A and 10B, a plurality of crucibles 1000 overlap each other, and the spaces of the crucibles are independently partitioned by a partition plate portion 1030 corresponding to the bottom surface of each crucible 1000. A lid 1010 is provided to close the opening of the uppermost crucible 1000.
The fluoride raw material purified by these multi-stage crucibles takes the shape of each chamber and takes the shape of a disk. The disc-shaped fluoride raw material is easy to carry, and because of its small size, is easily formed.
Further, these multi-stage crucibles have an advantage that heat transmitted from the outside of the crucible is transmitted to the partition plate 1030 and easily transmitted to the center of the space, and the fluoride raw material can be purified while effectively performing heat control.

In FIG. 10A, each crucible 1000 has a configuration in which gaseous impurities can be exhausted from the crucible chamber to the outside of the depressurized crucible through a small gap 1040 between another crucible 1000 and a lid 1010 which are overlaid. Has become.
Further, in FIG. 10B, the crucible has two holes 1020 symmetrically with respect to the center of the crucible on the side wall portion, and gaseous impurities can be easily removed through the holes 1020 than in the crucible shown in FIG. 10A. The pressure can be exhausted from the crucible chamber to the outside of the crucible whose pressure has been reduced.
Opening / closing means (not shown) may be provided in this hole to control opening and closing of the hole as appropriate.
Further, the crucible 1000 and the partition plate portion 1030 may be configured to be detachable from each other. In this case, the crucible 1000 may be cylindrical.

  The hole 1020 provided in the side wall portion of the crucible 1000 may be circular or other shapes. The position of the hole can be set as appropriate other than being provided at the upper part of the side wall portion. In addition, the number of holes may be appropriately determined as needed in addition to the two holes as shown in the drawing. When the number of holes is plural, each hole may be positioned at a symmetrical position with respect to the center of the crucible, or may be positioned at an asymmetrical position.

  Hereinafter, the present invention will be described in more detail with reference to examples.

Zinc fluoride was added as a scavenger to a general synthetic calcium fluoride raw material having a purity of 99% at a concentration of about 0.5 mol% with respect to calcium fluoride, followed by mixing. Next, the mixture was put into a crucible of a refining furnace shown in FIG. 4, and the inside of the furnace and the crucible were evacuated. Then, Ar was filled to a pressure of 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 inside of the furnace was evacuated to a vacuum, and the temperature was maintained at 2.66 × 10 ⁻⁴ Pa and the temperature was 1360 ° C. for 11 hours. Then, the growth crucible was lowered at a rate of 2 mm / h. The diameter of the obtained fluoride single crystal can be appropriately changed according to the inner diameter of the crucible of the single crystal growth furnace. For example, a single crystal having a diameter of 230 mm, 300 mm or more can be obtained.

  Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of zinc fluoride were put. The furnace was evacuated to raise the temperature of the crucible from room temperature to 900 ° C. at a rate of 100 ° C./h, and then kept at 900 ° C. for 20 hours. 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 transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 7 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 8 shows a transmission spectrum after gamma ray irradiation. As a result, a calcium fluoride crystal having excellent transmission characteristics in the vacuum ultraviolet region and having extremely little deterioration due to gamma ray irradiation was obtained.

The internal transmittance was measured with a vacuum ultraviolet spectrophotometer. The internal transmittance is a denominator of the amount of light entering the medium, that is, the fluoride, and is a percentage when the amount of light when the light travels a unit distance in the medium is a numerator. It is believed that the difference between the molecular values is caused by the absorption in the medium by the material constituting the medium. The deterioration rate was measured by irradiating a gamma ray of 1 × 10 ⁵ R / H for 1 hour and measuring the transmittance before and after the irradiation with a visible region spectrophotometer.

Zinc fluoride was added as a scavenger to a general synthetic calcium fluoride raw material having a purity of 99% at a concentration of about 0.5 mol% based on calcium fluoride. Next, this mixture was put into a crucible of a refining furnace shown in FIG. 4, and the furnace and the crucible were evacuated. At this time, the opening of the crucible was closed to reduce the degassing property. After the crucible was heated to 1360 ° C. to melt the raw materials, the crucible was moved to open the opening to increase the degassing property, and the degree of vacuum was set to 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.

  Subsequent steps were the same as in Example 1.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, as in the case of the first embodiment, a calcium fluoride crystal having excellent transmission characteristics in the vacuum ultraviolet region and having extremely little deterioration due to gamma ray irradiation was obtained.

Zinc fluoride was added as a scavenger to a general synthetic calcium fluoride raw material having a purity of 99% at a concentration of about 0.5 mol% based on calcium fluoride. Next, this mixture was put into a crucible of a refining furnace shown in FIG. 4, and the furnace and the crucible were evacuated. After heating the raw material to a temperature at which the scavenge reaction occurs, the crucible was moved to open the opening to increase the degassing property. Thereafter, the raw material was heated to the melting point or higher, and the degree of vacuum at the time of melting was set to 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.

  Subsequent steps were the same as in Example 1.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, as in the case of Example 1, a calcium fluoride crystal having excellent transmission characteristics in the vacuum ultraviolet region and having extremely little deterioration due to gamma ray irradiation was obtained.

(Comparative Example 1)
Zinc fluoride was added as a scavenger to a general synthetic calcium fluoride raw material having a purity of 99% at a concentration of about 0.5 mol% with respect to calcium fluoride, followed by mixing. Next, this mixture was put into a crucible of a refining furnace shown in FIG. 4, and the inside of the furnace was evacuated. At this time, the crucible was opened so that the degassing property was enhanced. After the crucible was heated to 1360 ° C. to melt the raw material, 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.

  Subsequent steps were the same as in Example 1.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, calcium fluoride was obtained, which had excellent transmission characteristics in the vacuum ultraviolet region, but was greatly deteriorated by gamma ray irradiation.

(Comparative Example 2)
Zinc fluoride was added as a scavenger to a general synthetic calcium fluoride raw material having a purity of 99% at a concentration of about 0.5 mol% with respect to calcium fluoride, followed by mixing. Next, this mixture was put into a crucible of a refining furnace shown in FIG. 4, and the inside of the furnace was evacuated. At this time, the opening of the crucible was closed to reduce the degassing property. After the crucible was heated to 1360 ° C. to melt the raw material, 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.

  The subsequent steps were the same as in Example 1.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, a transmission characteristic on the short wavelength side of less than 170 nm was reduced, and a calcium fluoride crystal which was greatly deteriorated by gamma ray irradiation was obtained.

(Comparative Example 3)
A calcium fluoride crystal was obtained in the same manner as in Comparative Example 1, except that a high-purity synthetic calcium fluoride raw material powder was used. Table 1 shows the difference in impurity concentration between the high-purity raw material and the general synthetic raw material. The price of a high-purity raw material is ten times that of a general synthetic raw material.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, despite the use of a high-purity synthetic calcium fluoride raw material, calcium fluoride crystals having excellent transmission characteristics in the vacuum ultraviolet region but being deteriorated by gamma ray irradiation were obtained.

(Comparative Example 4)
A calcium fluoride crystal was obtained in the same manner as in Comparative Example 2 except that a high-purity synthetic calcium fluoride raw material was used.

  The calcium fluoride crystal thus obtained was cut and polished to obtain a disk having a thickness of 10 mm, and the transmittance spectrum in the vacuum ultraviolet region and the deterioration rate of the internal transmittance due to gamma ray irradiation were measured. FIG. 11 shows a transmission spectrum in a vacuum ultraviolet region, and FIG. 12 shows a transmission spectrum after gamma ray irradiation. As a result, despite the use of a high-purity synthetic calcium fluoride raw material, a calcium fluoride crystal in which the transmission characteristics in the vacuum ultraviolet region decreased on the short-wave side from 170 nm and deteriorated by gamma ray irradiation was obtained.

4 is a flowchart of a fluoride purification process according to the present invention. It is a graph which shows the temperature program in a purification process. 5 is a graph showing the degassing property of the crucible, that is, the allowable range of the timing for switching the environment. It is a flowchart for explaining the example of a manufacturing process. It is a schematic diagram which shows the cross section of the refinery furnace used for a refinement | purification process. It is a schematic diagram which shows the cross section of the growth furnace used for a single crystal growth process. It is a schematic diagram which shows the cross section of the annealing furnace used for an annealing process. FIG. 2 is a schematic diagram of an exposure apparatus using the fluoride crystal of the present invention as an optical article. It is a schematic diagram which shows the projection optical system of the exposure apparatus using the fluoride crystal of this invention as an optical article. FIG. 1 is a schematic diagram showing an optical system of an excimer laser oscillator using a fluoride crystal of the present invention. FIG. 10A is a schematic view showing a cross section of another refining furnace used in the refining process, FIG. 10A is a multi-stage crucible, and FIG. 10B is a multi-stage crucible having a hole in a side wall. It is a graph which shows the transmission spectrum of the calcium fluoride crystal produced on various conditions. 5 is a graph showing transmission spectra of calcium fluoride crystals prepared under various conditions after gamma ray irradiation.

Explanation of reference numerals

21 Illumination light source part,
22 Exposure mechanism,
23 illumination light source,
24 mirrors,
25 concave lens,
26 convex lens,
27 Optical Integrator,
29 mirrors,
30 condenser lens,
31 reticles,
31a reticle holder,
32 projection optics,
33 wafers,
34 XY stage,
35 surface plate,
36 alignment means,
L1 to L11 lenses,
81 Reflector,
82 aperture hole,
83 resonators,
84 prism,
85 etalons,
201 Refining furnace chamber,
202 insulation,
203 heater,
204 crucibles,
205 fluoride,
206 crucible lowering mechanism,
207 crucible opening and closing mechanism,
208 gas introduction device,
301 Growth furnace chamber,
302 insulation,
303 heater,
304 rooms (crucible),
305 fluoride,
306 crucible lowering mechanism,
401 annealing furnace chamber,
402 insulation,
403 heater,
404 crucible,
405 Fluoride crystal 1000 Crucible 1010 Lid 1020 Hole 1030 Partition plate 1040 Gap

Claims (13)

Heating to dissolve the fluoride raw material to which the solid scavenger is added, loaded in the crucible installed in the refining furnace, and a heating step of maintaining the dissolved state;
A cooling step of cooling to solidify the dissolved fluoride raw material, and a fluoride purification method comprising:
In the middle of the heating step, a second environment in which the crucible communicates with the purification furnace from a first environment in which an opening connecting the inside of the crucible provided in the crucible and the inside of the purification furnace is closed. A method for purifying fluoride, wherein the method is changed to an environment as described above. The method for purifying fluoride according to claim 1, wherein the pressure in the refining furnace is equal in the first environment and the second environment. The method for purifying fluoride according to claim 1, wherein the pressure in the purification furnace is lower in the second environment than in the first environment. The method for purifying fluoride according to claim 3, wherein the pressure of the refining furnace is reduced in the second environment as compared with the first environment by increasing the evacuation capacity of the refining furnace. The method for purifying fluoride according to claim 3 or 4, wherein the pressure in the refining furnace in the first environment is 1 atm or more. The method according to any one of claims 1 to 5, wherein the pressure in the refining furnace in the second environment is ^10-3 Pa or less. The method for purifying fluoride according to claim 1, wherein the change from the first environment to the second environment is performed after a scavenger in the crucible is melted. The method for purifying fluoride according to claim 1, wherein the change from the first environment to the second environment is performed after the fluoride raw material in the crucible is melted. The method for purifying a fluoride according to any one of claims 1 to 8, wherein in the cooling step, the crystal is grown to grow simultaneously with the purification. 10. The method for purifying fluoride according to claim 1, wherein the crucible is internally partitioned into a plurality of spaces. A method for producing a fluoride crystal, comprising melting a fluoride refined by the purification method according to any one of claims 1 to 10 and growing the crystal to obtain the fluoride. An optical component obtained by molding the fluoride crystal obtained by the manufacturing method according to claim 11. An exposure apparatus comprising: an optical system having the optical component according to claim 12; an excimer laser light source; and a stage for mounting an object to be exposed.

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