JP2005330155A - Method for producing crystalline substance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide fluorite more improved in uniformity of refractive indices. <P>SOLUTION: The production method employs a crystal production apparatus provided with a fluid circulation path for changing temperature near a seed crystal to define temperature control in the vicinity of the seed crystal, and a temperature sensor for observation is provided on the periphery of the fluid circulation path in a crucible descending method, where the length of the seed crystal is elongated, or the joint part between the seed crystal and a crystallization part is processed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an apparatus for producing fluorite that is suitable for various optical elements, lenses, window materials, prisms and the like used in a wide wavelength range from a vacuum ultraviolet region to a far infrared region.

  Excimer lasers are attracting attention as the only high-power lasers that oscillate in the ultraviolet region, and are expected to be applied in the electronics, chemical and energy industries.

  Specifically, it is used for processing, chemical reaction, etc. of metals, resins, glass, ceramics, semiconductors and the like. In recent years, the development of a light source for ultrafine lithography that makes use of short wavelength characteristics has been particularly advanced.

  In the lithography process, a method of transferring a pattern drawn on a mask onto a wafer with a lens is mainly performed. This is called optical lithography. In general, the resolution of a transfer pattern increases in proportion to the numerical aperture (NA) of a lens and the inverse of the wavelength of light. However, there is a manufacturing problem regarding the numerical aperture of the lens, and it is effective to shorten the wavelength of light in order to increase the resolution. For this reason, as a light source for photolithography, g-line (436 nm) to i-line (365 nm), and KrF excimer laser (248 nm) have been realized. A reduction projection type exposure apparatus using a KrF excimer laser as a light source has achieved a resolution of 0.23 μm.

  In an apparatus using a KrF excimer laser or a light having a shorter wavelength, particularly, a wavelength of 200 nm or less, that is, a so-called vacuum ultraviolet wavelength region as a light source, it can be polished with a large aperture as an optical material used for this. Conventionally, it has been considered that such synthetic quartz glass is suitable. As the crystal, lithium fluoride crystal, magnesium fluoride crystal, and calcium fluoride crystal (fluorite) can be considered. However, the lithium fluoride crystal is not practical because it has significant deliquescence and is difficult to polish. In addition, since the magnesium fluoride crystal is a biaxial crystal, it has an optically anisotropic property and a birefringence phenomenon occurs. For this reason, it can be used for polarizing elements such as polarizing prisms utilizing the birefringence phenomenon, and optical materials that do not require high imaging performance such as vacuum window materials, but such as lenses and prisms used in optical lithography. It is not suitable as an optical material that requires high imaging performance. Fluorite crystals do not show deliquescence and optical anisotropy, and have been regarded as promising as being excellent UV transmissive materials and usable in precision optical systems.

  Since it is used as an optically transparent material for fluorite crystal, single crystallizing, improvement of homogeneity of refractive index, and reduction of birefringence (distortion) are particularly serious problems.

  As a conventional fluorite growth method, there is a method called a Bridgman method in which a melt is sequentially solidified from one end by moving in a furnace having a temperature distribution. InP crystals and GaAs crystals used for optical devices are also produced by the above growth method.

Control of the temperature gradient in the solidification direction, that is, the temperature gradient in the vicinity of the solid-liquid interface where the melt is solidified and becomes a solid, and the crystal growth direction are important for single crystallization and birefringence (strain) reduction. Fluorite is known to have a lower birefringence when light is transmitted from the (111) direction of the crystal than when light is transmitted from other surface directions. Therefore, when producing a crystal, growth is performed using a seed crystal having a normal plane orientation (111).
Japanese Patent Laid-Open No. 9-315893 JP 2000-191322 A

  However, in the Bridgman method, the seed crystal is arranged in the crucible as shown in FIG. 1, and the crystal is solidified from the bottom, so the seed crystal must be arranged below the crystal part. Making the plane orientation of the seed crystal the same as the plane orientation of the crystal, that is, the connection of the seed crystal, cannot be realized with good reproducibility unless the temperature in the vicinity of the seed crystal is precisely controlled. In particular, if the temperature is raised too much, the seed crystal is completely melted, so the desired plane orientation cannot be obtained.

  Since the temperature in the vicinity of the seed crystal is governed by the temperature distribution in the entire furnace, if the temperature of the seed crystal is controlled precisely, the temperature control range in the furnace becomes small. Figure 2 shows the temperature distribution near the seed crystal connection. Ideally, it is desirable that the temperature of the unmelted portion is considerably lower than that of the molten portion. In the present invention, by disposing a fluid in the vicinity of the seed crystal, the control temperature range of the temperature of the seed crystal is widened, so that the unmelted portion of the seed crystal is increased, the temperature range of the connection between the seed crystal and the crystal, and Increase the probability.

  FIG. 3 shows the seed crystal temperature control apparatus of the present invention. 14 is a fluid circuit. The fluid is monitored by temperature sensor 15a. The temperature and flow rate of the fluid are controlled so that the temperature sensor has a desired temperature. This makes it possible to increase the temperature control range in the vicinity of the connectable seed crystal. In addition, the temperature can be controlled in a wider range by changing the type of fluid.

  An object of the present invention was made in view of the above-mentioned unsolved problems of the conventional techniques, and an object of the present invention is to provide a fluorite in which the homogeneity of the refractive index and the reduction of the birefringence are further improved. did.

  Still another object of the present invention is to provide a fluorite suitable for an optical component for an excimer laser, particularly an optical component for an excimer laser of an exposure apparatus for photolithography.

  Still another object of the present invention is to provide fluorite that can be a highly reliable optical article.

  Still another object of the present invention is to provide fluorite that can be produced at a relatively low cost.

  Still another object of the present invention is to provide an optical component for an excimer laser that does not deteriorate its optical characteristics even when it is repeatedly irradiated with high-power light at a short wavelength for a long period of time.

  The present invention relates to a fluorite having a plane orientation (111) and having a birefringence of 2 nm or less by increasing the length of a seed crystal or processing a connecting portion between a seed crystal and a crystal part in a crucible descent method It is characterized by manufacturing.

  According to the present invention, the following various effects can be achieved.

  It is possible to provide a fluorite with improved refractive index homogeneity.

  Fluorite with less birefringence (distortion) can be provided.

  A fluorite suitable for an optical component for excimer laser, particularly for an optical component for excimer laser of an exposure apparatus for photolithography can be provided.

  Fluorite that can be a reliable optical article can be provided.

  A fluorite that can be produced at a relatively low cost can be provided.

  It is possible to provide an optical component for an excimer laser in which optical characteristics are not deteriorated even when repeatedly irradiated with high-power light at a short wavelength for a long period of time.

  The present invention can obtain the same effect not only in the conventional Bridgman method but also in the Bridgman method using a multi-stage crucible having slits in the crucible.

  The present inventor manufactured many fluorites by changing the production conditions at the time of fluorite production.

  FIG. 4 shows a flowchart of a preferred manufacturing process example of the present invention.

  First, a fluoride raw material is prepared in order to mix the hooker raw material and the scavenger. For that purpose, calcium carbonate and hydrogen fluoride are prepared, and these calcium carbonate and hydrogen fluoride are reacted to synthesize powdered calcium fluoride.

  Calcium fluoride is produced by the following reaction.

CaCO ₃ + 2HF → CaF ₂ + H ₂ O + CO ₂
In this synthesis, it is preferable to dry the CaF ₂ generated by the above reaction and then calcinate to remove moisture.

  The calcium fluoride raw material thus obtained is vacuum packed so as not to be exposed to the atmosphere as much as possible.

  Then, calcium fluoride and scavenger are mixed. At this time, calcium fluoride and a scavenger may be placed in a container and mixed by rotating the container.

  As the scavenger, zinc fluoride, bismuth fluoride, sodium fluoride, lithium fluoride, or the like that is more easily bonded to oxygen than the growing fluoride is desirable.

  A substance that reacts with the oxide mixed in the synthetic fluoride raw material to become an oxide that is easily vaporized is selected. Zinc fluoride is particularly desirable.

  For example, a zinc fluoride scavenger changes calcium oxide generated by the presence of moisture into calcium fluoride.

CaF ₂ + H ₂ O → CaO + 2HF (300 ° C)
CaO + ZnF ₂ → CaF ₂ + ZnO ↑
The addition rate of the scavenger is 0.05 mol% or more and 5.00 mol% or less, more preferably 0.1 to 1.0 mol%.

  The calcium fluoride powder and scavenger mixture thus obtained is placed in a crucible of a refining furnace. Thereafter, the heater is energized to melt the mixture. Subsequently, the crucible is lowered by the above-mentioned Bridgman method to grow crystal of molten calcium fluoride.

(Purification process)
This step does not require the temperature control as the single crystal growth step described later. Therefore, there may be a grain boundary of the obtained crystal.

  The upper part of the crystal thus obtained, that is, the last crystallized part with time is removed. Since this portion tends to concentrate impurities, impurities that adversely affect the characteristics are removed by this removal step.

  This crystal is again put in the crucible, and a series of steps of melting, crystallization, and removal of the upper part is repeated a plurality of times.

(Single crystal manufacturing process)
The fluoride purified in the purification process is melted by heating the crucible from about 1390 to about 1450 ° C., and then gradually cooled. In this slow cooling, it is preferable that the crucible is lowered at a speed of 0.1 to 5.0 mm per hour to cool slowly.

(Annealing process)
The crystal-grown fluoride single crystal is heat-treated in an annealing furnace. In this step, the crucible is heated to 900-1300 ° C. The heating time is 20 hours or more, more preferably 20 to 30 hours.

  The fluoride single crystal thus obtained can have oxygen of 25 ppm or less and undesirable amounts of impurities such as water, iron (Fe), nickel (Ni) and chromium (Cr), respectively, of 10 ppm or less.

  The fluoride single crystal obtained by the above process is subjected to refractive index homogeneity measurement and birefringence (strain) measurement.

  Embodiments of the present invention will be described below with reference to the drawings. The region for storing the raw material in the apparatus of the illustrated embodiment may be other than the case illustrated.

  In FIG. 1, this apparatus includes a casing 5 forming a furnace chamber 6 and a side heater 3a made of graphite disposed in the furnace chamber. The side heater is supplied with power from a side heater power source 7. The crucible 1 is made of graphite or platinum. Further, a heat insulating member 4 made of graphite is installed inside the case 5 to protect the case 5 from high heat. A crucible support rod 2 is installed so as to penetrate the casing 5 from the lower side of the casing, and supports the crucible 1. The crucible lifting / lowering motor 2a is supplied with a current from the crucible lifting / lowering power supply 2b, and its power is controlled by the lifting / lowering control device 9. The temperature environment in the furnace is managed by the temperature sensor 15b.

  The seed crystal storage unit 12 is disposed below the crucible 1. A fluorite seed crystal is placed here. All heater parts are made of carbon or molybdenum. The structure near the seed crystal is shown in FIG. The fluid is introduced through the fluid circuit 14. The temperature sensor 15a is arranged at the boundary between the seed crystal melting portion and the unmelted portion, and controls the temperature and flow rate of the fluid so as to reach a desired temperature.

  Calcium carbonate and hydrofluoric acid were reacted to obtain powdered calcium fluoride.

This and ZnF ₂ were added at 0.7% by weight with respect to calcium fluoride, and both were mixed.

  Next, this mixture was put in a crucible of a refining furnace, heated to 1450 ° C., and then cooled to obtain molten calcium fluoride.

Next, the block was placed in the crucible of the single crystal growth furnace of FIG. As a scavenger, ZnF ₂ was added to the crucible in an amount of 0.1% by weight of calcium fluoride.

  The seed crystal was placed in the seed crystal storage section of FIG. 3 using the seed crystal with the surface orientation (111) as the top surface.

  No fluid was introduced near the seed crystal. The temperature sensor of the fluid circuit at that time was 1450 ° C.

The degree of vacuum in the furnace was 6 × 10 ⁻⁴ Torr. The temperature was raised from room temperature to 1450 ° C., and the degree of vacuum was 2 × 10 ⁻⁶ Torr and the temperature was 1450 ° C. and maintained for 11 hours.

  Next, the crucible was lowered at a speed of 2 mm / h. The temperature drop rate at this time corresponds to about 100 ° C./h.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of ZnF ₂ with respect to calcium fluoride were added. The furnace was evacuated and the temperature of the crucible was increased from room temperature to 900 ° C. at a rate of 100 ° C./h, and then held at 900 ° C. for 20 hours. Then, the temperature was lowered at a rate of 6 ° C / h and cooled to room temperature.

  This operation was repeated 20 times each.

  Next, as a result of this example, the average value of the seed connection rate (%) (the number of crystals whose upper surface is oriented (111) / number of trials) and birefringence (strain) (nm / cm) is shown in Table 1 below. Show.

  Calcium carbonate and hydrofluoric acid were reacted to obtain powdered calcium fluoride.

This and ZnF ₂ were added at 0.7% by weight with respect to calcium fluoride, and both were mixed.

  Next, this mixture was put in a crucible of a refining furnace, heated to 1450 ° C., and then cooled to obtain molten calcium fluoride.

Next, the block was placed in the crucible of the single crystal growth furnace of FIG. As a scavenger, ZnF ₂ was added to the crucible in an amount of 0.1% by weight of calcium fluoride.

  The seed crystal was placed in the seed crystal storage section of FIG. 3 using the seed crystal with the surface orientation (111) as the top surface.

  Water was introduced into the fluid circulation path, and the temperature near the seed crystal was precisely controlled, and the temperature of the part defining the melted and unmelted part of the seed crystal was fixed at 1400 ° C.

The degree of vacuum in the furnace was 6 × 10 ⁻⁴ Torr. The temperature was raised from room temperature to 1450 ° C., and the degree of vacuum was 2 × 10 ⁻⁶ Torr and the temperature was 1450 ° C. and maintained for 11 hours.

  Next, the crucible was lowered at a speed of 2 mm / h. The temperature drop rate at this time corresponds to about 100 ° C./h.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of ZnF ₂ with respect to calcium fluoride were added. The furnace was evacuated and the temperature of the crucible was increased from room temperature to 900 ° C. at a rate of 100 ° C./h, and then maintained at 900 ° C. for 20 hours. Then, the temperature was lowered at a rate of 6 ° C / h and cooled to room temperature.

  This operation was repeated 20 times each.

  Next, as a result of this example, the average value of the seed connection rate (%) (the number of crystals whose upper surface is oriented (111) / number of trials) and birefringence (strain) (nm / cm) is shown in Table 1 below. Show.

  Calcium carbonate and hydrofluoric acid were reacted to obtain powdered calcium fluoride.

This and ZnF ₂ were added at 0.7% by weight with respect to calcium fluoride, and both were mixed.

  Next, this mixture was put in a crucible of a refining furnace, heated to 1450 ° C., and then cooled to obtain molten calcium fluoride.

Next, the block was placed in the crucible of the single crystal growth furnace of FIG. As a scavenger, ZnF ₂ was added to the crucible in an amount of 0.1% by weight of calcium fluoride.

  The seed crystal was placed in the seed crystal storage section of FIG. 3 using the seed crystal with the surface orientation (111) as the top surface.

  Nitrogen gas was introduced into the fluid circulation path, the temperature near the seed crystal was precisely controlled, and the temperature of the part defining the melted and unmelted part of the seed crystal was fixed at 1400 ° C.

The degree of vacuum in the furnace was 6 × 10 ⁻⁴ Torr. The temperature was raised from room temperature to 1450 ° C., and the degree of vacuum was 2 × 10 ⁻⁶ Torr and the temperature was 1450 ° C. and maintained for 11 hours.

  Next, the crucible was lowered at a speed of 2 mm / h. The temperature drop rate at this time corresponds to about 100 ° C./h.

Next, a calcium fluoride single crystal grown in a crucible of an annealing furnace and 0.1% by weight of ZnF ₂ with respect to calcium fluoride were added. The furnace was evacuated and the temperature of the crucible was increased from room temperature to 900 ° C. at a rate of 100 ° C./h, and then maintained at 900 ° C. for 20 hours. Then, the temperature was lowered at a rate of 6 ° C / h and cooled to room temperature.

  This operation was repeated 20 times each.

  Next, as a result of this example, the average value of the seed connection rate (%) (the number of crystals whose upper surface is oriented (111) / number of trials) and birefringence (strain) (nm / cm) is shown in Table 1 below. Show.

  As described above, according to the present invention, it is possible to provide a method for producing a crystal article for an optical component which is excellent in terms of seed connection rate and birefringence (distortion).

It is a figure of a pulling-down growth furnace. Temperature distribution near the seed crystal connection. Seed crystal temperature control device. The flowchart for demonstrating the example of a manufacturing process of the optical fluorite.

Explanation of symbols

1 Crucible (crystal part)
2 Crucible support rod
2a Crucible lifting motor
2b Power supply for crucible lifting motor
3a side heater
3b Side heater support
3c Side heater support legs
4 Side and top insulation
5 Housing (chamber)
6 Furnace room
7 Power supply for side heater heating
8 Heater control device
9 Control device for crucible lifting motor
Control device for 10 types of cooling
11 crucible holder
12 seed crystal storage
13 Carbon material
14 Fluid circuit
15a Temperature sensor (for fluid circuit)
15b Temperature sensor (for furnace environment)

Claims (9)

  A method for producing a crystalline substance, characterized in that, in a crucible descent method, temperature control in the vicinity of a seed crystal is defined in a crystal production apparatus that crystallizes by cooling a raw material of a crystalline substance that has been melted in a crucible.   In the crucible descent method, in a crystal manufacturing device that crystallizes by cooling the raw material of the crystalline material melted in the crucible, a fluid circulation path is provided to change the temperature in the vicinity of the seed crystal, and the surrounding area is for observation A method for producing a crystalline substance, characterized in that a temperature sensor is installed.   3. The method for producing a crystalline substance according to claim 1, wherein the temperature control fluid in the vicinity of the seed crystal is a gas or a liquid.   3. The method for producing a crystalline substance according to claim 1 or 2, wherein the flow rate of the fluid according to claim 3 is variable.   4. The method for producing a crystalline substance according to claim 3, wherein the temperature of the fluid according to claim 3 is controlled.   An apparatus for projecting and exposing a mask pattern image onto a substrate using a projection optical system, the projection optical system including the fluorite material according to claim 1 and forming the mask pattern image on the substrate. A projection exposure apparatus.   An apparatus for projecting and exposing a mask pattern image onto a substrate using a projection optical system, the projection optical system including the fluorite material according to claim 1 and forming the mask pattern image on the substrate. A projection exposure apparatus.   An apparatus for projecting and exposing a mask pattern image onto a substrate using a projection optical system, the projection optical system including the fluorite material according to claim 1 and forming the mask pattern image on the substrate. A projection exposure apparatus.   9. The projection exposure apparatus according to claim 6, wherein the illumination optical system and / or the projection optical system includes a fluorite optical material and quartz glass.

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