JP2005330123A - Method for producing caf2 crystal - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain good light quantity of an optical system by improving the transmittance of a lens system of an aligner. <P>SOLUTION: A mixed gas of an inert gas and gaseous fluorine or carbon fluoride is used as a gas scavenger in at least one process of a purification process, a growth process and an annealing process for producing the fluoride crystal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

The present invention relates to a CaF ₂ crystal used in an exposure apparatus for photolithography.

  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.

An apparatus that generates excimer laser light is known as an excimer laser oscillation apparatus. A laser gas such as Ar, Kr, Xe, KrF, ArF, or F ₂ filled in the manifold is excited by electron beam irradiation or discharge. The excited atoms then combine with the ground state atoms to produce molecules that exist only in the excited state. This molecule is called an excimer. Since excimer is unstable, it immediately emits ultraviolet light and falls to the ground state. This is called a bond-free transition, and an excimer laser oscillation device is a device that multiplies ultraviolet light obtained by this transition in an optical resonator composed of a pair of mirrors and extracts it as laser light.

ArF laser or F ₂ laser Among excimer laser light is light in a wavelength range wavelengths is called 193 nm, and the vacuum ultraviolet region such as 157 nm, not used as the transmittance of light in such a wavelength region is high in the optical system I must.

CaF ₂ crystal is an optimal optical material for such purposes.

  On the other hand, as an optical lens material used in an exposure apparatus for photolithography, it is necessary that the refractive index is uniform, the birefringence is small, and the polishing accuracy is increased.

  Patent Document 1 proposes a calcium fluoride crystal production method using a solid scavenger that has a high permeability of calcium fluoride crystals and is advantageous in safety.

Patent Document 2 proposes a heat treatment apparatus and a heat treatment method for a fluorite single crystal using a solid scavenger (fluorinating agent).
Japanese Patent Laid-Open No. 9-227293 Japanese Patent Laid-Open No. 11-240800

However, although the conventional CaF ₂ crystal shows satisfactory performance as a normal visible light optical article, the CaF ₂ crystal as a lens material used in an exposure machine in the vacuum ultraviolet wavelength region has a high internal transmittance. In addition, optical properties may deteriorate when repeatedly irradiated with high-power light at short wavelengths for a long period of time, such as excimer lasers, homogeneity of refractive index that affects imaging performance, and birefringence. There are problems such as high rates.

In particular, among the excimer lasers, in the case of an optical component used in an exposure machine for photolithography using an F ₂ excimer laser having a short wavelength, the optical performance required for the CaF ₂ crystal as a lens material is very severe.

As described above, the optical performance required for the CaF ₂ crystal as a lens material is internal transmittance, laser durability, refractive index homogeneity, and birefringence.

  Furthermore, the crystal material must have surface accuracy in the polishing process for forming a lens.

  While searching for the cause, the present inventors discovered that the decrease in the internal transmittance of the crystal is greatly affected by the oxidation of the crystal and the residual scavenger used to prevent oxidation, and the internal transmittance was reduced. I noticed how important the action of the scavenger is to keep it from falling.

The present invention has been made in view of the technical problems described above, and has as its main object to provide an optical material CaF ₂ crystal that can be used in an optical system that requires high transmittance at a short wavelength.

Another object of the present invention is to provide a CaF ₂ crystal 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 a CaF ₂ crystal that can be a highly reliable optical article.

The CaF ₂ crystal production method of the present invention is a non-fluorine gas scavenger and a non-fluorine gas scavenger in a purification process for melt-solidifying a high-purity CaF ₂ powder raw material, a crystal growth process for growing a single crystal, and an annealing process for removing strain in the crystal. A mixed gas of active gas is supplied to the target calcium fluoride to improve the optical characteristics of the calcium fluoride single crystal.

  According to the present invention, by using a mixed gas of inert gas and fluorine or fluorocarbon as a gas scavenger used in one or more of the purification process and growth process and annealing process of fluoride crystal production, Impurities consisting of scavenger constituent elements remaining in the crystal were eliminated, and the internal transmittance in the vacuum ultraviolet wavelength region and the laser durability were improved.

  This makes it possible to improve the transmittance of the lens system of the target exposure machine and obtain a good light quantity of the optical system.

Even in the case of repeatedly irradiating light with high output at a short wavelength for a long period of time, it is possible to provide a CaF ₂ crystal whose transmittance characteristics are hardly deteriorated.

From the above, it is possible to provide a CaF ₂ crystal suitable for an optical component for excimer laser, particularly an optical component for excimer laser of an exposure apparatus for photolithography, and has high imaging performance and high reliability. CaF ₂ crystals that can be used as articles can be provided.

The present inventors have produced a number of CaF ₂ crystals by changing the scavenger addition conditions during CaF ₂ crystal production, optical properties (spectral transmittance, gamma ray durability) of the crystals was measured.

  For the spectral transmittance, a spectroscope capable of measuring a vacuum ultraviolet wavelength region of 120 nm to 300 nm and a spectroscope for ultraviolet-visible measurement for evaluating the generation of a color center after γ-ray irradiation were used.

  Gamma ray durability is simpler than laser durability, and it is possible to know whether or not a color center has occurred in the entire large crystal.

In addition, it is possible to determine the tendency of residual scavenger elements in the crystal and trace oxidation. As irradiation conditions, γ rays with an irradiation intensity of 1 × 10E ⁶ R were irradiated for 1 hour to obtain a total amount of 1 × 10E ⁶ R.

  From these evaluations, it has been found that crystals having good internal transmittance have little residual impurities, especially scavenger constituent elements, and less oxidative deterioration of the crystals.

  We also found that it is important to optimize the reaction process of the scavenger in order to prevent oxidation in the annealing process and to contaminate the crystal with the material in the furnace.

  Needless to say, it is important for the raw material that an element having particularly poor transmittance and laser durability, such as rare earth, is contained in the raw material, and as a result of the raw material selection, the above evaluation results were obtained.

  Hereinafter, a preferred manufacturing process of the present invention will be described with reference to the drawings.

First, a high purity CaF ₂ synthetic raw material is prepared as a raw material.

  Then, the calcium fluoride raw material and the scavenger are mixed. At this time, calcium fluoride and a scavenger may be put in a container and mixed by rotating the container. As the scavenger, zinc fluoride, bismuth fluoride, sodium fluoride, lithium fluoride and the like that are more easily bonded to oxygen than the growing fluoride are desirable.

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

  For example, the zinc fluoride scavenger is important as an action of converting calcium oxide generated by the presence of moisture into calcium fluoride and an action of converting calcium oxide in the calcium fluoride raw material into calcium fluoride.

CaF + H ₂ O → CaO + 2HF
CaO + ZnF ₂ → CaF ₂ + ZnO ↑
In Japanese Patent Laid-Open No. 9-227293, calcium oxide is changed to calcium fluoride by adding a solid scavenger, but if zinc metal remains in the crystal, there is a demand for a lens to be used for an exposure machine for photolithography using an F ₂ excimer laser. The performance cannot be satisfied.

  For this reason, it is necessary to develop a scavenger addition method and optimization, and a purification, growth, and annealing process that minimizes the amount of scavenger addition.

  Japanese Patent Laid-Open No. 11-240800 discloses an annealing apparatus and annealing method for a fluorite single crystal, in which a scavenger (fluorinating agent) is housed in a carbon container separated from a carbon container in which crystals to be annealed are arranged and annealed under atmospheric pressure. Has proposed.

Hereinafter, with reference to the accompanying drawings, a method for manufacturing the CaF ₂ crystal and the optical element of the present invention.

Figure 1 shows a flow chart of a method of manufacturing an optical element using CaF ₂ of the present invention. First, a synthetic raw material of high purity CaF ₂ is prepared as a raw material, and the CaF ₂ raw material and a scavenger are mixed. A synthetic raw material for high-purity CaF ₂ is produced by treating calcium carbonate with hydrofluoric acid. Although the present invention does not exclude a method of removing impurities (eg, SiO ₂ ) by treating CaF ₂ rough with hydrofluoric acid, high-purity CaF ₂ is a powder unlike the rough and has a bulk density of about (approximately 10 to about 20μ), which is very small. It is preferable that when mixing the CaF ₂ material and scavenger put CaF ₂ raw material and the scavenger in mixing vessel to ensure uniform mixing to rotate.

  As the scavenger, zinc fluoride, cadmium fluoride, manganese fluoride, bismuth fluoride, sodium fluoride, lithium fluoride and the like that are more easily bonded to oxygen than the growing fluoride, and are more easily decomposed and evaporated are desirable. A substance that reacts with the oxide mixed in the fluoride raw material to be easily vaporized is selected. In particular, zinc fluoride is desirable. Here, it is important to control the amount of scavenger added.

The amount of scavenger added in the purification step in the present embodiment is 0.005% by weight or more and 0.5% by weight or less, and more preferably 0.05% by weight. When the addition amount is large, the refractive index homogeneity is lowered due to the residual amount of the scavenger, and the internal transmittance and the laser durability are lowered. In other words, since the amount of the added scavenger is small, the amount of impurities and crystal defects (particularly, dislocation density) contained in the CaF ₂ crystal are reduced, and a high-quality CaF ₂ crystal is provided.

  In addition to scavenger addition in the crystal growth process, by using the mixed gas of the fluorine-based gas and the inert gas of the present invention, it is possible to eliminate the residue of the metal element which is a scavenger constituent element found in the case of a solid scavenger in the grown crystal. it can.

  In FIG. 2, reference numeral 102 denotes a chamber of the refining furnace 100, which is connected to a vacuum exhaust system including a dry pump 150, a mechanical booster pump 160, a turbo molecular pump 170, and an abatement system 180. The exhaust pumps 150 to 170 are not limited to these types. For the abatement system (trap), both a cooling trap that cools the exhaust gas and deposits and removes harmful components in the exhaust gas, and a high-temperature trap that thermally decomposes the exhaust gas can be applied. .

  The chamber 102 is insulated by a heat insulating material 108 and accommodates the crucible 104 and the heater 106. The crucible 104 is made of, for example, carbon, has a substantially cylindrical shape, and is rotatably supported by the crucible support mechanism 140. If necessary, crucible support mechanism 140 is configured to allow crucible 104 to be lowered. The rotation by the crucible support mechanism 140 is performed in order to make the temperature of the crucible 104 uniform. The chamber 102 is also connected to a cooling pipe 130 for temperature adjustment. The temperature in the chamber 102 can be controlled by the heater 106 and the cooling pipe 130. The thermocouple 110 is made of platinum, for example, and measures the temperature of the crucible 104 from the vicinity of the outer wall of the crucible 104.

Thereafter, the heater 106 is energized to heat the mixture 120 in the crucible 104 to perform dehydration and other adsorbate removal. The purification furnace 100 is evacuated to a vacuum level of 1 × 10 ⁻³ Pa or more by a vacuum exhaust system.

Subsequently, the CaF ₂ raw material is completely melted through 350 ° C. to 1100 ° C. which is a temperature range of the scavenge reaction.

  After melting, the molten state is maintained for several hours to several tens of hours in order to remove residual harmful elements.

  During this time, the solid scavenger added to the calcium fluoride is harmful to the optical properties if it remains in the crystal, and is thus completely removed by evaporation in the melt.

Subsequently, the crucible 104 is lowered and the molten CaF ₂ raw material is gradually cooled to grow crystals. Since it is not necessary to arrange the lattice arrangement of the CaF ₂ crystal, the descending speed of the crucible does not have to be slow. Although the present invention includes a case where the crucible 104 is not lowered, the effect of removing impurities is improved by lowering the crucible 104.

  Since this step does not require temperature control as the single crystal growth step described later, the resulting crystal may be polycrystalline or have grain boundaries. The upper part of the crystal thus obtained, that is, the last crystallized part with time is removed. Since this portion is a portion where impurities easily collect (that is, segregation), removing this portion removes impurities that adversely affect the characteristics. Again, this crystal is put in the crucible 104, and a series of steps of melting, crystallization, and upper removal is performed. Note that an inert gas may be introduced into the chamber 102 if necessary.

The single crystal growth step is a step of growing a single crystal of CaF ₂ to improve the quality of the crystal (that is, adjusting the lattice arrangement). As a growth method, an appropriate method is selected according to the size of the crystal and the purpose of use. The purified crystal 220 is put in a crucible 204 housed in a chamber 202 of a crystal growth furnace 200 shown in FIG.

  In FIG. 3, reference numeral 202 denotes a chamber of the crystal growth furnace 200, which is connected to a vacuum exhaust system including a dry pump 250, a mechanical booster pump 260, a turbo molecular pump 270, and an abatement system 280. The chamber 202 is insulated by a heat insulating material 208 and houses the crucible 204 and the heater 206. The crucible 204 is made of, for example, carbon, has a substantially cylindrical shape, has high hermeticity, and is supported by the crucible support / pull-down mechanism 240 so as to be rotatable and vertically movable. The rotation by the crucible support / pull-down mechanism 240 is performed in order to make the temperature of the crucible 204 uniform. In this embodiment, the crucible 310 and the crucible 204 in the vacuum baking furnace of FIG. 5 are the same, but they may be separate members (that is, the mixture in the crucible 304 is transferred to the crucible 204). The chamber 202 is also connected to a cooling pipe 230 for temperature adjustment. A desired temperature gradient can be formed in the vertical direction shown in FIG. 3 by the heater 206 and the cooling pipe 230. The thermocouple 210 measures the temperature of the crucible 204 from the vicinity of the outer wall of the crucible 204.

Then, by energizing the heater 206 to heat the CaF ₂ secondary material (crystal) 220 in the crucible 204 up to about 1420 ° C., to completely melt the CaF ₂ crystal. Thereafter, the crucible 204 is gradually lowered at a speed of 1 mm / h (passing through a predetermined temperature gradient), and the molten CaF ₂ crystal is gradually cooled to grow a single crystal.

Subsequently, the fluoride grown single crystal is heat-treated in an annealing furnace 400 shown in FIG. 4 (annealing step). The annealing step is a step of heat-treating the grown CaF ₂ single crystal to remove distortion that causes crystal cracking. The grown single crystal 420 is put into a crucible 404 housed in a chamber 402 of an annealing furnace 400 shown in FIG.

  In FIG. 4, reference numeral 402 denotes a chamber of the annealing furnace 400, which is connected to a vacuum exhaust system including a dry pump 450, a mechanical booster pump 460, a turbo molecular pump 470, and an abatement system 480. The chamber 402 is insulated by a heat insulating material 408 and accommodates the crucible 404 and the heater 406. The crucible 404 is made of, for example, carbon, has a substantially cylindrical shape, is configured in a multistage manner, and is supported by a crucible support member 440. The chamber 402 is also connected to a cooling pipe 430 for temperature adjustment. The temperature of the chamber 402 can be controlled by the heater 406 and the cooling pipe 430. The thermocouple 410 is made of, for example, platinum, and measures the temperature of the crucible 404 from the vicinity of the outer wall of the crucible 404.

In the annealing step, the crucible 404 is heated uniformly to about 900 ° C. to about 1000 ° C., and the strain of the CaF ₂ crystal is removed in the solid state. The heating time is about 20 hours or more, more preferably about 20 to about 30 hours. In the annealing step, crystal dislocations are reduced through annealing. Thereafter, the temperature of the CaF ₂ crystal is returned to room temperature while maintaining the state in which the strain disappears.

  Thereafter, it is molded into a required optical article shape (convex lens, concave lens, disk shape, plate shape, etc.) (molding process).

Since the dislocation density in the CaF ₂ crystal is small at the time of polishing, the partial decrease in surface accuracy is very small, and high-accuracy processing is possible with an allowable value or less.

  By combining various lenses thus obtained, an illumination optical system suitable for an excimer laser, particularly an ArF excimer laser, can be constructed (an optical system assembly step).

  An exposure apparatus for photolithography can be configured by combining an excimer laser light source, an optical system having a lens made of calcium fluoride, and a stage that can move the substrate.

A necessary amount of a commercially available high-purity calcium fluoride powder raw material and ZnF ₂ as a scavenger were added in an amount of 0.05% by weight with respect to calcium fluoride, and both were mixed and dispersed uniformly, and then filled in a carbon crucible.

  Subsequently, it transfers to a refinement furnace and heat-melts to 1420 degreeC.

  The melting time is set to an appropriate time, for example, 30 hours, in order to remove harmful elements from the constituent elements of the solid scavenger.

  After removing harmful unnecessary elements, it was gradually cooled to solidify the raw material.

  The harmful and unnecessary portions of the calcium fluoride block surface layer are removed to obtain a secondary material before crystal growth.

Incidentally, put ZnF ₂ to 0.005 weight percent crucible as a scavenger.

The furnace was evacuated, heating the crucible, vacuum 6 × 10 -4 ^Torr, the temperature is raised to 1420 ° C., dissolving the crucible calcium fluoride, the degree of vacuum to degas 2 × 10 ^{- 6} Torr, temperature was 1420 ° C. and held for 20 hours.

Next, the crucible was lowered at a speed of 1 mm / h to grow a good quality CaF ₂ single crystal.

  Since the pulling rate at this time preferably corresponds to the crystal growth rate, it goes without saying that it is necessary to consider the size and shape of the crystal to be manufactured.

  In general, as the crystal size increases, the pulling speed needs to be reduced.

  Next, the grown calcium fluoride single crystal was put in a crucible of the annealing furnace, the inside of the furnace was evacuated, and kept at 300 ° C. for about 24 hours to remove moisture in the furnace.

When the degree of vacuum in the furnace is 5 × 10 ⁻⁴ Pa or less, 10 cc of a mixed gas of fluorocarbon gas and argon gas generated by heating Teflon (registered trademark) powder from the scavenger supply system shown in FIG. / Min. The temperature of the crucible was raised from room temperature to 900 ° C. at a rate of 100 ° C./h, and then maintained at 900 ° C. for 50 hours. The mixed gas supply of carbon fluoride and argon gas was stopped 20 hours after the temperature was raised to 900 ° C.

  After 50 hours at 900 ° C., the temperature was lowered at a rate of 5 ° C./h and cooled to room temperature.

  At this time, the cooling rate needs to be reduced according to the increase in the size of the crystal.

  In other words, it is difficult to make the birefringence very small unless slow.

A necessary amount of a commercially available high-purity calcium fluoride powder raw material and ZnF ₂ as a scavenger were added in an amount of 0.05% by weight with respect to calcium fluoride, and both were mixed and dispersed uniformly, and then filled in a carbon crucible.

  Next, degassing is carried out at 150 ° C. for 24 hours in a vacuum drying furnace, transferred to a refining furnace, and heated to 1420 ° C. for melting.

  The melting time is set to an appropriate time, for example, 30 hours, in order to remove harmful elements from the constituent elements of the solid scavenger.

  After removing harmful unnecessary elements, it was gradually cooled to solidify the raw material.

  The harmful and unnecessary portions of the calcium fluoride block surface layer are removed to obtain a secondary material before crystal growth.

The refined secondary material is filled in a crucible for crystal growth, set in a growth furnace, the inside of the furnace is evacuated, and the crucible is heated at 300 ° C. for 24 hours, so that the degree of vacuum is 5 × 10 ⁻⁴ Pa. After evacuation, a mixed gas of fluorocarbon gas and argon gas generated by heating Teflon (registered trademark) powder from the scavenger supply system shown in FIG. The temperature was increased to a crucible temperature of 1420 ° C. at a rate of temperature increase of 100 ° C./h, and then maintained for 50 hours.

  The mixed gas of carbon fluoride and argon gas was stopped 20 hours after the temperature was raised at 1420 ° C.

Next, the crucible was lowered at a speed of 1 mm / h to grow a good quality CaF ₂ single crystal.

  Since the pulling rate at this time preferably corresponds to the crystal growth rate, it goes without saying that it is necessary to consider the size and shape of the crystal to be manufactured.

  In general, as the crystal size increases, the pulling speed needs to be reduced.

  Next, the grown calcium fluoride single crystal was put in a crucible of the annealing furnace, the inside of the furnace was evacuated, and kept at 300 ° C. for about 24 hours to remove moisture in the furnace.

When the degree of vacuum in the furnace is 5 × 10 ⁻⁴ Pa or less, a mixed gas of fluorocarbon gas and argon gas generated by heating Teflon (registered trademark) powder from the scavenger supply system shown in FIG. 5 is 10 cc / min. . The temperature of the crucible was raised from room temperature to 900 ° C. at a rate of 100 ° C./h, and then maintained at 900 ° C. for 50 hours. The mixed gas supply of carbon fluoride and argon gas was stopped 20 hours after the temperature was raised to 900 ° C.

  After 50 hours at 900 ° C., the temperature was lowered at a rate of 5 ° C./h and cooled to room temperature.

  At this time, the cooling rate needs to be reduced according to the increase in the size of the crystal.

  In other words, it is difficult to make the birefringence very small unless slow.

It is a flowchart showing a manufacturing method of the CaF ₂ crystal of the present invention. It is a schematic sectional drawing of the refiner | purifier used for the refinement | purification process of the manufacturing method shown in FIG. It is a schematic sectional drawing of the crystal growth furnace used for the single crystal growth process of the manufacturing method shown in FIG. It is a schematic sectional drawing of the annealing furnace used for the annealing process of the manufacturing method shown in FIG. It is a schematic sectional drawing of the scavenger supply system used for the annealing process of the manufacturing method shown in FIG.

Explanation of symbols

100 Refining furnace 200 Crystal growth furnace 300 Annealing furnace 400 Scavenger supply system

Claims (2)

During the manufacturing process of the calcium fluoride single crystal, in the purification process for melting and solidifying the high-purity CaF ₂ synthetic powder raw material, the single crystal growth process using the high-purity purified raw material, and the annealing process for removing strain in the single crystal, A calcium fluoride single crystal manufacturing method and a calcium fluoride single crystal characterized by using fluorine or a mixed gas of fluorocarbon and an inert gas as a scavenger for preventing oxidation of calcium fluoride. 2. The fluorine-containing gas is fluorine or fluorocarbon gas, and the inert gas is argon. CaF ₂ single crystal method of manufacturing.

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