JP2006016242A - Manufacturing method of fluoride crystal, optical part and exposure unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of a fluoride single crystal little in the reduction of transmittance when an ultraviolet light is irradiated in vacuo by the reduction of the density of structural faults, and requiring no precision control in the stage of crystal growth. <P>SOLUTION: This manufacturing method of a fluoride single crystal has a melt process in which a fluoride filled in a crucible is heated in a vacuum furnace to make a fluoride melt, a carbon monoxide removing process in which carbon monoxide is removed from the fluoride melt, and a crystallization process in which the fluoride melt after carbon monoxide removing process is cooled and crystallized. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a fluoride crystal, and more particularly to a method for producing a high-quality fluoride single crystal suitable for an optical member for an exposure apparatus using vacuum ultraviolet light.

In recent years, VLSI has been highly integrated and highly functional, and a fine processing technique on a wafer is required. As the processing method, a method by photolithography is generally performed. At present, the exposure wavelength gradually becomes shorter, and projection exposure apparatuses using ArF excimer laser light (wavelength 193 nm) as a light source are also on the market following projection exposure apparatuses (steppers and scanners) using KrF excimer laser light (wavelength 248 nm). Has appeared. There are very few optical materials that can be used in a lithography apparatus that uses light having a wavelength shorter than 250 nm, and two types of calcium fluoride (CaF ₂ ) single crystal and synthetic quartz glass are mainly used.

In order to perform further fine processing, attempts have been made to shorten the wavelength of the light source, and the demand for an optical lithography technique using F ₂ laser light (wavelength 157 nm) is increasing in recent years. However, at the wavelength of F ₂ laser light, synthetic quartz glass is considered difficult to use anymore.
It is believed that only calcium fluoride single crystals can be used.

  Further, only a calcium fluoride single crystal having a high quality can be used for an optical member of a projection exposure apparatus, and a calcium fluoride single crystal having a high transmittance at an exposure wavelength is required.

  In order to improve the transmittance of the calcium fluoride single crystal, it is effective to advance the purification of the raw material. In addition, it is now indispensable to purify a raw material during the manufacturing process of calcium fluoride crystals in order to manufacture an optical material that can be used for photolithography at a wavelength of 250 nm or less.

  Here, an example of the manufacturing method of a calcium fluoride single crystal is shown.

  In the case of a calcium fluoride single crystal used in the ultraviolet region or the vacuum ultraviolet region, natural fluorite is not used as a raw material, and a high-purity raw material produced by chemical synthesis is generally used. The raw material can be used as a powder, but since the powder has a low bulk density and a significant volume reduction when melted, a semi-molten product with a high bulk density or its pulverized product (cullet) is used as a raw material. Is normal. These raw materials having a high bulk density are generally called pretreated products.

In the process of manufacturing the fluoride pre-treatment product, a fluorinating agent such as lead fluoride (PbF ₂ ) or carbon tetrafluoride (CF ₄ ) is added to the raw material powder to remove impurities in the raw material powder. Is done. These fluorinating agents are also called scavengers, and have the effect of substituting the elements contained as impurities in the raw material powder with fluorine and further removing the substituted impurity elements as volatile compounds. For example, in the process of manufacturing a calcium fluoride (CaF ₂ ) semi-molten product,
After lead fluoride is added to the raw material powder, it is heated in a heating furnace to form a melt, and oxygen contained as calcium oxide (CaO) in the raw material powder is removed as volatile lead oxide (PbO).

CaO + PbF ₂ → CaF ₂ + PbO ↑
If the molten state of the raw material is kept for a certain period of time and then cooled and crystallized, a polycrystalline body with improved bulk density and reduced impurity concentration, that is, a pre-processed product can be obtained.

  Next, a method for producing a fluoride single crystal will be described.

  The most commonly used method for producing fluoride single crystals is the vertical Bridgman method. In the vertical Bridgman method, a crucible is placed in a heating furnace having a high-temperature part and a low-temperature part independently controlled in temperature, and the fluoride melted by heating in the crucible is gradually moved from the high-temperature part to the low-temperature part together with the crucible. This is a method of crystal growth by lowering. A single crystal manufacturing apparatus having such a configuration may be referred to as a vertical Bridgman furnace.

  Crystal growth by the vertical Bridgman method is performed as follows.

  First, a raw material is filled in a crucible of a single crystal manufacturing apparatus. As the fluoride raw material to be filled, it is preferable to use the above-mentioned pre-treated product. However, the raw material powder which has not been reduced in impurity concentration and bulk density may be directly filled in the crucible. In this case, by mixing the fluorinating agent with the raw material powder to be filled in the crucible, or by supplying the fluorinating agent from the outside, in the raw material heating stage before the crucible is lowered, The raw material can be purified.

After the crucible is filled with the raw material, the inside of the single crystal manufacturing apparatus is kept at a vacuum of 10 ⁻³ Pa or less using an evacuation apparatus. Next, the temperature in the apparatus is raised to the melting point or higher of the fluoride to melt the raw material in the crucible. When the crystal is grown, the temperature of the heater installed on the lower side is relatively lowered with respect to the temperature of the heater installed on the upper side, and a steep temperature gradient is provided between the two. Then, the heater or the temperature in the vicinity of the heater is measured, and the power supplied to the heater is controlled so that a temperature corresponding to the melting point of the fluoride is formed between the two heaters. At this time, in order to minimize the drift of the temperature distribution in the growth apparatus, it is common to perform highly accurate PID control.

  In the single crystal growth step, the single crystal is gradually grown from the lower part of the melt in the crucible by pulling down the crucible at a speed of about 0.1 to 5 mm / hour. The growth is completed when the melt is crystallized to the top of the melt, and simple slow cooling is performed while avoiding rapid cooling. When the temperature in the growth apparatus drops to about room temperature, the apparatus is opened to the atmosphere and a single crystal (ingot) is taken out.

  In the case of a single crystal used for a small-sized optical component or a window material that does not require homogeneity, the ingot is cut into a desired size and then processed to a final product through a process such as rounding. On the other hand, in the case of a single crystal that is used for a projection lens of a projection exposure apparatus and requires high homogeneity, heat treatment is performed in order to relieve distortion after ingot or cut into a desired size. There are many cases.

In addition, the manufacturing method of the calcium fluoride single crystal which is a kind of fluoride is described in detail in Patent Document 1.
Japanese Patent Laid-Open No. 11-157982

  According to the above-described conventional crystal growth apparatus and crystal growth method, it is inevitable that a certain degree of structural defects are included in the manufactured single crystal ingot. Structural defects are not confirmed by microscopic observation, but can be confirmed by the naked eye as scatterers from microscopic defects at the atomic level that can be confirmed from the decrease in light transmittance when irradiated with high-energy short-wavelength light. Even such relatively large defects exist. The latter relatively large structural defect includes a negative crystal and a bubble form containing gas. Since these structural defects cause a decrease in light transmittance, it is necessary to reduce the defect density in order to improve the yield of the optical member collected from the ingot.

  According to the crystal growth theory, to reduce the structural defect density, it is considered effective to keep the crystal growth rate low and to uniformly control the temperature conditions at the solid-liquid interface. It is difficult to precisely control the conditions.

  It is an object of the present invention to provide a method for producing a fluoride single crystal that has a low transmittance decrease upon irradiation with vacuum ultraviolet light and does not require precise control at the stage of crystal growth.

  As a result of studying means for solving the above problems, the present inventor has found that it is effective to sufficiently remove carbon monoxide (CO) from a fluoride melt in a pretreatment product or a single crystal manufacturing process. . Until now, it was thought that oxygen impurities in the fluoride raw material were removed as volatile oxides by adding a scavenger. However, according to the inventor's research, some of the oxygen impurities in the raw material react with carbon, which is a constituent material such as a crucible, to produce carbon monoxide, and the gasified carbon monoxide is foamed in the melt. Thus, it has been found that this becomes a factor for generating structural defects in the single crystal. The approximate course is as follows.

  The present inventor first measured the inclusion of relatively large bubbles in a single crystal with a mass spectrometer in order to investigate the impurity component that becomes the starting point of the defect. As a result, carbon monoxide (CO) was detected as the main component of the bubble inclusion gas.

Carbon monoxide reacts with carbon, which is a crucible material, when a small amount of residual oxygen, moisture, and the like are removed by a scavenge reaction, for example, PbO + C ⇒ Pb + CO ↑.
It is possible to generate as follows.

  It is also conceivable that carbon monoxide is generated from the fluoride raw material itself. Fluoride raw materials produced by chemical synthesis include those that pass through carbon-containing compounds such as carbonates in the synthesis and purification process, and carbon monoxide can be generated using carbon or carbonate radicals derived from such processes as the source. Sex cannot be denied.

  Carbon monoxide in the fluoride melt is expressed in the form of bubbles, but if the removal is insufficient, it will be cooled while remaining as bubbles in the melt. It is thought that this leads to a decrease in light transmittance. Therefore, it is important to sufficiently remove carbon monoxide from the fluoride melt in the fluoride pretreatment product manufacturing process or single crystal manufacturing process.

  The present invention newly provides a carbon monoxide removal step in the fluoride crystal production process, and enables production of such a fluoride single crystal with reduced light transmittance reduction during irradiation with vacuum ultraviolet laser light. It is.

  That is, the method for producing a fluoride crystal according to claim 1 of the present invention includes a melting step in which a fluoride filled in a crucible is heated in a vacuum furnace to obtain a fluoride melt, and from the fluoride melt to carbon monoxide. It has a carbon monoxide removal process which removes (CO), and a crystallization process which cools and crystallizes the fluoride melt which passed through the carbon monoxide removal process.

  The method according to claim 1 can be used as a manufacturing process of a pre-processed product for manufacturing a single crystal, and a single crystal can be directly manufactured if the crystallization process is a single crystal growth process. Accordingly, the method for producing a fluoride single crystal according to claim 2 is characterized in that, in the method for producing a fluoride crystal according to claim 1, the crystallization step is a single crystal growth step for growing a single crystal. To do.

  The pretreatment product of fluoride produced by removing carbon monoxide by the method according to claim 1 can be used as a raw material in a conventional single crystal production method, but the pretreatment product is a single crystal production process. In some cases, moisture is adsorbed from the storage environment until it is introduced into the container, and when melted in the single crystal manufacturing process, bubbles of carbon monoxide are generated again by reaction with carbon constituting the crucible. Therefore, in order to further reduce the structural defects in the single crystal, it is preferable to provide a carbon monoxide removing step also in the single crystal manufacturing step. Accordingly, the method for producing a fluoride single crystal according to claim 3 is a melting step in which the fluoride crystal produced by the method according to claim 1 is filled in a crucible and heated in a vacuum furnace to obtain a fluoride melt. A carbon monoxide removal step for removing carbon monoxide (CO) from the fluoride melt, and a single crystal growth step for growing the single crystal by cooling the fluoride melt that has undergone the carbon monoxide removal step. It is characterized by having.

  It is very difficult to determine whether carbon monoxide has been sufficiently removed from the fluoride melt by directly analyzing the hot melt. Therefore, as a result of various studies, the present inventor has found that judgment based on the residual gas composition in the vacuum furnace is effective.

By keeping the fluoride in a molten state, carbon monoxide in the melt gradually moves into the gas phase and is discharged out of the system by an exhaust device of a vacuum furnace. At this time, the residual gas in the vacuum furnace is analyzed, and if the carbon monoxide (CO) partial pressure is 1 × 10 ⁻⁴ Pa or less, it can be determined that the carbon monoxide in the melt has been sufficiently removed. Therefore, in the method for producing a fluoride crystal or a fluoride single crystal according to claim 4 of the present invention, in addition to the feature according to any one of claims 1 to 3, the carbon monoxide removing step includes: The fluoride is maintained in a molten state until the partial pressure of carbon monoxide (CO) in the vacuum furnace becomes 1 × 10 ⁻⁴ Pa or less.

  The above invention is particularly effective when the crucible in direct contact with the fluoride melt contains carbon (C). Therefore, the invention described in claim 5 is characterized in that, in addition to the characteristics described in any one of claims 1 to 4, the crucible contains carbon (C).

By using the above method, it is possible to produce a fluoride single crystal in which the structural defect density is reduced and the decrease in transmittance during irradiation with vacuum ultraviolet laser light is suppressed. Of the fluorides, especially calcium fluoride (CaF ₂ ) is transparent at the vacuum ultraviolet wavelength and excellent in durability, and therefore can be suitably used as an optical member of an exposure apparatus. Accordingly, the invention described in claim 6 is characterized in that, in addition to the feature described in any one of claims 1 to 5, the fluoride is calcium fluoride (CaF ₂ ).
The invention according to claim 7 and claim 8 is an optical member for vacuum ultraviolet optical system comprising the calcium fluoride single crystal produced by the method according to claim 6, and for vacuum ultraviolet optical system according to claim 7. An exposure apparatus provided with an optical member.

  Since the fluoride crystal produced by the production method according to the present invention has sufficiently removed carbon monoxide, if it is used as a raw material for producing a single crystal, the generation of structural defects in the melting process can be reduced. A high-quality single crystal can be produced in which the light transmittance does not decrease in a short time even when irradiated with energy vacuum ultraviolet laser light.

[First embodiment]
The first embodiment of the present invention relates to a method for producing a fluoride crystal as a pretreatment product for producing a single crystal.

FIG. 1 is an example of a manufacturing apparatus used for manufacturing a fluoride crystal according to the present invention. The vacuum furnace 1 is provided with a total pressure gauge 7, a partial pressure gauge 2 and an exhaust port 3, and is evacuated by an unillustrated exhaust device attached to the exhaust port 3. A heat insulating material 4 and a heater 5 are disposed in the vacuum furnace 1, and the crucible 6 can be heated to an arbitrary temperature.

The partial pressure gauge 2 is for performing residual gas analysis in the vacuum furnace 1. The type of the partial pressure meter is preferably a mass spectrometric type. In particular, the quadrupole mass spectrometer is small in size and has a required accuracy and is easy to use. However, in a mass spectrometer, since carbon monoxide (CO) and nitrogen (N ₂ ) give main peaks having almost the same mass number, it is necessary to pay attention to fragments and isotope peaks and to estimate the influence of nitrogen. According to the experiment results of the inventor, the nitrogen partial pressure in the residual gas in a normal production apparatus is negligible.

  The vacuum vessel 1 is required to have sufficient mechanical strength to withstand the atmospheric pressure applied during evacuation. It is also necessary to have a certain degree of corrosion resistance against reactive gases introduced into the vacuum container or possibly generated inside the vacuum container. Therefore, it is desirable that the vacuum vessel 1 is made of stainless steel, which is a material having these properties.

  The heater 5 is controlled to a predetermined temperature by a control system (not shown). The control system of the heater 5 may be a general system including a temperature sensor, a temperature controller, a power controller, and the like, but it is necessary that the crucible 6 can be controlled to a temperature higher than the melting point of fluoride.

  Although the crucible 6 is supported inside the heater 5 by the support member 8, the upper part of the crucible 6 and the support member 8 is heated to a temperature equal to or higher than the melting point of the fluoride, and must be made of a material that can withstand high temperatures. Further, since the crucible 6 is in direct contact with the fluoride, it is required to be a material that is taken in as an impurity and does not have an adverse effect. As a material that satisfies the above conditions, it is desirable that the crucible 6 and the upper part of the support member 8 are made of a carbon material (carbon material). The present invention has a particularly large effect when the crucible 6 is made of a material containing carbon.

  Next, an example of producing fluoride crystals using this production apparatus will be described.

First, prepare a fluoride raw material. The fluoride raw material is preferably one having as few impurities as possible, and one produced by chemical synthesis should be used. A scavenger is added to the prepared fluoride raw material and filled in the crucible 6 of the manufacturing apparatus. The scavenger contains lead fluoride (P
bF ₂ ), cobalt fluoride (CoF ₂ ), manganese fluoride (MnF ₂ ), or the like can be used. The amount of scavenger added is usually about 0.1 to 1.0 mol%.

  Next, the vacuum furnace 1 is evacuated. When the inside is exhausted sufficiently, the heater 5 is energized while heating is continued, and heating is started. Most of the moisture and the like adhering to the fluoride raw material by evacuation is desorbed, but desorption is further promoted by heating while evacuating.

  When the temperature reaches a temperature lower by about 400 ° C. to 600 ° C. than the melting point of fluoride, the temperature is maintained for a predetermined time and the scavenge reaction proceeds. The holding time is usually about 10 hours.

  After the elapse of a predetermined time, heating is started again and heated to a temperature equal to or higher than the melting point of fluoride. By this operation, the fluoride in the crucible 6 is melted.

  When the content of the crucible 6 exceeds the melting point of fluoride and the contents are melted, the partial pressure gauge 2 continuously measures the partial pressure of carbon monoxide in the vacuum furnace 1 while maintaining the molten state. Remove. Carbon monoxide generated from the melt is discharged into the vacuum furnace 1 through a small hole 9 a formed in the lid 9 of the crucible 6.

As the carbon monoxide removal process proceeds, the carbon monoxide partial pressure in the vacuum furnace 1 gradually decreases. This is because carbon monoxide in the melt decreases and the release rate from the melt into the vacuum furnace decreases. Therefore, the progress of the carbon monoxide removal step can be followed by monitoring the partial pressure in the vacuum furnace. If the carbon monoxide partial pressure decreases to 1 × 10 ⁻⁴ Pa or less, it can be determined that the carbon monoxide has been sufficiently removed from the melt.

If the partial pressure of carbon monoxide in the vacuum furnace 1 is 1 × 10 ⁻⁴ Pa or less, it can be judged that the removal of carbon monoxide is sufficient, but if the partial pressure of carbon monoxide continues to decrease, the molten state remains as it is. The carbon monoxide can be reliably removed by continuing heating and evacuation until the partial pressure converges to a constant value. The amount of leakage in the vacuum furnace is considered to be almost constant, and the contribution to the carbon monoxide partial pressure due to degassing from vacuum furnace components such as stainless steel is considered to be negligibly small. This is because the pressure component can be regarded as being generated from the crucible contents. The convergence value of the partial pressure may be a value at the time when the fluctuation of the partial pressure value becomes equal to the noise level in consideration of the S / N ratio of the partial pressure meter used for measurement.

  After the carbon monoxide in the melt is sufficiently removed by the above steps, the crucible is gradually cooled to crystallize the contents, thereby obtaining a fluoride crystal suitable as a pretreatment product for producing a single crystal.

The fluoride crystal produced according to this embodiment has oxygen impurities and carbon monoxide sufficiently removed, so that when used as a pretreatment product for producing a single crystal, there is little decrease in light transmittance when irradiated with vacuum ultraviolet laser light. A fluoride single crystal can be produced.
[Second Embodiment]
In the second embodiment of the present invention, a single crystal is manufactured using the fluoride crystal manufactured in the first embodiment as a raw material.

  The single crystal manufacturing apparatus used in the second embodiment is a vertical Bridgman furnace having a structure shown in FIG. In the single crystal manufacturing apparatus of FIG. 2, the heater is divided into an upper heater 15 and a lower heater 16. The temperature of the upper heater 15 and the lower heater 16 is independently controlled. The upper heater 15 constitutes a high temperature part, and the lower heater 16 constitutes a low temperature part. The heater control system may be a general system including a temperature sensor, a temperature controller, a power controller, and the like, but it is necessary that at least the fluoride can be controlled to a constant temperature equal to or higher than the melting point.

A crucible 17 that contains fluoride is disposed inside the heater. The crucible 17 is driven by a pull-down mechanism 19 through a crucible support member 18. The upper part of the crucible 17 and the support member 18 may be heated to a temperature higher than the melting point of calcium fluoride, and must be made of a material that can withstand high temperatures. Further, since the crucible 17 is in direct contact with the fluoride, it is required to be a material that is taken in as an impurity and does not adversely affect the crucible 17. As a material satisfying the above conditions, it is desirable that the crucible 17 and the upper portion of the support member 18 are made of a carbon material (carbon material).

  Next, a method for producing a fluoride single crystal using the single crystal production apparatus of FIG. 2 will be described.

  As a raw material, a fluoride crystal (pretreated product) manufactured by the method of the first embodiment is used. In the pre-processed product manufactured according to the first embodiment, carbon monoxide is sufficiently removed in the manufacturing process, so that the amount of released gas is small during remelting in the single crystal manufacturing process, and the purity is high and the structural defects are small. A fluoride single crystal can be produced.

After the prepared fluoride crystal is filled in the crucible 17, the vacuum furnace 11 is sealed, and the inside of the vacuum furnace 11 is exhausted through the exhaust port 13 by an exhaust device (not shown). After the inside is sufficiently evacuated, the upper heater 15 and the lower heater 16 are energized, and the crucible 17 is heated to the melting point or higher of the fluoride to melt the filled fluoride into a melt.

  When the content of the crucible 17 exceeds the melting point of the fluoride and melts, the molten state is maintained while continuously measuring the partial pressure of carbon monoxide in the vacuum furnace 11 by the partial pressure gauge 12, and the carbon monoxide from the melt is maintained. Remove. Carbon monoxide generated from the melt is discharged into the vacuum furnace 11 through a small hole 21 a formed in the lid 21 of the crucible 17. Since the carbon monoxide removal step is the same as that in the first embodiment, a detailed description thereof will be omitted. However, in this embodiment, since the raw material from which carbon monoxide has been previously removed is used, the monoxide generated from the melt is used. The amount of carbon is small, and the carbon monoxide removal step is completed in a short time.

  When carbon monoxide is sufficiently removed, the upper heater 15 and the lower heater 16 are adjusted to a predetermined temperature to form a temperature gradient in the furnace. The temperature gradient is such that the upper heater 15 side is at a high temperature and the lower heater 16 side is at a low temperature so that the melting point of fluoride can be obtained at an intermediate point between the two.

  When a predetermined temperature distribution is obtained in the furnace, a single crystal growth process is started. In the single crystal growth process, the crucible 17 is gradually pulled down by the crucible pulling mechanism 19. The fluoride melt in the crucible passes through the melting point as it is pulled down, and a single crystal grows from the bottom of the crucible 17. The crucible 17 is pulled down to the bottom, and when the crucible contents are completely solidified, the crucible 17 is cooled to room temperature, and the fluoride single crystal formed in the crucible 17 is taken out.

  The fluoride single crystal thus produced can be cut into a desired shape and subjected to heat treatment, polishing, surface treatment, etc. as necessary to obtain an optical member.

The present invention can be applied to general single crystal manufacturing methods involving melting of raw materials, such as the Czochralski method, in addition to the Bridgeman method described above.
[Third embodiment]
The third embodiment of the present invention relates to an exposure apparatus provided with an optical member made of a fluoride single crystal according to the present invention.

  FIG. 3 shows an example of an exposure apparatus according to the present embodiment, and a wafer stage on which a substrate 38 coated with a photosensitive agent 37 placed on the surface 33a (the whole is simply referred to as “substrate W”) can be placed. 33. Exposure light is supplied to the illumination optical system 31 and the illumination optical system 31 for transferring the mask pattern (reticle R) prepared on the substrate W by irradiating vacuum ultraviolet light having a wavelength prepared as exposure light. The first surface P1 (object surface) on which the mask R for projecting an image of the pattern of the mask R on the substrate W and the surface of the substrate W coincide with the surface of the substrate W. And a projection optical system 35 placed between (image plane).

  The illumination optical system 31 also includes an alignment optical system 110 for adjusting the relative position between the mask R and the wafer W, and the mask R can move in parallel to the surface of the wafer stage 33. It is arranged on the reticle stage 32. The reticle exchange system 200 exchanges and transports a reticle (mask R) set on the reticle stage 32. The reticle exchange system 200 includes a stage driver for moving the reticle stage 32 in parallel with the surface 33 a of the wafer stage 33. The projection optical system 35 has an alignment optical system applied to a scan type exposure apparatus.

The exposure apparatus according to the present embodiment includes an optical member made of a fluoride single crystal manufactured by the manufacturing method according to the present invention. Specifically, the exposure apparatus shown in FIG. 3 includes the fluoride single crystal according to the present invention on one or both of the optical member 39 of the illumination optical system 31 and the optical member 30 of the projection optical system 35. An optical member.

  In FIG. 3, 300 is a stage control system for controlling the wafer stage 3, and 400 is a main control unit for controlling the entire apparatus.

  In this exposure apparatus, since the optical member made of the fluoride single crystal according to the present invention is used, the light transmittance does not decrease in a short time even when irradiated with high energy vacuum ultraviolet laser light, and the exposure performance is improved. It can be kept stable.

  In this example, a calcium fluoride pretreated product was produced by the production method according to the present invention, and a calcium fluoride single crystal was produced by a conventional production method using this pretreated product as a raw material.

A manufacturing apparatus shown in FIG. 1 was used for manufacturing the pre-processed product. 1 mol% of lead fluoride was added as a scavenger to chemically synthesized calcium fluoride as a raw material, and the crucible 6 of the manufacturing apparatus was filled. The vacuum furnace 1 was sealed and the inside was evacuated to 3 × 10 ⁻⁴ Pa, then the heater 5 was energized, and the temperature was raised to 800 ° C. while evacuation was continued. After holding the scavenge reaction for 10 hours at 800 ° C., the heating was started again while measuring the partial pressure of carbon monoxide with the partial pressure gauge 2, and the temperature was maintained at 1450 ° C., which is a temperature higher than the melting point. The calcium was completely melted.

Due to the melting of calcium fluoride, the carbon monoxide partial pressure in the vacuum furnace 1 rapidly increased and reached a maximum of 8 × 10 ⁻⁴ Pa. Thereafter, when the holding at 1450 ° C. was continued, the carbon monoxide partial pressure in the vacuum furnace 1 rapidly decreased in the first few hours, and then gradually decreased to 8 × 10 ⁻⁵ Pa in about 80 hours. Converged to. When 96 hours had elapsed after melting, heating was stopped, and the mixture was gradually cooled to room temperature to crystallize calcium fluoride to obtain a pretreated product.

Using this pretreated product as a raw material, a calcium fluoride single crystal was produced by the apparatus shown in FIG. The pretreatment product was filled in the crucible 17 and the inside of the apparatus was evacuated. When the degree of vacuum reached 5 × 10 ⁻⁵ Pa, the crucible 17 was positioned at the top, the upper heater 15 was set to 1450 ° C., and the lower heater 16 was set to 1300 ° C. to melt calcium fluoride. After the calcium fluoride was completely melted, the crucible was pulled down after being held for 12 hours, and the crucible 17 was pulled down at a constant speed of 1.0 mm / hour to grow a single crystal. The carbon monoxide partial pressure in the vacuum furnace 1 at the start of the crucible pull-down was 3 × 10 ⁻⁴ Pa.

  After the contents of the crucible 17 were completely crystallized, it was gradually cooled to near room temperature, and then the inside of the production apparatus was opened to the atmosphere to take out a single crystal (ingot).

In the obtained single crystal, defects (including bubbles and negative crystals) detectable with the naked eye were evaluated. The evaluation is to apply a liquid with the same refractive index as the crystal (hereinafter referred to as immersion liquid) to the ground surface of the block, extract the scattering points from the image taken by the CCD camera through the light of the condenser lamp, and count the number of them. Went by. Since the volume of the region to be counted can be known from the beam size of the condenser lamp, the same measurement was performed at a plurality of locations, and the average number of defects per unit volume was calculated. The number of defects in the single crystal produced in this example was 103/1000 cm ³ .

Further, a test piece for measuring transmittance was produced from this ingot. The parallel two surfaces of the test piece facing each other were mirror-polished, and the ridge height was 30 seconds or less, and the surface roughness was 0.5 nm RMS or less. This test piece was irradiated with an ArF excimer laser at a fluence of 50 mJ / cm ² per pulse, and the light transmittance at a wavelength of 193 nm after irradiation with 100,000 pulses was measured. The internal transmittance excluding the surface reflection of the test piece was 99.4% per 1 cm of the optical path length.

  In Example 2, a calcium fluoride single crystal was manufactured by the single crystal manufacturing method according to the present invention using a calcium fluoride pretreated product manufactured by a conventional method as a raw material.

  The manufacturing process of the pre-processed goods by the conventional method is as follows.

Using the manufacturing apparatus shown in FIG. 1, 1 mol% of lead fluoride was added as a scavenger to chemically synthesized calcium fluoride as a raw material, and the crucible 6 of the manufacturing apparatus was filled. The vacuum furnace 1 was sealed and the inside was evacuated to 5 × 10 ⁻⁵ Pa, and then the heater 5 was energized and heated to 800 ° C. After maintaining the scavenge reaction at 800 ° C. for 10 hours, the temperature is raised to 1450 ° C. to melt the calcium fluoride, and then kept for 8 hours, and then cooling is started to crystallize. did. The carbon monoxide partial pressure in the vacuum furnace 1 at the start of cooling was 8 × 10 ⁻⁴ Pa.

  Next, a calcium fluoride single crystal was manufactured by the single crystal manufacturing method according to the present invention using the pre-processed product manufactured by the above-described conventional method as a raw material and using the single crystal manufacturing apparatus of FIG.

First, the above-mentioned pretreatment product was filled in the crucible 17 of the single crystal manufacturing apparatus, and the inside of the vacuum furnace 11 was evacuated. The degree of vacuum in the vacuum furnace 11 is measured by the total pressure gauge 20, and when the degree of vacuum reaches 3 × 10 ⁻⁴ Pa, the crucible 17 is positioned at the top, the upper heater 15 is set to 1450 ° C., and the lower heater 16 is set to 1300 ° C. The calcium fluoride in the crucible 17 was melted by setting. At the same time, monitoring of the carbon monoxide partial pressure by the partial pressure gauge 12 was started.

As the calcium fluoride in the crucible 17 melted, the carbon monoxide partial pressure inside the vacuum furnace 11 temporarily increased and reached a maximum of 5 × 10 ⁻⁴ Pa. Thereafter, when the holding at 1450 ° C. was continued, the carbon monoxide partial pressure in the vacuum furnace 1 rapidly decreased in the first few hours, and then gradually decreased to 8 × 10 ⁻⁵ Pa in about 80 hours. Converged to.

  When 96 hours had elapsed after melting, the crucible pulling was started, and the crucible 17 was pulled down at a constant speed of 1.0 mm / hour to grow a single crystal. After the contents of the crucible 17 were completely crystallized, it was gradually cooled to near room temperature, and then the inside of the production apparatus was opened to the atmosphere to take out a single crystal (ingot).

When the manufactured ingot was evaluated for the same number of defects as in Example 1, the result was 12 pieces / 1000 cm ³ .

A test piece was prepared from the manufactured ingot by the same procedure as in Example 1, and ArF excimer laser was irradiated at a fluence of 50 mJ / cm ² per pulse, and the light transmittance at a wavelength of 193 nm after irradiation of 100,000 pulses was measured. did. The internal transmittance excluding the surface reflection of the test piece was 99.8% per 1 cm of the optical path length.

  In this example, the pretreatment product produced in Example 1 was used as a raw material to fill the crucible 17 of the single crystal production apparatus (FIG. 2), and thereafter a single crystal was produced by the same process as in Example 2. The holding time from the melting of the raw material to the start of the crucible pulling was 96 hours.

Since the carbon monoxide has already been removed from the raw material used in this example (produced in Example 1), the carbon monoxide partial pressure in the melting step during the production of the single crystal is lower than in Example 2. The maximum was 3 × 10 ⁻⁴ Pa and the convergence value was 8 × 10 ⁻⁵ Pa.

When the manufactured ingot was evaluated for the same number of defects as in Example 1, the result was 0 pieces / 1000 cm ³ .

  A test piece was produced from the obtained ingot and the transmittance after ArF excimer laser irradiation was measured in the same procedure as in each example. The internal transmittance excluding the surface reflection of the test piece was 99. It was 9%.

Comparative example

  As a comparative example, a calcium fluoride pretreated product was produced by a conventional method, and a single crystal was produced using the pretreated product as a raw material.

The same calcium fluoride raw material as in each example was prepared, 1 mol% of lead fluoride was added as a scavenger, and the crucible 6 of the manufacturing apparatus shown in FIG. 1 was filled. The vacuum furnace 1 was sealed and the inside was evacuated to 3 × 10 ⁻⁴ Pa, then the heater 5 was energized, and the temperature was raised to 800 ° C. while evacuation was continued. After maintaining the scavenge reaction at 800 ° C. for 10 hours, the temperature was raised to 1450 ° C. to melt the calcium fluoride, kept for 8 hours, and then cooled and crystallized to obtain a pretreated product. The carbon monoxide partial pressure in the vacuum furnace 1 at the start of cooling was 8 × 10 ⁻⁴ Pa.

Using this pretreated product as a raw material, the crucible 17 of the apparatus shown in FIG. 2 was filled, and the inside of the apparatus was evacuated. When the degree of vacuum reached 5 × 10 ⁻⁵ Pa, the crucible 17 was positioned at the top, the upper heater 15 was set to 1450 ° C., and the lower heater 16 was set to 1300 ° C. to melt calcium fluoride. After the calcium fluoride was completely melted, the crucible was pulled down after being held for 12 hours, and the crucible 17 was pulled down at a constant speed of 1.0 mm / hour to grow a single crystal. The carbon monoxide partial pressure in the vacuum furnace 1 at the start of the crucible pull-down was 2 × 10 ⁻⁴ Pa.

  After the contents of the crucible 17 were completely crystallized, it was gradually cooled to near room temperature, and then the inside of the production apparatus was opened to the atmosphere and the ingot was taken out.

When the manufactured ingot was evaluated for the same number of defects as in Example 1, the result was 1089 pieces / 1000 cm ³ .

  A test piece was produced from the obtained ingot and the transmittance after ArF excimer laser irradiation was measured in the same procedure as in each example. The internal transmittance excluding the surface reflection of the test piece was 99. 0%.

It is an example of the manufacturing apparatus of a fluoride crystal. It is an example of the manufacturing apparatus of a fluoride single crystal. It is an example of exposure apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Vacuum furnace, 2 ... Partial pressure gauge, 3 ... Exhaust port, 4 ... Heat insulating material, 5 ... Heater, 6 ... Crucible, 7 ... Total pressure gauge, 8 ... Support member, 11 ... Vacuum furnace, 12 ... Partial pressure gauge, 13 ... Exhaust port, 15 ... upper heater, 16 ... lower heater, 17 ... crucible, 18 ... support member, 19 ... lowering mechanism, 30 ... optical member, 31 ... illumination optical system, 35 ... projection optical system, 39 ... optical member, 100 ... Vacuum ultraviolet light source, R ... Mask, W ... Wafer

Claims (8)

A melting process in which the fluoride filled in the crucible is heated in a vacuum furnace to form a fluoride melt, a carbon monoxide removal process for removing carbon monoxide (CO) from the fluoride melt, and a carbon monoxide removal process. And a crystallization step of cooling and crystallizing the fluoride melt that has passed through the method. 2. The method for producing a fluoride crystal according to claim 1, wherein the crystallization step is a single crystal growth step for growing a single crystal. A melting step of filling a crucible with the fluoride crystal produced by the method according to claim 1 and heating it in a vacuum furnace to form a fluoride melt, and removing carbon monoxide (CO) from the fluoride melt. A method for producing a fluoride single crystal, comprising: a carbon monoxide removal step; and a single crystal growth step of growing the single crystal by cooling the fluoride melt that has undergone the carbon monoxide removal step. 5. The carbon monoxide removal step is to hold the fluoride in a molten state until a carbon monoxide partial pressure in the vacuum furnace becomes 1 × 10 ⁻⁴ Pa or less. The manufacturing method of the fluoride crystal or fluoride single crystal as described in any one. The method for producing a fluoride crystal or a fluoride single crystal according to any one of claims 1 to 4, wherein the crucible contains carbon (C). The method for producing a fluoride crystal or a fluoride single crystal according to any one of claims 1 to 5, wherein the fluoride is calcium fluoride (CaF ₂ ). A method for producing a single crystal. The optical member for vacuum ultraviolet optical systems which consists of a calcium fluoride single crystal manufactured by the method of Claim 6. An exposure apparatus comprising the optical member for a vacuum ultraviolet optical system according to claim 7.

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