JP2005145727A - Method for removing cloudiness from fluorite, optical fluorite, optical system and aligner - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reducing or removing cloudiness in fluorite. <P>SOLUTION: In the method, a fluorite material 10 is placed inside an airtight heat-treatment furnace 15, the airtight heat-treatment furnace 15 is evacuated, the evacuation is completed after the pressure inside the airtight heat-treatment furnace 15 has reached a value equal to or below the desired pressure, and temperature elevation is initiated after evacuation completion T1. By adjusting the pressure inside the airtight heat-treatment furnace 15 to ≤1 Pa at temperature elevation initiation T2, oxygen inside the fluorite material 10 is diffused into the exterior of the fluorite material 10 after temperature elevation initiation. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a method for removing dust from fluorite by heat-treating fluorite used in an optical system, and an optical fluorite from which dust is removed using the method, an optical system, and an exposure apparatus.

  An optical lithography technique for exposing and transferring a fine pattern of an integrated circuit on a wafer such as silicon has been well known. In recent years, semiconductors have been highly integrated, and accordingly, an optical lithography apparatus with higher resolution is required.

In order to increase the resolution of the light source of this photolithography apparatus, the wavelength has been reduced from g-line (436 nm) to i-line (365 nm), KrF excimer (248 nm), ArF excimer (193 nm), and F ₂ laser.

  Here, an optical glass material currently used as a material of an optical member such as a lens of an illumination optical system or a projection optical system of an optical lithography apparatus using a light source having a wavelength longer than that of i-line has a shorter wavelength region than that of i-line. Then, the light transmittance is drastically lowered, and light is not transmitted through most optical glasses, particularly in a wavelength region of 250 nm or less. Therefore, the lens material of the optical system of the photolithography apparatus using an excimer laser or the like as a light source is currently limited to a part of materials that can transmit light of such a short wavelength. As one of the materials, fluorite (calcium fluoride crystal) is known.

  As described above, fluorite used in the imaging optical system of an optical lithography apparatus using a light source such as an excimer laser is required to have a very high refractive index homogeneity and a small distortion, and has an excellent light transmittance. Need to be. Regarding the light transmittance, for example, a projection exposure apparatus requires a lens having a very large curvature for aberration correction, and the optical path length of the entire projection optical system may reach as much as 1000 mm. In this case, in order to keep the transmittance as a whole of the projection optical system at 80% or more, the internal transmittance per 10 mm of the optical member needs to be 99.8% or more. Furthermore, the high transmittance needs to be maintained over the entire aperture of the lens.

  In recent years, the specifications for refractive index homogeneity and distortion have become very strict. In order to produce higher-quality fluorite optical components, the grown fluorite is annealed to produce fluorite. It has been practiced to improve the homogeneity by reducing the strain remaining in the interior (for example, see Patent Document 1 below).

In this annealing treatment, for example, fluorite is placed in a heat treatment furnace together with a fluorinating agent, the gas in the furnace is exhausted, and various volatile impurities, gases, moisture, etc. present in the atmosphere in the heat treatment furnace are removed. However, the temperature inside the furnace is increased by raising the temperature to a fluorine atmosphere, followed by predetermined heating and cooling.
JP-A-10-231194

  One of the internal defects in fluorite is a phenomenon called nigari. This dust is observed as a white scatterer when the light beam passes through the fluorite.

  The details of the cause of the dust are not clear. Conventionally, even if the fluorite has not been confirmed to be dusty, if the distortion inside the fluorite is removed by annealing, the dust may be generated. When this dust is generated, problems such as a decrease in transmittance and flare generation due to scattering occur. Therefore, in order to produce a higher quality fluorite optical member, it is desired to reduce or remove dust in the fluorite. Yes.

  Accordingly, the present invention provides a method capable of reducing or removing dust existing in fluorite, an optical meteorite with less dust, and an optical system and an exposure apparatus using the same. And

  The present inventors conducted research on fluorite dust and found that the cause of the dust was due to oxygen. In particular, oxygen was found to be localized in the subgrain boundary or dislocation network, which is a crystal defect of fluorite. Moreover, it has been found that some conventional annealing treatments cause oxygen to agglomerate due to heating, resulting in deterioration of the dust.

  Then, the oxygen localized in the fluorite is diffused to the outside of the fluorite and released, so that this dust can be reduced, and further, when the pressure around the fluorite is sufficiently reduced and heat-treated, It was discovered that the diffusion of oxygen inside the stone was facilitated, and it became possible to eliminate the dust.

  That is, in the invention according to claim 1, the fluorite material is accommodated in an airtight heat treatment furnace, the gas in the airtight heat treatment furnace is exhausted, and after the pressure in the airtight heat treatment furnace reaches a desired pressure or lower, Ending the exhaust, and starting the temperature rise after the end of the exhaust, and performing a heat treatment, the pressure inside the hermetic heat treatment furnace at the start of the temperature rise is reduced to 1 Pa or less, After starting, oxygen in the fluorite material is diffused to the outside of the fluorite material.

  The invention according to claim 2 is characterized in that, in addition to the configuration according to claim 1, the pressure in the hermetic heat treatment furnace at the end of the exhaust is made lower than the pressure at the start of temperature rise.

The invention described in claim 3 is characterized in that, in addition to the structure described in claim 2, the pressure in the hermetic heat treatment furnace at the end of the exhaust is 1 × 10 ⁻² Pa or less.

  According to a fourth aspect of the present invention, in addition to the configuration of the first to third aspects, the pressure increase in the hermetic heat treatment furnace during a predetermined time from the end of the exhaust to the start of the temperature rise is within a predetermined range. The airtightness of the airtight heat treatment furnace is ensured so that the heat treatment is performed.

  According to a fifth aspect of the present invention, in addition to the structure according to any one of the first to fourth aspects, the heat treatment is performed at a temperature range of 800 ° C. or higher and 1300 ° C. or lower after the temperature increase starts. The temperature is raised to a constant temperature and maintained at a constant temperature for a predetermined time, and then the temperature is gradually lowered.

  According to a sixth aspect of the present invention, in addition to the configuration according to any one of the first to fifth aspects, the temperature is raised by storing a fluorinating agent together with the fluorite material in the hermetic heat treatment furnace. Thus, the heat treatment is performed in a fluorine atmosphere by vaporizing the fluorinating agent in the hermetic heat treatment furnace.

  According to a seventh aspect of the present invention, in addition to the configuration according to any one of the first to sixth aspects, the fluorite material is disposed on a support portion in the hermetic heat treatment furnace. The heat treatment is performed in a state in which the bottom portion is separated from the floor surface of the hermetic heat treatment furnace.

  According to an eighth aspect of the present invention, there is provided an optical fluorite according to any one of the first to seventh aspects.

  An optical system according to a ninth aspect includes the optical fluorite according to the eighth aspect.

  An exposure apparatus according to a tenth aspect has the optical system according to the ninth aspect.

  According to the first to seventh aspects of the present invention, oxygen in the fluorite material is diffused to the outside of the fluorite material by performing the heat treatment with the pressure in the hermetic heat treatment furnace at the start of raising the temperature being 1 Pa or less. Therefore, oxygen localized in the subgrain boundary or dislocation network, which is a crystal defect of the fluorite material, can be easily diffused to the outside of the fluorite material, and the dust can be reduced.

  According to the invention described in claim 2 or claim 3, since the pressure in the hermetic heat treatment furnace at the end of exhaust is made lower than the pressure at the start of temperature rise, moisture in the atmosphere around the fluorite material at the time of heat treatment And oxygen can be reduced, oxygen present in the fluorite material can be easily released to the outside, and dust can be further reduced.

  According to the invention described in claim 4, the airtightness of the hermetic heat treatment is ensured so that the increase in pressure in the hermetic heat treatment furnace within a predetermined range between the end of exhaust and the start of temperature rise is within a predetermined range. Therefore, it is possible to keep the air pressure in the hermetic treatment furnace sufficiently low during the heat treatment, heat the meteorite in an atmosphere with a small amount of oxygen and moisture, and more easily reduce dust. can do.

  According to the invention described in claims 8 to 10, it is possible to provide an optical fluorite with a small amount of dust, and since there is little dust of the optical meteorite, a higher transmittance and a small flare can be obtained. It is possible to provide an optical optical system or an exposure apparatus that can obtain excellent resolution.

  Embodiments of the present invention will be described below. In this embodiment, the pre-manufactured fluorite material is heat-treated using a heat treatment furnace as shown in FIG. 1 to reduce or remove the dust existing inside the fluorite material, thereby reducing the optical meteorite. An example of obtaining the above will be described.

  First, the optical fluorite obtained in this embodiment is suitable as an optical element that can be used for an optical system that is irradiated with light of a short wavelength, such as a KrF excimer laser, an ArF excimer laser, or an F2 laser, or a material thereof. Fluorite that can be used for

  Further, the fluorite material is, for example, a calcium fluoride crystal manufactured using a known manufacturing method such as the Bridgman method, and may be a crystal lump after growth. What removed the residual distortion at the time of a growth by annealing treatment may be used. Further, it may be processed into various optical element shapes. In this embodiment, a crystal lump that has been grown by the Bridgman method and has dust inside is used.

  As shown in FIG. 2A, this dust is caused by, for example, oxygen mixed during the growth of the fluorite material 10, and oxygen 11 mixed during the growth of the fluorite material 10 is a subgrain having a crystal structure defect. It is presumed that these oxygens 11 exist locally in the gaps 12 of the crystal lattice such as the boundary or the dislocation network, become a scattering center when the light of the collector is incident, and are observed as dust.

  Such dust is removed using an airtight heat treatment furnace 15 shown in FIG. Here, the atmosphere in the hermetic heat treatment furnace 15 is set to a sufficiently low pressure, and the heat treatment is performed while maintaining the low pressure state. In addition, since the fluorite material 10 is a grown crystal mass, an annealing process for removing strain remaining in the fluorite material 10 is also performed in the heat treatment for removing the stagnation.

  The hermetic heat treatment furnace 15 is disposed in a casing 16 and is hermetically sealed and can be evacuated and decompressed, a carbon container 18 disposed in the hermetic container 17, and the inside of the hermetic container 17 and the carbon container 18. And a heater 19 that can be heated from the outer periphery.

  In order to heat-treat the fluorite material 10 using the airtight heat treatment furnace 15, first, a fluorite bottom support portion 21 on which the fluorite material 10 can be placed is placed inside the carbon container 18. By placing the fluorite material 10 on the bottom support 21, the bottom surface 10 a of the fluorite material 10 is accommodated while being separated from the floor surface 15 a of the heat treatment furnace 15. Thereby, the surface of the fluorite material 10 is not covered with other members as much as possible, and the meteorite material 10 is arranged so as to be in direct contact with the atmosphere inside the carbon container 18 as much as possible. In addition, the fluorinating agent 22 is housed in the carbon container 18 together with the fluorite material 10. As the fluorinating agent 22, for example, a substance that can vaporize the fluorine component by raising the temperature, such as Teflon (registered trademark), acidic ammonium fluoride, or the like can be used.

  Next, in a state where the fluorite material 10 and the fluorinating agent 22 are accommodated in the carbon container 18, the hermetic container 17 is sealed, and the exhaust path 17a is evacuated by a vacuum pump (not shown). FIG. 3 shows the pressure change in the airtight container 17 during the exhaust process.

When the pressure in the airtight container 17 and the carbon container 18 reaches a desired pressure or less due to this exhaust, the exhaust by the vacuum pump is finished. The pressure P1 in the airtight heat treatment furnace 15 at the end of exhaust T1 is preferably 1 × 10 ⁻² Pa or less. Thereby, the amount of oxygen and water vapor remaining in the atmosphere in the hermetic heat treatment furnace 15 can be sufficiently reduced, and a small amount of oxygen present in the fluorite material 10 can be sufficiently diffused.

  Next, after exhausting is completed, the heater 19 starts to raise the temperature in the hermetic container 17 and the carbon container 18 to perform heat treatment.

  In many cases, the airtight container 17 includes a very small amount of leak and a volatile component from the container. As a result, in this hermetic heat treatment furnace 15, the pressure P1 in the hermetic heat treatment furnace 15 at the end of exhaust T1 is different from the pressure P2 in the hermetic heat treatment furnace 15 at the start of temperature rise T2, and is different from the pressure P1 at the end of exhaust T1. The pressure P2 at the start of temperature increase T2 increases.

  If the leak in the hermetic vessel 17 is large, it becomes difficult to maintain the atmosphere around the fluorite material 10 during the heat treatment. Therefore, in this embodiment, the leak amount of the hermetic treatment furnace 15 is examined and its hermeticity is confirmed. Confirm. This airtightness must be able to maintain an oxygen content and a water content that are low enough to diffuse oxygen present in the fluorite material 10 to the outside during the heat treatment. It is possible to secure an atmosphere that can remove water.

  Here, the pressure increase in the hermetic heat treatment furnace 15 is set to be within a predetermined range during a predetermined time from the exhaust end T1 to the temperature rise start T2. More specifically, after the passage of time of 60 minutes or more from the end T1 of exhaust, the pressure P2 in the hermetic heat treatment furnace 15 at the start of temperature rise T2 can be maintained within 100 times the pressure P1 at the end of exhaust T1. It is particularly preferred to have

  Further, in addition to such airtightness, in the present invention, the pressure P2 in the airtight heat treatment furnace 15 at the start of temperature increase T2 is set to 1 Pa or less. Without such a pressure, it is difficult to diffuse oxygen in the fluorite material 10 to the outside after starting the temperature rise, and the oxygen dispersed in the fluorite material 10 may be aggregated and the dust may be easily deteriorated. Because there is.

  At this time, the partial pressure of oxygen and the partial pressure of water vapor remaining in the hermetic container 17 and the carbon container 18 of the hermetic heat treatment furnace 15 at the start of temperature increase T2 are at least 1 Pa or less, and in particular, the oxygen partial pressure is set to 0. If it is 2 Pa or less and the water vapor partial pressure is 0.05 Pa or less, oxygen in the fluorite material 10 is more easily diffused, which is preferable.

  In this temperature increase, the fluorite material 10 is heated to a temperature range of 800 ° C. or higher and 1300 ° C. or lower. During the temperature rise, the inside of the hermetic heat treatment furnace 15 becomes a fluorine atmosphere by the fluorinating agent 22 accommodated in the hermetic heat treatment furnace 15. In this state, the heat treatment is performed by maintaining the temperature at a constant temperature for a predetermined time and then gradually lowering the temperature.

  When such a heat treatment is performed, the inside of the hermetic treatment furnace 15 has a sufficiently low pressure, so that the oxygen 11 in the fluorite material 10 is maintained during the temperature rise or at a constant temperature, as shown in FIG. It passes through gaps 12 in crystal lattices such as subgrain boundaries and dislocation networks that exist in a network, and diffuses to the outside of the low pressure around the fluorite material 10. As a result, the amount of oxygen present in the fluorite material 10 is reduced, the turbidity is reduced and most preferably removed completely.

  At the same time, in this heat treatment, a constant temperature is maintained within a predetermined temperature range, and then the temperature is gradually lowered. Therefore, an annealing treatment for reducing residual strain in the fluorite material 10 can be performed at the same time. Therefore, fluorite with better homogeneity can be obtained.

  When the fluorite material 10 is heat-treated as described above, the oxygen P11 in the fluorite material 10 is fluorinated by performing the heat treatment with the pressure P2 in the hermetic heat treatment furnace 15 at the start of temperature rise T2 being 1 Pa or less. Since it diffuses to the outside of the stone material 10, it is easy to diffuse the oxygen 11 localized between the crystal gaps 12 of the fluorite material 10 to the outside of the fluorite material 10, and to reduce the dust. Is possible.

  Further, since the pressure P1 in the hermetic heat treatment furnace 15 at the end of exhaust T1 is made lower than the pressure P2 at the start of temperature rise T2, moisture, oxygen, etc. in the atmosphere around the fluorite material 10 at the time of heat treatment are reduced. This makes it easier to release the oxygen present in the fluorite material 10 to the outside and to further reduce the dust.

  Furthermore, if the airtightness in which the pressure increase in the airtight heat treatment furnace 15 during the predetermined time from the exhaust end time T1 to the temperature rising start time T2 is within a predetermined range can be secured, the leakage of the airtight processing furnace 15 is reduced. In addition, it is possible to maintain an atmosphere with a sufficiently small amount of oxygen and moisture during heat treatment, and it is easier to reduce dust.

  In the above-described embodiment, the meteorite material 10 has been described as having a dust, but even a meteorite material 10 that has no dust and may cause a dust due to a conventional annealing process. The present invention can be applied in exactly the same manner.

  Further, in the above, the heat treatment for removing the dust is performed together with the annealing treatment for removing the residual strain of the fluorite material 10, but the treatment for removing the dust may be performed separately from the annealing treatment. .

  In that case, for example, as shown in FIG. 4A to FIG. 4C, first, annealing treatment can be performed, and then heat treatment for removing dust can be performed.

First, as shown in FIG. 4A, in the meteorite material 10 made of the crystal lump after the growth, oxygen mixed at the time of growth is dispersed in the entire fluorite material 10 and no dust is observed. The fluorite material 10 is depressurized to about 10 ⁻¹ Pa at the end of evacuation as before, and after the end of evacuation, the temperature rise is started in a state where the internal pressure is about several Pa to 10 Pa and annealed. When the treatment is performed, as shown in FIG. 4B, oxygen aggregates in the subgrain boundary and the dislocation network in the fluorite material 10, and a dust is generated. Then, if the meteorite material 10 in which the dust is generated is heat-treated in the same manner as in the above embodiment, as shown in FIG. 4C, oxygen in the meteorite material 10 is diffused to the outside to remove the dust. It becomes possible.

  Next, an exposure apparatus using the optical meteorite obtained by removing dust as described above will be described. FIG. 5 shows the basic structure of the exposure apparatus.

  The exposure apparatus 30 includes at least a light source 31 for supplying an excimer laser or the like as exposure light, an illumination optical system 32 for supplying the exposure light to the mask R, and an image of the pattern of the mask R on the substrate W to be exposed. A projection optical system 33 for projecting is included.

  The illumination optical system 32 includes an alignment optical system 34 for adjusting the relative position between the mask R and the substrate W to be exposed. The mask R is disposed on a mask stage 36 whose position is controlled by a mask exchange system 35. The substrate to be exposed W is arranged on a wafer stage 38 whose position is controlled by a stage control system 37. Further, the light source 31, the alignment optical system 34, the mask exchange system 35, and the stage control system 37 are controlled by the main control unit 39.

  In such an exposure apparatus 30, although not shown in detail, a large number of optical members are arranged in the illumination optical system 32 and / or the projection optical system 33. An optical member made of optical fluorite may be used for at least some of these many optical members for various reasons, and the exposure apparatus 30 of this embodiment is also made of optical fluorite. An optical member is disposed. As the optical fluorite, fluorite from which dust is reduced or removed as described above is used.

  According to the exposure apparatus 30 having such an illumination optical system 32 and / or the projection optical system 33, since the optical member made of optical meteorite is less turbid, higher transmittance can be obtained and a higher resolution can be obtained. Can be obtained.

[Examples 1 to 5 and Comparative Examples 1 and 2]

  Using a fluorite material 10 made of crystal grains after growth having substantially the same shape, the airtightness of the airtight heat treatment furnace 15 and the pressure at the end of exhaustion are varied, and the exhaust and temperature are raised under the conditions shown in Table 1. Heat treatment was performed, and dust was observed before and after the heat treatment.

  Here, in Example 3, the fluorinating agent 22 and the fluorinating agent 22 were disposed in the hermetic heat treatment furnace 15, but in other examples and comparative examples, heat treatment was performed without being disposed. Further, in Example 4, the bottom support 10 is disposed and the fluorite material 10 is placed, so that the bottom surface 10a of the fluorite material 10 is separated from the floor surface 15a of the hermetic heat treatment furnace 15, but other In the examples and comparative examples, the heat treatment was performed without arranging the bottom upper support portion 22.

  The dust was visually evaluated by irradiating each fluorite with the same parallel light. Evaluation was made into 5, 4, 3, 2, 1 in five steps from the one where darkness was deep, 3 or less was accepted, and 4 or more was rejected.

  The results are shown in Table 1.

In Example 1, after exhausting to 10 ⁻² Pa or less and exhausting, the pressure at the start of the temperature rise was reduced to about 0.9 Pa and the heat treatment was performed, and the dust density passed 3 and passed. On the other hand, in Comparative Example 1, after exhausting to 10 ⁻¹ Pa or less in the prior art and ending the exhaust, when heat treatment was performed at a pressure at the start of temperature increase of about 5 Pa, the concentration of dust after the heat treatment remained at 5 It was rejected.

In Example 2, after exhausting to 10 ⁻² Pa or less and ending the exhaust, when the heat treatment is performed by lowering the pressure at the start of temperature rise to about 0.2 Pa, the density of the dust becomes 2 that is thinner than Example 1 became.

In Example 3, after placing the fluorinating agent 22 in the hermetic heat treatment furnace 15 and exhausting it to 10 ⁻² Pa or less and ending the exhaust, the pressure at the start of temperature increase is lowered to about 0.9 Pa and the fluorine is increased in the temperature increasing process. When the heat treatment agent 22 was vaporized and heat treatment was performed with the inside of the hermetic heat treatment furnace 15 in a fluorine atmosphere, the density of the dust became 2 which was thinner than in the example, which was acceptable.

In Example 4, after exhausting to 10 ⁻² Pa or less according to the present invention and ending the exhaust, the heat treatment in which the pressure at the start of temperature increase was reduced to about 0.2 Pa was performed. In order to further promote the diffusion of oxygen, the bottom portion 10a of the fluorite material 10 is placed on the support portion 22 made of fluorite and subjected to a heat treatment with a gap (hereinafter referred to as “bottom heat treatment”) to provide oxygen. It is now possible to increase the diffusion path of, promote diffusion and make the darkness density 1.

In Example 5, after passing the fluorite optical member having passed the density of 3 and passing to 10 ⁻² Pa or less of the present invention to finish the exhaustion, the pressure at the start of temperature increase was lowered to about 0.2 Pa. When the heat treatment is performed, the density of the dust becomes 2 which is thinner, whereas in Comparative Example 2, the pressure at the start of the temperature rise is about 5 Pa after exhausting to 10 ⁻¹ Pa or less and ending the exhaust. After the heat treatment, the density of dust after the heat treatment deteriorated to 5 and was rejected.
[Example 6]

  Dust was observed under the conditions shown in Table 2 in the same manner as described above, except that the grown crystal mass was annealed to remove strain and then heat-treated to remove dust.

  The results are shown in Table 2.

In this Example 6, after exhausting to 10 ⁻¹ Pa or less in the prior art and ending the exhaust, annealing was performed with a pressure at the start of temperature increase of about 5 Pa. However, after exhausting to 10 ⁻² Pa or less and ending the exhaust, when the heat treatment at which the pressure at the start of the temperature increase was reduced to about 0.2 Pa, the density of the dust became 2 and passed.

It is sectional drawing which shows the airtight heat processing furnace of embodiment of this invention. It is a figure which shows the structure of a fluorite typically, and shows the dust produced at the time of a growth. It is a figure which shows the structure of a fluorite typically, and has shown the state which the dust produced at the time of a growth disappeared by heat-processing in embodiment of this invention. It is a graph which shows the pressure change in the airtight container 17 of an exhaust process. It is a figure which shows the structure of a fluorite typically, and has shown the state which was not seen after growth. It is a figure which shows the structure of a fluorite typically, and has shown the state which the dust generate | occur | produced by performing the conventional annealing. It is a figure which shows the structure of a fluorite typically, and has shown the state from which the dust which generate | occur | produced by annealing disappeared by heat-processing in embodiment of this invention. It is a figure which shows the basic structure of the exposure apparatus of embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Fluorite material 11 Oxygen 12 Crevice 15 Airtight heat treatment furnace 17 Airtight container 18 Carbon container 19 Heater 21 Bottom support 22 Fluorinating agent

Claims (10)

The fluorite material is housed in an airtight heat treatment furnace, the gas in the airtight heat treatment furnace is exhausted, and after the pressure in the airtight heat treatment furnace reaches a desired pressure or less, the exhaust is terminated, and from the end of the exhaust A method of starting the temperature rise later and performing heat treatment,
Fluorite that diffuses oxygen in the fluorite material to the outside of the fluorite material after the temperature rise is started by setting the pressure in the hermetic heat treatment furnace at the start of the temperature rise to 1 Pa or less How to remove dust. 2. The method for removing fluorite debris according to claim 1, wherein the pressure in the airtight heat treatment furnace at the end of the exhaust is made lower than the pressure at the start of the temperature increase. 3. The method for removing fluorite dust according to claim 2, wherein the pressure in the hermetic heat treatment furnace at the end of the exhaust is 1 × 10 ⁻² Pa or less. Ensuring the hermeticity of the hermetic heat treatment furnace and performing the heat treatment so that the pressure increase in the hermetic heat treatment furnace during a predetermined time from the end of the exhaust to the start of the temperature rise is within a predetermined range. The method for removing fluorite dust according to any one of claims 1 to 3. The heat treatment is characterized in that after the start of the temperature increase, the temperature of the fluorite material is increased to a temperature range of 800 ° C. or higher and 1300 ° C. or lower and maintained at a constant temperature for a predetermined time, and then the temperature is gradually decreased. The method for removing fluorite dust according to any one of claims 1 to 4. The fluorinating agent is housed in the hermetic heat treatment furnace together with the fluorite material and the temperature is raised, thereby vaporizing the fluorinating agent in the hermetic heat treatment furnace and performing the heat treatment in a fluorine atmosphere. 6. The method for removing dust from fluorite according to any one of claims 1 to 5. The heat treatment is performed in a state where the bottom portion of the fluorite material is separated from the floor surface of the hermetic heat treatment furnace by disposing the fluorite material on a support in the hermetic heat treatment furnace. The method for removing fluorite dust according to any one of claims 1 to 6. 8. An optical fluorite, wherein the fluorite dust removal method according to any one of claims 1 to 7 is applied. An optical system comprising the optical fluorite according to claim 8. An exposure apparatus comprising the optical system according to claim 9.

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