JP2007308349A - METHOD FOR HEAT-TREATING BaLiF3 CRYSTAL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance the transparency of a crystal by efficiently removing crystallized particles of LiF which are apt to generate abundantly in the crystal when the BaLiF<SB>3</SB>crystal useful as a last lens for an liquid immersion type aligner is produced by a melt method. <P>SOLUTION: The method for heat-treating the BaLiF<SB>3</SB>crystal comprises having a temperature gradient which satisfies all of the following requirements of (1)-(3), when the BaLiF<SB>3</SB>crystal is subjected to a highest temperature. (1) The temperature difference between one side surface part and the other side surface part is at least greater than 10°C, preferably within the range of 20-70°C. (2) The temperature of the high temperature side surface part is lower than 857°C, preferably not higher than 855°C, and particularly not higher than 850°C. (3) The temperature of the low temperature side surface part is higher than 775°C, preferably not lower than 780°C, more preferably not lower than 785°C, and particularly not lower than 790°C. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a heat treatment method for BaLiF ₃ crystal. More specifically, the present invention relates to a heat treatment method that is promising as a last lens of an immersion type exposure apparatus and can remove turbidity of a BaLiF ₃ crystal and improve its transmittance.

  In lithography processes carried out in the field of manufacturing electronic materials such as semiconductor integrated circuits, there is an increasing demand for miniaturization of the transfer pattern on the exposure substrate, and an improvement in the resolution of the exposure apparatus is being studied to realize this requirement. .

  In general, it is known that in an exposure apparatus, as the exposure wavelength is smaller and the numerical aperture of the lens is larger, the resolution can be improved by reducing the resolution line width. For this reason, light in the vacuum ultraviolet region with a wavelength of 200 nm or less (for example, ArF excimer laser; oscillation wavelength 193 nm, F2 excimer laser; oscillation wavelength 157 nm, etc.) can be used as a light source, and can handle such short wavelength light New optical system designs and lens materials are being developed.

  In parallel with these attempts, by filling a liquid between the exposure substrate and the last lens of the exposure apparatus, the wavelength of light on the exposure substrate surface is substantially shortened to improve the resolution. Research on immersion exposure equipment is also underway.

  An immersion exposure apparatus is an apparatus that includes at least a light source, an illumination optical system, a mask (reticle), a projection optical system, and a liquid supply / recovery device. Is an apparatus that performs exposure in a state in which a liquid is filled between a lens (last lens) provided on the substrate and an exposure substrate having a resist film. The last lens of such an immersion type exposure apparatus has a high refractive index and transmittance at the light wavelength of the light source, low or no intrinsic birefringence or stress birefringence, and durability against the light of the light source. Various performances are required, such as being durable and resistant to the liquid used.

The present inventors have studied a material having the above-described physical properties that can be used as a last lens of an immersion type exposure apparatus. As a result, the present inventors have found that a BaLiF ₃ single crystal has excellent physical properties and have already proposed (Japanese Patent Application No. 2005-152231).

Further, as applications other than the last lens of an immersion exposure apparatus, it has already been proposed to use a BaLiF ₃ single crystal as an optical member of a vacuum ultraviolet light apparatus such as an exposure apparatus (for example, Patent Documents 1 and 2). reference).

As disclosed in these patent documents, a BaLiF ₃ single crystal is produced directly from a raw material of BaF ₂ and LiF in a molar ratio of 1: 1 in melt growth such as the Czochralski method and the Bridgman method. It is necessary to prepare and manufacture a melt with excess LiF.

This reason can be understood from the phase equilibrium diagram of BaF ₂ and LiF shown in FIG. That is, as shown in FIG. 1, even when a molten liquid having a composition of BaF ₂ and LiF in a molar ratio of 1: 1 is cooled, the upper limit temperature at which BaLiF ₃ crystals can exist at the molar ratio. BaF ₂ is precipitated at a temperature about 50 to 60 ° C. higher than 857 ° C. or higher. Therefore, the ratio of LiF is higher than the theoretical molar ratio of 1: 1, thereby adjusting the raw material melt in which the BaF ₂ precipitation temperature is close to the BaLiF ₃ precipitation temperature, and a seed crystal composed of BaLiF ₃ crystals is prepared. It is necessary to grow a BaLiF ₃ single crystal in contact. In other words, no method has yet been found for directly producing a BaLiF ₃ single crystal from a molten raw material having a molar ratio of BaF ₂ and LiF of 1: 1. Will be used.

JP 2002-228802 A JP, 2003-119096, A Claims, 0008 paragraph, Example 3

However, when a BaLiF ₃ single crystal is produced from a molten raw material containing LiF in excess of the theoretical molar ratio as described above, LiF is phase-separated and precipitated in the BaLiF ₃ crystal, and thus the obtained crystal It has been clarified by examination of the present inventors that the material often appears cloudy.

Such turbidity results in a decrease in transmittance, and as a result, when using a BaLiF ₃ single crystal as an optical material as described above, it becomes a very serious problem.

Therefore, the present invention improves the light transmittance by removing or reducing the turbidity of the crystal in which LiF is precipitated in the BaLiF ₃ crystal, which is obtained from a molten raw material containing excessive LiF, and causing turbidity. It is an object of the present invention to provide a method.

  The inventors of the present invention conducted intensive studies to solve the above-mentioned problems, and as a result, found that turbidity was reduced by performing a heat treatment that generates a temperature gradient in the crystal, and as a result of further investigation, The present invention has been completed.

That is, the present invention is a method for heat-treating a BaLiF ₃ crystal, wherein the temperature distribution state when the crystal is brought to the highest temperature has a temperature gradient that satisfies the following conditions (1) to (3): This is a heat treatment method for a BaLiF ₃ crystal, which is carried out so as to have a state of having.
(1) There is a temperature difference of at least 10 ° C. between one surface portion and the opposite surface portion. (2) The temperature of the high temperature side surface portion is less than 857 ° C. (3) The temperature of the low temperature side surface portion is 775 ° C. Higher than

When the BaLiF ₃ crystal having turbidity due to LiF precipitated in the crystal by the above method is heat-treated, the LiF is discharged out of the crystal, thereby reducing the turbidity of the BaLiF ₃ crystal. As a result, a crystal with high transmittance can be obtained stably.

  Such a crystal having a good transmittance, particularly a single crystal, is useful as a material that transmits vacuum ultraviolet light, such as the last lens of the immersion exposure apparatus as described above.

The BaLiF ₃ crystal as an object of the heat treatment in the present invention may be manufactured by any method, but is a crystal manufactured by a melt growth method such as the Czochralski method or the Bridgman method. In many cases, LiF precipitates in the crystal body, and thus the effect of the present invention is remarkably obtained.

A method for producing a BaLiF ₃ crystal by the Czochralski method (CZ method) or the Bridgman method is disclosed in the above-mentioned Japanese Patent Application Laid-Open Nos. 2002-228802 and 2003-1119096, but more specifically. In particular, a method for producing a BaLiF ₃ crystal by the CZ method will be described below.

As BaF ₂ and LiF used as raw materials, those having as little impurities as possible are preferably used, and the concentration of metal impurities other than alkaline earth metals is preferably 10 ppm or less, more preferably 5 ppm or less, and particularly preferably 1 ppm or less. In addition, it is preferable to use a raw material from which moisture and oxides (BaO, Li ₂ O, etc.) are removed as much as possible.

Such a raw material BaF ₂ and LiF are mixed at a molar ratio of about 35:65 to 48:52, and a seed crystal composed of a BaLiF ₃ single crystal is brought into contact with the molten liquid and gradually raised to obtain a crystal. As described above, with the melt growth method, it is practically impossible to obtain a BaLiF ₃ crystal having a size that can be used as an optical material with reproducibility unless an LiF-excess raw material is used.

When pulling up, the raw material BaF ₂ and LiF mixed in the above molar ratio are filled in a carbon crucible such as a high-density graphite sintered body, a platinum crucible or the like, and the temperature is raised to the melting temperature or higher in a CZ furnace. The melting temperature varies depending on the molar ratio of BaF ₂ and LiF as shown in FIG. As raw materials to be filled in the crucible, those in powder form may be used, or raw materials mixed in advance at a predetermined ratio and further heated to form a sintered body or a melt-solidified body may be used. Prior to melting in the CZ furnace, evacuation (preferably about 10 ⁻⁵ to 10 ⁻² Pa) of the furnace up to about 600 to 650 ° C. can also remove volatile impurities such as adsorbed moisture. preferable. Further, it is also preferable to introduce a fluorine-based gas such as HF or CF ₄ after evacuation or without evacuation, and to heat in the atmosphere from the viewpoint that moisture and oxide can be efficiently removed.

A seed crystal composed of a BaLiF ₃ single crystal is brought into contact with a sufficiently melted raw material melt and gradually pulled up. The seed crystal can be arbitrarily selected according to the orientation of the crystal to be pulled up. For example, <100> or <111> can be used.

The pulling can be performed in an atmosphere of a fluorine-based gas such as HF or CF ₄ , an inert gas such as Ar, He, Ne, or N ₂ , or a fluorine-based gas diluted with the inert gas. .

  The pulling speed is usually 0.1 to 20 mm / h.

After pulling up the BaLiF ₃ crystal of a desired length, it is cooled to about room temperature and taken out from the CZ furnace. During cooling, the higher the cooling rate, the less likely to cause phase separation. On the other hand, when the cooling rate is extremely high, there may be a problem that a crystal produced (pulled up) is cracked by thermal shock. Therefore, the cooling rate is preferably 5 to 500 ° C./hr.

The BaLiF ₃ crystal thus obtained is often a BaLiF ₃ single crystal having white turbidity presumed to be caused by precipitated LiF.

In the heat treatment method of the present invention, the BaLiF ₃ crystal obtained as described above may be heat treated as it is, but in order to increase the efficiency of the heat treatment, it is processed into a predetermined size by cutting, grinding or the like. It is preferable to perform from. That is, when the final application is, for example, a lens of an exposure apparatus, after processing into a disk shape that matches the size of the lens, the heat treatment of the present invention is performed, and then the final lens is processed. Is preferred. Specifically, although it depends on the size of the lens, it is usually processed into a disk shape having a diameter of 30 to 400 mm and a thickness of about 10 to 300 mm. Of course, any dimension may be used according to the shape of the final processed product.

After processing into a desired size, in the present invention, heat treatment is performed in a state where a specific temperature gradient is generated in the interior. That is, it is necessary that the temperature distribution state when the crystal to be heat-treated is at the highest temperature has a temperature gradient that satisfies all the following conditions (1) to (3).
(1) It has a temperature difference of at least 10 ° C. or more between one surface portion and the opposite surface portion.
(2) The temperature of the surface portion on the high temperature side is less than 857 ° C.
(3) The temperature of the low-temperature side surface is higher than 775 ° C.

  The reason is unknown, but if the temperature gradient from one surface of the crystal toward the other surface does not exist in the crystal so as to satisfy the condition (1), heat treatment can be performed. It is almost impossible to remove turbidity.

In addition, since BaLiF ₃ has a melting point of 857 ° C., when it is heated further, the crystal starts to melt, and in many cases, as shown in the phase equilibrium diagram (FIG. 1), BaF ₂ and liquid phase state Phase separation.

On the other hand, at a temperature of 775 ° C. or lower, LiF and BaLiF ₃ are phase-separated, but in such a temperature range, the LiF is not discharged out of the crystal but aggregates in the crystal, and rather turbidity increases There is.

  Conventionally, heat treatment of a crystal body is often performed in order to remove strain and the like, but such heat treatment is performed so as not to generate a temperature distribution in the crystal body as much as possible from the viewpoint of strain removal. However, a temperature difference of 10 ° C. or less, usually 5 ° C. or less is inevitably generated between the vicinity of the surface and the center portion, and a temperature gradient is positively generated as in the present invention. It is not a thing.

  The temperature difference between the two surfaces may be 10 ° C. or more, but from the viewpoint of turbidity removal efficiency, there should be a temperature difference, preferably 20 ° C. or more, more preferably 30 ° C. or more, and 40 ° C. or more. Is particularly preferred.

  On the other hand, the upper limit of the temperature difference is less than 82 ° C. (difference between 857 ° C. and 775 ° C.). Considering temperature control errors, it is preferably 75 ° C. or lower, more preferably 70 ° C. or lower, and more preferably 60 ° C. or lower.

The temperature of the surface portion on the high temperature side needs to be less than 857 ° C. as described above. However, it is more preferable to set the temperature to 855 ° C. or less, considering the BaF ₂ phase separation and considering the temperature control error. More preferably, it is as follows.

  The temperature of the low temperature side surface portion is preferably 780 ° C. or higher, more preferably 785 ° C. or higher, and particularly preferably 790 ° C. or higher.

  The time for maintaining the temperature gradient in the crystal body is 30 minutes to 60 hours, usually 1 to 48 hours, although it depends on the thickness of the crystal body (the length in the direction having the temperature gradient). It is. The thicker the thickness, the easier it is to obtain good results when the state having the temperature gradient is kept for a long time.

  The temperature gradient may exist in any direction of the crystal body, but in order to efficiently remove turbidity, the temperature gradient is preferably present in a direction in which the length decreases.

  The rate of temperature rise to such a state having a temperature gradient as described above is such that the crystal is 700 ° C. or higher and 775 ° C. or lower during the temperature rise, and in particular, the state of 750 ° C. or higher is not prolonged. It is preferable to make it. If the temperature is kept within the temperature range for a long time, LiF aggregates as described above, and it becomes difficult to remove the aggregated LiF even if the LiF is held so as to have the temperature gradient state as described above. On the other hand, if the temperature is raised too rapidly, the crystal may be damaged by thermal shock. For these reasons, it is preferable that the temperature rising rate exceeding 700 ° C. up to the maximum temperature of the heat treatment is 5 to 20 ° C./hr.

  On the other hand, the cooling rate may be slow. When the temperature is lowered, all or most of the excess LiF has already been removed from the crystal body, and therefore the state at 775 ° C. or lower may be present for a long time. The slower the temperature drop rate, the higher the effect of removing the turbidity of the crystal body, and the strain is less likely to remain in the crystal body. The temperature decreasing rate is usually about 0.1 to 5 ° C./hr, particularly about 0.1 to 2 ° C./hr up to about 700 to 750 ° C.

The above heat treatment may be performed under normal pressure or under reduced pressure. However, LiF is easily discharged out of the crystal body under reduced pressure, further under reduced pressure exhaust, particularly under vacuum exhaust. It is preferable to carry out with. The higher the degree of decompression, the better. However, the pressure in the heat treatment furnace is usually in the range of 1 × 10 ⁻⁶ to 10 Pa, particularly 1 × 10 ⁻⁵ to 1 × 10 ⁻² Pa.

In addition, when performing at normal pressure or under reduced pressure, the atmosphere is preferably a non-oxidizing atmosphere, and fluorine-based gases such as HF and CF ₄ , inert gases such as Ar, He, Ne, and N ₂ , Or it can carry out in atmosphere, such as a fluorine-type gas diluted with this inert gas.

  The method for heat treatment so as to have a temperature gradient as in the present invention is not particularly limited, but for example, it is possible to use the apparatus shown in FIG.

That is, the heater (2) is disposed above the chamber (1) constituting the heat treatment furnace, and the side, bottom, and ceiling between the chamber (1) and the heater (2) are insulated wall (7 ) To form a hot zone. The BaLiF ₃ crystal (3), which is the object of heat treatment, is placed on the heat sink (4), and on the upper surface thereof, the heating from the heater (2) is uniform so that it is uniform in the lateral direction. A hot plate (5) is arranged. Further, the crystal body (3) is surrounded by an annular heat insulating material (6) on the side portion.

  In the embodiment of FIG. 2, a cylindrical support member (8) extending from the furnace bottom to a space (hot zone) surrounded by the heat insulating wall (7) through a hole provided in the bottom heat insulating wall is disposed. The heat radiating plate (4), the crystal body (3) and the like are held in the hot zone by the support member (8).

  When the soaking plate (5) is heated by the heater (2), the upper surface of the crystal (3) is heated, but heat is radiated from the heat radiating plate (4) brought into contact with the lower surface of the crystal. Therefore, a temperature gradient is generated in the crystal (3).

  The temperatures of the upper surface and the lower surface of the crystal body (3) can be measured by placing thermocouples (not shown) at the corresponding positions.

  The annular heat insulating material (6) suppresses heat transfer due to radiation from the soaking plate (5) to the heat radiating plate (4) and prevents heating due to radiation from the side to the crystal body (3). It has the effect of facilitating the temperature difference between the upper and lower parts of the crystal (3).

  When other conditions are the same, the temperature gradient can be changed by changing the surrounding thickness (the length in the left-right direction in FIG. 2) of the annular heat insulating material (6). That is, if the surrounding thickness is increased, the effect of suppressing heat transfer by radiation from the heat equalizing plate (5) to the heat radiating plate (4) becomes higher, so the temperature gradient becomes steep, and conversely the surrounding thickness is reduced. If it is made thinner, the temperature gradient becomes gentler.

  The temperature gradient can also be controlled by controlling the heat dissipation of the heat sink (4). For example, as shown in the figure, if a plurality of heat radiating plates (4) are provided with a space between them, the temperature gradient can be relaxed by increasing the number of sheets, and conversely, there is little or no provision. This can make the temperature gradient steep.

  The temperature gradient can also be controlled by changing the thermal conductivity of the heat sink (4). If a material with high thermal conductivity is used, the temperature gradient can be sharpened, and if a low material is used, it can be made loose.

  Other methods of changing the temperature gradient include lowering the temperature outside the space surrounded by the heat insulating material wall (7) and the heat sink (4) (for example, cooling the chamber with cooling water) It is also possible by forcibly cooling 4).

  In FIG. 2, by providing a through-hole in the bottom heat insulating material (7), the temperature below the crystal body (3) and the soaking plate (5) is relatively lowered. A method of increasing the heat dissipation from the bottom heat insulating material so as to easily cause a temperature difference by thinning all or part thereof may be used.

  Furthermore, a method of embedding a crystal in a through or non-through hole provided in the bottom heat insulating material, or a rod-like heat radiating member is projected into a space surrounded by a heat insulating material wall through a hole in the bottom heat insulating material, and a crystal is formed thereon. A method of arranging the body can also be adopted.

  Further, the heat radiating plate (4) and the heat equalizing plate (5) are not necessarily required, and an embodiment in which they are not in direct contact with the crystal body (3) is possible, but an appropriate temperature gradient in the vertical (vertical) direction is also possible. On the other hand, it is preferable to make contact as described above in that it is easy to obtain lateral thermal uniformity.

  In the above, the aspect of the position where the heater (2) and the crystal body (3) shown in FIG. 2 are vertically arranged has been described, but the heater is down, the crystal body is up, or a heater is provided in the center, There may be any positional relationship such as arranging the crystals vertically.

  When the heat treatment of the present invention is performed in the furnace having the above-described structure, even in the temperature lowering process, the crystal body has a temperature difference. Therefore, it is preferable to perform a second heat treatment (hereinafter sometimes referred to as a second heat treatment) from the viewpoint of removing distortion of the crystal. This second heat treatment may be performed in the same manner using the same apparatus as the heat treatment (annealing) for removing strain of a conventionally known fluoride single crystal.

  That is, a crystal body that has been heat-treated to generate the above temperature gradient is placed in a conventionally known annealing furnace in which the temperature of the hot zone in the furnace becomes as uniform as possible, and is less than 857 ° C., usually 750 to 855 ° C. , Preferably after heating to 800-850 ° C., gradually cooled. In order to remove the strain efficiently, it is preferable that there is no temperature gradient inside the crystal body. However, when the temperature increasing / decreasing rate during the second heat treatment is high, a temperature difference is likely to occur between the vicinity of the surface and the central portion. . The temperature difference generated at the time of rapid temperature rise can be set to 10 ° C. or less, preferably 5 ° C. or less, more preferably 3 ° C. or less by maintaining the maximum temperature for an appropriate time. The holding time is usually from 1 minute to 60 hours, and in many cases from 10 minutes to 48 hours, depending on the size of the crystal. If the crystal is small, the temperature is increased slowly, or the initial temperature decrease is sufficiently slow (for example, 1 ° C./hr or less), the temperature is immediately decreased without maintaining the maximum temperature after the temperature increase. You may do.

  The temperature is raised to the maximum temperature and, if necessary, kept at the temperature, and then lowered. In order not to cause a large temperature difference between the central portion and the vicinity of the surface, and thus no new strain is generated, it is preferably about 700 ° C., more preferably about 600 ° C., 0.1 to 20 ° C./hr, especially 0 The temperature may be lowered at a rate of about 1 to 5 ° C./hr. After the crystal body becomes the above temperature or lower, the temperature may be lowered at a higher rate.

  What is necessary is just to process into a desired shape after performing the 2nd heat processing for distortion removal as mentioned above. For example, when a lens of an exposure apparatus is used, it may be processed into a lens shape according to a known method.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited to these Examples.

Powdered BaF ₂ and LiF were mixed at a molar ratio of BaF ₂ : LiF = 43: 57, accommodated in a platinum crucible (inner diameter 130 mm, height 180 mm), and accommodated in a CZ crystal growth furnace. Next, the inside of the furnace was kept at a vacuum of 1 × 10 ⁻³ Pa or less and the crucible was heated to 600 ° C. over 24 hours, and then CF ₄ gas with a purity of 99.999% was introduced into the furnace. Atmospheric pressure. Thereafter, the temperature of the crucible was raised to 900 ° C. over 2 hours to melt the mixture.

Next, a seed crystal whose contact surface is the <100> orientation of BaLiF ₃ is brought into contact with the raw material melt in the crucible, and this seed crystal is pulled up at a speed of 1.0 mm / hr while rotating at 10 rpm. _Three single crystals were grown. The obtained single crystal had a total length of 150 mm, a length of the straight body portion of 100 mm, and a diameter of the straight body portion of 50 mm.

The straight body portion of the BaLiF ₃ single crystal is cut into round pieces, and the surface is polished until the surface roughness is 0.5 nm or less by RMS to prepare a sample having a thickness of 10 mm. The transmittance was measured in the range of 140 to 200 nm using a rate measuring device (manufactured by JASCO Corporation, KV-201; measured in a nitrogen atmosphere with an oxygen content of 0.2 ppm or less). The transmittance (including surface reflection) at 193 nm, which is the wavelength of the ArF laser, was 18%. The transmittance at other wavelengths is shown in FIG. Further, it was clear that this sample was clouded even by visual observation after irradiation with halogen light.

Since the surface reflection loss calculated from the refractive index of 1.64 at 193 nm of BaLiF ₃ corresponds to about 11%, the theoretical transmittance is about 89%. Therefore, the internal transmittance at 193 nm before the heat treatment calculated from the transmittance including the surface reflection is 20% / cm.

  Subsequently, the same sample was placed in a heat treatment furnace having a structure shown in FIG. 2 (however, the heater was annularly arranged near the side heat insulating material wall). The diameter of the hole provided in the bottom heat insulating material wall is 300 mm, the heat radiating plate (4) and the soaking plate (5) are both made of a high density graphite sintered body, the diameter is 230 mm, and the thickness is 5 mm. Further, as an annular heat insulating material (6) surrounding the crystal body (3), carbon felt (GF-20-10F manufactured by Nippon Carbon Co., Ltd.) is cut into a disk shape having a thickness of 10 mm and a diameter of 100 mm, and a diameter is formed at the center. A 50 mm hole is formed. Moreover, in order to measure the temperature of the upper surface and the lower surface of the crystal body, the upper center portion of the soaking plate (5) and the lower center portion of the heat radiating plate (4) in contact with the crystal body (3) A thermocouple was placed in

While evacuating the inside of the furnace using a diffusion pump and an oil rotary pump (1 × 10 ⁻² Pa or less), the temperature was raised at 60 ° C./hr until the indicated temperature of the upper thermocouple reached 700 ° C., and 700 ° C. The temperature was raised to 850 ° C. at 12.5 ° C./hr and kept at this temperature for 24 hours. At this time, the indicated temperature of the bottom thermocouple was 805 ° C.

  Next, the temperature was lowered to 750 ° C. at 1.4 ° C./hr, and then lowered to room temperature over 24 hours.

  After the temperature was lowered to room temperature, the crystal was taken out from the furnace. As a result, white powder adhered to the surface of the crystal (3) and the heat radiating plate (4). When this powder was observed with a microscope, it was assumed that the powder had a dice-like shape and had a cubic system. Further, when analyzed by EDS, a metal element having an atomic number equal to or higher than boron was not detected. From these facts, the powder was judged to be LiF.

  The surface of the crystal was again polished by RMS until it became 0.5 nm or less to obtain a sample for VUV transmittance measurement (thickness: 9.5 mm), and transmittance in the range of 140 to 200 nm was measured. The transmittance (including surface reflection) at 193 nm, which is the wavelength of the ArF laser, was 55%. The transmittance of other wavelengths is shown in FIG. 3 together with the result before the heat treatment.

  Since the internal transmittance calculated from the transmittance including the surface reflection of the 9.5 mm-thickness-treated sample is 62% / cm, the transmittance is greatly improved by the heat treatment given the temperature gradient. I understand that. In addition, it was confirmed by visual observation that the white turbidity was significantly reduced compared with that before the heat treatment.

It is a phase diagram of LiF and BaF _2. It is a schematic diagram of the typical heat processing furnace used in order to perform the heat processing of this invention. Before and after performing the heat treatment of the present invention, a vacuum ultraviolet spectral data of BaLiF ₃ single crystal.

Explanation of symbols

1. 1. Chamber of heat treatment furnace Heater 3. BaLiF ₃ crystal 4. 4. Heat sink Soaking plate6. 6. Annular heat insulating material Insulation wall 8. Support member

Claims (6)

A method of heat treating a BaLiF ₃ crystal so that the temperature distribution when the crystal is at the highest temperature has a temperature gradient that satisfies the following conditions (1) to (3): A heat treatment method for a BaLiF ₃ crystal, characterized in that:
(1) There is a temperature difference of at least 10 ° C. between one surface portion and the opposite surface portion. (2) The temperature of the high temperature side surface portion is less than 857 ° C. (3) The temperature of the low temperature side surface portion is 775 ° C. Higher than The heat treatment method for a BaLiF ₃ crystal according to claim 1, wherein the heat treatment is performed under reduced pressure. A heat treatment method for a BaLiF ₃ crystal, wherein the heat treatment is performed again under the condition that the maximum temperature difference in the BaLiF ₃ crystal is less than 10 ° C. after the heat treatment by the method according to claim 1 or 2. A method of heat-treating a BaLiF ₃ crystal, wherein the surface is heated from one surface and dissipates heat from the opposite surface, and the temperature of the heated surface is less than 875 ° C. The BaLiF ₃ crystal is heat-treated by heating until the temperature of the surface becomes higher than 775 ° C. and then cooling to room temperature. A method for manufacturing a last lens of an immersion type exposure apparatus, wherein the BaLiF ₃ crystal body heat-treated by the method according to claim 1 is processed into a lens shape. A BaLiF ₃ crystal is pulled up by a Czochralski method from a melt of a raw material in which BaF ₂ and LiF are mixed at a molar ratio of 0.35: 0.65 to 0.48: 0.52, BaLiF ₃ method for producing a single crystal of performing heat treatment at claims 1 to 4, wherein the method.

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