JP2008201644A - MANUFACTURE PROCESS OF BaLiF3 SINGLE CRYSTAL - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stably, simply and inexpensively manufacture an optical member which is composed of BaLiF<SB>3</SB>crystal having the light-transmitting principal axis direction in the <100> direction, useful as the last lens in an immersion exposure apparatus for the manufacture of semiconductors and has a good uniformity in the refractive index. <P>SOLUTION: An ingot is manufactured by growing the crystal in the <111> direction by Czochralski method. The ingot is sliced in the diagonal direction to obtain a plate-like article having two parallel faces composed of ä100} faces. The plate-like article has a more uniform stress distribution (distortion) than the one grown in the <100> direction and sliced horizontally and gives a member with a good refractive index uniformity. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a method for producing an optical member made of a BaLiF ₃ single crystal. More specifically, the present invention relates to a manufacturing method that is promising as a last lens of an immersion type exposure apparatus and that can improve the optical characteristics of a BaLiF ₃ crystal whose principal axis direction for transmitting light is the <100> direction.

  In the lithography process implemented 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 improvement of the resolution of the exposure apparatus is being studied to realize this requirement. Yes.

  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.

As a material having the above physical properties that can be used as the last lens of such an immersion exposure apparatus, a BaLiF ₃ single crystal has been proposed (see Patent Document 1).
Further, as applications other than the last lens of the immersion type 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 2 and 3). reference).

When using a BaLiF ₃ single crystal as a last lens of an immersion type exposure apparatus, in order to prevent a reduction in exposure resolution due to intrinsic birefringence characteristics of the BaLiF ₃ single crystal, design of an optical system, A device is devised for the pattern shape drawn on the mask (reticle). In the design of the optical system, for example, the projection optical system is configured in combination with a lens material having intrinsic birefringence different from that of a BaLiF ₃ single crystal such as a CaF ₂ single crystal, thereby canceling the influence of intrinsic birefringence. Although improvement of exposure accuracy is being studied, in optical design of such an exposure apparatus, BaLiF ₃ single crystal used as a last lens of an immersion type exposure apparatus transmits light. It is said that a single crystal having a crystal orientation <100> having 4-fold symmetry with respect to the principal axis direction is preferred.

The BaLiF ₃ single crystal can be produced by melt growth such as the Czochralski method or the Bridgman method as disclosed in the above-mentioned patent documents.

  Usually, when a single crystal for an optical member such as a lens is manufactured by the Czochralski method, the single crystal is grown in the same direction as the principal axis direction through which light is transmitted from the viewpoint of the simplicity of the method and the processing efficiency. After obtaining a lump of crystal body (hereinafter referred to as “ingot”), the ingot is cut along a plane perpendicular to the crystal growth direction to obtain a plate-like body having a desired crystal direction (hereinafter, “ (Referred to FIG. 1).

  In the Czochralski method, by selecting the crystal orientation of the seed crystal, it is possible to easily control the crystal growth direction of the single crystal ingot, and the ingot whose crystal growth direction is the <100> direction is It is possible to manufacture the seed crystal by pulling up the seed crystal after bringing the seed crystal facing the <100> direction with respect to the crystal growth direction into contact with the raw material melt.

JP 2007-005777 A JP 2002-228802 A Japanese Patent Laid-Open No. 2003-119096

However, when an ingot grown in the <100> direction by the Czochralski method as described above is cut perpendicularly to the growth direction to obtain a BaLiF ₃ single crystal disc, It has been clarified by the present inventors that refractive index homogeneity (also referred to as homogeneity) often deteriorates.

  Refractive index homogeneity is a necessary characteristic when used as the last lens of an immersion type exposure apparatus. If the refractive index homogeneity of the last lens is low, the resolution at the time of exposure is lowered, which is a very serious problem. Become.

As a result of further investigations by the present inventors, when the strain distribution state was observed by a method called crossed Nicols for the BaLiF ₃ single crystal disk produced by the above method, the heterogeneous strain distribution was found to be a crystal plane. Since the portion where the distortion was inhomogeneous coincided with the region where the refractive index was disturbed in the disk surface, the distortion was inhomogeneously distributed. Is one of the causes of the decrease in refractive index homogeneity. The inhomogeneous distribution of strain observed in such a crossed Nicol is partially relaxed by the stress generated in the crystal during the growth of the BaLiF ₃ single crystal being slipped on the crystal plane, and the other parts. It is presumed that it was caused by the change in the state of distortion.

Therefore, the present invention suppresses slip during BaLiF ₃ single crystal growth, thereby providing high refractive index homogeneity required as a characteristic of the last lens of the immersion type exposure apparatus and a principal axis direction through which light is transmitted is < and to provide a method for producing an optical member made of BaLiF ₃ single crystal 100> direction.

  The present inventors have intensively studied to solve the above-mentioned problems, and as a result, by growing the ingot in the <111> direction by the Czochralski method, slip occurs on the crystal plane during the growth, and strain is reduced. It is possible to suppress the distribution state from becoming inhomogeneous, and by cutting the optical member from the ingot grown in the <111> direction so that the principal axis direction for transmitting light is the <100> direction, As a result of finding that an optical member having high refractive index homogeneity can be produced and further studying it, the present invention has been completed.

That is, the present invention is a method for manufacturing an optical member that is made of a BaLiF ₃ single crystal and has a principal axis direction through which light is transmitted being the <100> direction in the single crystal, and the seed crystal is brought into contact with the BaLiF ₃ melt. A step of growing the ingot in the <111> direction by the Czochralski method of pulling up and obtaining a single crystal, and then processing the ingot to obtain a plate-like body having two parallel faces of {100} faces a BaLiF ₃ process for producing an optical member made of a single crystal which comprises a.

After the ingot is grown in the <111> direction by the Czochralski method by the above method, a plate-like body having a plane parallel to the {100} plane is cut out from the ingot, and an optical member made of a BaLiF ₃ single crystal body Thus, it is possible to suppress the occurrence of slip on the crystal plane during growth, and an optical member having high refractive index homogeneity can be obtained stably, simply and inexpensively.

An optical member made of a BaLiF ₃ single crystal having a <100> direction through which light is transmitted, manufactured according to the present invention, has high refractive index homogeneity and is used as a material for the last lens of an immersion exposure apparatus. Useful.

The optical member which is the object of the present invention is made of a BaLiF ₃ single crystal, and the principal axis direction through which the light is transmitted is the <100> direction of the BaLiF ₃ single crystal. Here, the principal axis direction is a direction serving as a reference axis for light transmitted through the optical member. When the optical member is a lens, the axis along which the centers of curvature of the lens spherical surfaces are arranged (also referred to as an optical axis). (See FIG. 2). In the optical member that is the object of the present invention, the main axis direction for transmitting the light does not need to be <100> strictly <100> direction, and may be substantially in that direction. The allowable range of the deviation is appropriately determined depending on the use of the optical member. Usually, the deviation of the angle between the main axis direction of the BaLiF ₃ single crystal and the <100> direction is within 10 °, preferably within 5 °, more preferably within 3 °, and particularly preferably within 1 °.

The optical member which is the object of the present invention is not particularly limited as long as it is made of a BaLiF ₃ single crystal and the direction of transmitting light is the <100> direction in the single crystal. A lens for an exposure apparatus, particularly a lens for an immersion type exposure apparatus or a lens blank is preferable because of high demand for an optical member having high refractive index homogeneity. Usually, such a lens is manufactured by first manufacturing a disk (disk) -shaped lens blank obtained by cutting an ingot manufactured by the Czochralski method, and then processing the lens.

  Hereafter, the manufacturing method of the optical member of this invention is demonstrated in detail.

First, in the production of the optical member, a method of growing an ingot of a BaLiF ₃ single crystal in the <111> direction by the Czochralski method will be specifically 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 raw materials BaF ₂ and LiF are mixed at a molar ratio of about 35:65 to 48:52, and a seed crystal made of BaLiF ₃ single crystal is brought into contact with the molten liquid and gradually pulled to obtain a single crystal. . As described in Patent Documents 1 and 2, etc., in the melt growth method, unless a LiF-excess raw material is used, a BaLiF ₃ single crystal having a size that can be used as an optical material is reproducible. It is virtually impossible to get specially.

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, etc., and the temperature is raised to the melting temperature or higher in a Czochralski furnace (CZ furnace). Warm up. Here, when a double crucible described in JP-A-2006-199577 is used as the crucible, the dust can be easily removed when the melt floats on the melt surface when the melt is melted. It is preferable because melt convection during growth can be stabilized and mixing of bubbles and the like can be prevented in the crystal.

The melting temperature of the raw material 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 of the furnace to about 600 to 650 ° C. (preferably about 10 ⁻⁵ to 10 ⁻² Pa) 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 single crystal composed of a BaLiF ₃ single crystal is brought into contact with a sufficiently melted raw material melt and gradually pulled up. In order to set the crystal growth direction to <111>, as the seed single crystal, a crystal whose pulled-up axial direction coincides with the <111> direction is used. Further, the cut surface when cut perpendicular to the axis is the {111} plane. The deviation of the seed crystal from the axial direction may be within 10 °, preferably within 5 °, more preferably within 3 °, and particularly preferably within 1 °. Usually, a cylindrical or prismatic seed crystal having such a crystal direction is used. The shape of the tip of the seed crystal, that is, the portion in contact with the raw material melt is not particularly limited, and may be any shape such as a flat shape, a cone shape, or the like. The pulling can be performed under normal pressure, reduced pressure or increased pressure. It is preferable to carry out under reduced pressure because a BaLiF ₃ single crystal with few crystal defects such as a negative crystal can be easily obtained. The atmosphere may be 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. From the viewpoint of easily eliminating the influence of oxygen, it is preferable to carry out in an atmosphere of a fluorine gas diluted with a fluorine gas or an inert gas. The pulling speed is usually 0.1 to 20 mm / h.

A BaLiF ₃ single crystal having a desired length is pulled up, 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, problems such as cracks may occur in the manufactured (pulled up) single crystal due to thermal shock. Therefore, the cooling rate is preferably 1 to 500 ° C./hr, more preferably 3 to 50 ° C./hr.

In this way, an ingot of BaLiF ₃ single crystal is obtained by the Czochralski method. Usually, an ingot is obtained as a single crystal, but even when there is a portion that has been polycrystallized due to fluctuations during growth, a single crystal can be obtained by cutting out the portion of the single crystal from the ingot.

In the production method of the present invention, cutting and grinding of a plate-like body (disk) having two parallel planes of {100} planes from an ingot of a BaLiF ₃ single crystal grown in the <111> direction by the above-described method. It is obtained by processing such as. Usually, a disc having two faces consisting of {100} faces is cut out and then processed to have two parallel faces consisting of {100} faces by grinding and polishing (see FIG. 4). In addition, the manufactured single crystal body is cut thickly in the horizontal direction with respect to the growth direction (substantially becomes a disk having a {111} plane), and then is obliquely ground to produce the {100} plane. May be.

By processing into a plate-like body having a plane parallel to the {100} plane in this way, it is possible to easily align the main axis direction through which light of the optical member to be finally manufactured to <100>. Since the step of cutting out in a plane parallel to the {100} plane is a step that enables the operation of aligning the crystal direction of the final optical member, the cut out plane is substantially parallel to the {100} plane. The angle between the perpendicular of the cut surface and the <100> direction vector of the BaLiF ₃ single crystal is within 10 °, preferably within 5 °, more preferably within 3 °, and particularly preferably within 1 °. is there.

The angle formed between the perpendicular of the cut surface and the <100> direction vector of the BaLiF ₃ single crystal can be specified using a known evaluation method such as crystal orientation measurement using X-rays. Further, in order to achieve sufficient optical performance in the stage of processing into the final optical member shape, the direction of the principal axis through which light is transmitted may be adjusted more precisely.

  Any known method may be used as a processing method when cutting an ingot, and cutting is generally performed using a cutting device equipped with a peripheral blade or a cutting blade such as an endless so-called band saw or wire saw. In particular, it is preferable to use an endless cutting blade such as a band saw or a wire saw in consideration of the loss of the workpiece during cutting.

  In the case where the ingot is easily broken during the cutting process, it is also possible to prevent the occurrence of cracking by cutting gradually from the end of the ingot using a cylindrical grinder, and to cut out the desired shape.

  The plate-like body (disk) thus obtained can be used as a lens blank as it is, or the outer peripheral portion can be processed into a disk shape as necessary to form a lens blank. Furthermore, if the lens blank is processed into a lens shape according to a known method, a lens can be obtained.

  When the lens is used for an exposure apparatus, it is preferable to remove the distortion by performing an annealing process before the lens processing so that a sufficient resolution can be obtained.

For the annealing treatment, a known method known as a method for annealing a fluoride single crystal may be applied. Specifically, it may be heated to a temperature lower by about 5 to 50 ° C. than the melting point of BaLiF ₃ (857 ° C.), and then gradually lowered. The temperature lowering rate is about 0.1 to 5 ° C./hr, particularly about 0.1 to 2 ° C./hr. It is also effective to slowly lower the temperature in a higher temperature range and gradually increase the temperature lowering rate as the temperature decreases.

  The annealing process may be performed in an ingot state, but it is more effective to perform the annealing process after processing to a size close to the final part shape. For example, it may be performed before the outer peripheral portion or the like is processed into a disk shape, or after the outer peripheral portion is processed into a disk shape, annealing is performed, and then the surface is further ground and polished to form a lens blank. It can also be.

  Moreover, as an optical member other than a lens or a lens blank, it is also preferable to apply the manufacturing method of the present invention to the manufacture of a window material for vacuum ultraviolet light.

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

Example 1
The bulk LiF raw material and the BaF ₂ raw material obtained by melting and solidifying the LiF powder and the BaF ₂ powder, respectively, have a molar ratio of LiF: BaF ₂ of 0.57: 0.43, and the total of the raw materials The mixture is mixed so that the amount becomes 3 kg, and accommodated in a double structure crucible composed of an inner crucible and an outer crucible disclosed in JP 2006-199577 A, etc., inside a Czochralski crystal growth furnace (CZ furnace) Housed in. The inner diameter of the outer crucible constituting the double crucible was 120 mm, and the inner diameter of the inner crucible was 84 mm. Next, the inside of the furnace was kept at a vacuum degree of 1 × 10 ⁻³ Pa or less and the crucible was heated to 600 ° C. over 24 hours, and a CF ₄ gas having a purity of 99.999% was introduced into the furnace. The pressure was 80 kPa. Thereafter, the temperature of the crucible was raised to 900 ° C. over 2 hours to melt the mixture.

Next, the raw material melt in the crucible is brought into contact with a seed crystal made of BaLiF ₃ single crystal and the vertical direction is the <111> direction, and the seed crystal is pulled up at a speed of 1.0 mm / h while rotating at 15 rpm. Thus, an ingot made of a BaLiF ₃ single crystal was grown. A BaLiF ₃ single crystal ingot was grown to a predetermined size, and then the ingot was separated from the melt. Subsequently, after cooling the CZ furnace over 36 hours, the ingot was taken out from the CZ furnace. The obtained ingot had a total length of 130 mm, a length of the straight body portion of 100 mm, and a diameter of the straight body portion of 50 mm.

About said ingot, it cut | disconnected from the straight body part by two surfaces parallel to the {100} surface, and obtained the plate-shaped member, and also the side surface of the obtained plate-shaped member was cylindrical-grinded into the disk shape Later, the cut surface was further polished to obtain a disk of BaLiF ₃ single crystal having a diameter of 30 mm and a thickness of 10 mm.

When the strain state of the obtained BaLiF ₃ single crystal disk was observed with crossed Nicols, the strain distribution was small over the entire disk, and there was almost no linear trace in which the strain distribution state changed drastically. It was not confirmed (FIG. 5). Further, refractive index homogeneity was evaluated using a light source having a wavelength of 633 nm with a Zygo Mark III GPI (6-inch interferometer). The refractive index homogeneity after removing Zernike's 36 terms was 258 ppb in terms of least mean square (RMS). FIG. 6 shows the refractive index distribution after removing 36 items of Zernike.

Comparative Example 1
A BaLiF ₃ single crystal ingot was grown by the Czochralski method in the same manner as in Example 1 except that the BaLiF ₃ single crystal in the <100> direction, which is the vertical direction of the seed crystal, was used. The obtained ingot had a total length of 130 mm, a length of the straight body portion of 100 mm, and a diameter of the straight body portion of 50 mm. From the straight body portion of the above ingot, cut in a plane parallel to the {100} plane perpendicular to the growth direction of the ingot (plane perpendicular to the crystal pulling direction) to form a disk, and then cylindrically grind the side surface. Further, the cut surface was polished to obtain a BaLiF ₃ disk having a diameter of 45 mm and a thickness of 10 mm. Furthermore, when the strain state of the obtained BaLiF ₃ disk was observed with crossed Nicols in the same manner as in Example 1, a linear trace in which the strain was unevenly distributed was confirmed (FIG. 5). . Furthermore, the refractive index homogeneity was evaluated in the same manner as in Example 1. The refractive index homogeneity after removing 36 items of Zernike was 493 ppb in RMS. FIG. 6 shows the refractive index distribution after removing 36 items of Zernike.

The schematic diagram which shows the conventional method of obtaining a plate-shaped body (disk). The schematic diagram which shows the principal axis (optical axis) direction in a lens. Phase equilibrium diagram of LiF and BaF _2. The schematic diagram which shows the process example of the ingot in this invention. The crossed Nicols image of the plate-shaped body obtained by the Example and the comparative example. The refractive index distribution map of the plate-shaped body obtained by the Example and the comparative example.

Claims (4)

A method for producing an optical member comprising a BaLiF ₃ single crystal, wherein the principal axis direction through which light is transmitted is the <100> direction in the single crystal, and is pulled up after bringing the seed crystal into contact with the BaLiF ₃ melt A step of growing an ingot in the <111> direction by a ski method to obtain a single crystal, and then processing the ingot to obtain a plate-like body having two parallel surfaces of {100} planes. A method for producing an optical member made of a BaLiF ₃ single crystal. A method of manufacturing a plate-like optical member made of a BaLiF ₃ single crystal and having two parallel planes of {100} planes, which is ingot by the Czochralski method of pulling up after bringing a seed crystal into contact with a BaLiF ₃ melt. In the <111> direction to obtain a single crystal, and then processing the ingot to obtain a plate-like body having two parallel faces consisting of {100} faces. Method.   The manufacturing method according to claim 1, wherein the optical member is a lens blank.   A manufacturing method of a lens for an exposure apparatus, wherein a lens blank is manufactured by the method according to claim 3, and then the lens blank is processed into a lens.

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