JP6264106B2 - Calcium fluoride optical member and manufacturing method thereof - Google Patents

Description

  The present invention relates to a calcium fluoride optical member and a method for producing the same, and more particularly to a calcium fluoride single crystal optical member and a method for producing the same.

A calcium fluoride (CaF ₂ , fluorite) single crystal has a high transmittance for light in a wide wavelength range from the vacuum ultraviolet region to the infrared region. Therefore, CaF ₂ single crystal is widely used as an optical member, and is used, for example, as a material for an arc tube of an excimer lamp (Patent Document 1).

JP 2009-163965 A

An optical member formed of a CaF ₂ single crystal is used in various environments having different temperatures and pressures, but it has been found that the strength is not sufficient in a high temperature / high pressure environment. Therefore, depending on the application and use environment of the optical member, there is the optical member formed by CaF ₂ single crystals are not suitable.

Therefore, an object of the present invention is to provide a CaF ₂ single crystal optical member that can be used reliably even at high temperatures and high pressures.

According to the first aspect of the present invention,
A calcium fluoride optical member,
It is formed of single crystal calcium fluoride and has a cylindrical shape,
The central axis of the cylinder and one of the three {100} crystal planes of the single crystal calcium fluoride orthogonal to each other are orthogonal to each other,
A radial thickness of the peripheral wall of the cylinder is 45 ° in one direction around the central axis with a plane parallel to any one of the three {100} crystal faces and including the central axis of the cylinder. Rotate the surface where the rotated surface and the peripheral wall intersect, and the plane parallel to the remaining one of the three {100} crystal planes and including the central axis of the cylinder by 45 ° in the one direction around the central axis A calcium fluoride optical member that is larger than the other part is provided at the place where the finished surface and the peripheral wall intersect.

According to the second aspect of the present invention,
A substantially rectangular tube-shaped optical member formed of single-crystal calcium fluoride and having a cylindrical space formed therein, wherein a pair of opposed outer surfaces of the substantially square-tube are made of single-crystal calcium fluoride. The first surface of the three orthogonal {100} surfaces is parallel to the other surface, and another set of opposed outer surfaces of the substantially square tube is parallel to the second surface of the {100} surfaces, An optical member is provided in which the bottom surface or top surface of the substantially rectangular tube is parallel to the third surface of the {100} surfaces.

According to a third aspect of the invention,
A method for producing a calcium fluoride optical member, comprising:
Identifying the direction in which three {100} crystal planes of single crystal calcium fluoride are orthogonal to each other;
Scraping the single crystal calcium fluoride into a cylindrical shape,
In the cutting, the central axis of the cylinder and any one of the three {100} crystal faces are orthogonal to each other, and the radial thickness of the peripheral wall of the cylinder is the three {100} crystals. A surface that is parallel to any one of the other surfaces and includes the central axis of the cylinder, rotated by 45 ° in one direction around the central axis, and a portion where the peripheral wall intersects, and the three {100} crystals A plane parallel to the remaining one of the planes and including the central axis of the cylinder is made to be larger than the other part at a point where the surface rotated by 45 ° in the one direction around the central axis and the peripheral wall intersect. The manufacturing method of the said calcium fluoride optical member is provided.

According to the present invention, a CaF ₂ single crystal optical member having sufficient strength even in a high temperature and high pressure environment is provided.

FIG. 1 is a perspective view of an optical member according to an embodiment of the present invention. FIG. 2 is an explanatory diagram showing position constraint conditions and load conditions in the simulation. 3A to 3C show the positional relationship between the optical member and the unit lattice (cubic lattice) of the CaF ₂ single crystal, and the positional relationship between the optical member and the principal axis x, principal axis y, and principal axis z of the unit lattice. Show. FIG. 4 is a table showing analysis conditions and analysis results for the optical member of the example and the optical member of the comparative example. FIGS. 5A to 5C are analysis results of the optical member of the example, and show the state of shear stress generated in the optical member when the internal pressure is 30 atm. 6A to 6C are analysis results of the optical member of the example, and show a state of shear stress generated in the optical member when the internal pressure is 90 atm. FIG. 7 is an explanatory diagram illustrating a position where shear stress is generated due to internal pressure in the optical member of the example. FIG. 8 is a graph showing the relationship between the critical decomposition shear stress (CRSS) of CaF ₂ single crystal and the temperature for each crystal plane. FIGS. 9A to 9C are analysis results of the optical member of the comparative example, and show the state of shear stress generated in the optical member when the internal pressure is 30 atm. FIGS. 10A to 10C are analysis results of the optical member of the comparative example, and show the state of shear stress generated in the optical member when the internal pressure is 90 atm. FIG. 11 is an explanatory diagram showing a position where shear stress due to internal pressure occurs in the optical member of the comparative example. 12A to 12C are analysis results of the optical member of the comparative example, and show the state of shear stress generated in the optical member when the internal pressure is 30 atm. FIGS. 13A to 13C are analysis results of the optical member of the comparative example, and show the state of shear stress generated in the optical member when the internal pressure is 90 atm. FIG. 14 is an explanatory view showing a position where shear stress due to internal pressure occurs in the optical member of the comparative example.

<First Embodiment>
A first embodiment of the present application will be described with reference to FIGS.

As shown in FIG. 1, the optical member 10 of the present embodiment is a cylindrical CaF ₂ tube having a central axis A, and has an inner peripheral surface 10i and an outer peripheral surface 10o forming the tube. The cross-sectional shape orthogonal to the central axis has a substantially square outer shape and a circular opening concentric with the central axis A. That is, the optical member 10 has a substantially quadrangular prism shape in which a cylindrical space is formed. The four corners C of the outer peripheral surface 10o are rounded with a predetermined curvature.

The optical member 10 is formed by CaF ₂ single crystal, the three {100} crystal faces that are orthogonal to each other of the CaF ₂ single crystal (first surface, second surface, third surface), i.e., (100) crystal plane, One of the (010) crystal plane and the (001) crystal plane exists in a direction perpendicular to the central axis A. As an example, FIG. 3A shows an optical member having a (001) crystal plane in a direction perpendicular to the central axis A together with a unit cell UL of CaF ₂ single crystal. Hereinafter, the {100} crystal plane, (100) crystal plane, (010) crystal plane, etc. are simply referred to as {100} plane, (100) plane, (010) plane, etc.

In the optical member 10, among the three {100} planes of the CaF ₂ single crystal that are orthogonal to each other, the remaining two {100} planes that are not orthogonal to the central axis A are two sets of opposing outer peripheral surfaces 10o, respectively. Each surface is parallel to each other. In the optical member 10 shown in FIG. 3 (a), the (010) plane exists in parallel with a pair of opposing surfaces of the outer peripheral surface 10o, and the (100) plane is another set of opposing outer peripheral surfaces 10o. It can be seen that it exists parallel to the surface.

Note that "CaF ₂ single crystal" as used herein, not only literal single crystal consisting of a single crystal, a CaF ₂ crystals of two or more of the number of degree counted crystal, CaF ₂ A CaF ₂ crystal having the same optical characteristics as a single crystal is also included. Also, in this specification, “a plane orthogonal to the axis (center axis)” and “a plane perpendicular to the axis (center axis)” are not only a plane orthogonal to the axis (“orthogonal plane”) but also orthogonal A surface obtained by rotating a surface by an angle of about 5 degrees or less in an arbitrary rotation direction around an arbitrary axis included in the orthogonal plane and intersecting the axis is also included. Further, the “plane including the axis (center axis)” includes a plane obtained by rotating the plane including the axis about the above-described arbitrary axis in an arbitrary rotation direction by an angle of about 5 degrees or less.

  The optical member 10 can be used in various fields. For example, in the field of semiconductor manufacturing apparatuses, the optical member 10 can be used in a light source of an exposure apparatus, a light source of a wafer inspection apparatus, and the like. In this case, after supplying gas to the inside of the optical member 10 from at least one of the openings 10a on both end faces 10e of the optical member 10, a lid (not shown) is attached to the openings 10a on both end faces 10e to seal the gas. To do. Then, the optical member 10 sealed with gas is held by an arbitrary holding member (not shown). The gas sealed in this state is irradiated with excitation light such as infrared rays to bring gas atoms (gas molecules) into a plasma state, and thereby plasma is generated outside the optical member 10 via the inner peripheral surface 10i and the outer peripheral surface 10o. Light can be extracted.

  Examples of the present invention will be described in more detail below, but the present invention is not limited thereto.

<Example 1>

The optical member 10 of Example 1 is a cylindrical member for a light source having a length of 50 mm in the direction of the central axis A, a circular shape with a cross section of an inner peripheral surface 10 i of 25 mm in diameter, and a substantially square shape with a cross section of an outer peripheral surface 10 o of 30 mm on a side. is there. Further, the corner C of the optical member 10 has a roundness of R = 5 mm. Further, the optical member 10 is formed by a narrowly defined CaF ₂ single crystal composed of one crystal, and the (001) plane and the (100) plane are the outer peripheral surface in the direction in which the (001) crystal plane is orthogonal to the central axis A. Each of them exists in parallel with two opposing faces of 10o (FIG. 3 (a)).

  In order to examine the strength against the gas pressure (internal pressure) accommodated in the optical member 10 of Example 1, that is, pressure resistance or durability, the following simulation (finite element analysis) was performed.

<Analysis conditions>
In the simulation, with respect to the optical member 10 described above, when an internal pressure is generated in the optical member 10, the value of the maximum shear stress generated in the {100} plane of the CaF ₂ single crystal forming the optical member 10 and the shear The position where stress was generated was analyzed. First, analysis conditions for this simulation will be described.

(1) Position restraint condition As shown in FIGS. 1 and 2, a virtual restraint point VC is set at the intersection of the surface including the end face 10e of the optical member 10 and the central axis A, and the position of the virtual restraint point VC is restrained. As a result, all the virtual constraint points of the optical member 10 were restrained. As a result, the optical member 10 is locked in such a manner that the optical member 10 can be freely deformed without any restriction regarding the shape change according to the load, and the entire position is not shifted by receiving the load. Restrained in state.

ここで、Ｃ_１１、Ｃ_１２、Ｃ_４４は、それぞれ弾性スティフネス［ＧＰａ］である。 (2) Material properties Since CaF ₂ single crystal is an orthotropic material, it was set to be an orthotropic material. The temperature of the optical member 10 (i.e. CaF ₂ single crystals) was to be 25 ° C.. The CaF ₂ single crystal is a cubic single crystal, and therefore the elastic matrix in the constitutive formula used for the analysis is represented by the following formula (1).

Here, C ₁₁ , C ₁₂ , and C ₄₄ are elastic stiffness [GPa], respectively.

The elastic stiffnesses C ₁₁ , C ₁₂ , and C ₄₄ used in this analysis are measured in the CaF ₂ single crystal according to JISR1602 “Fine ceramics elastic modulus test method” and “Dynamic elastic modulus test method ultrasonic pulse method”. The sound velocity [m / s] of the propagating ultrasonic wave is measured and calculated using this measured value. The elastic stiffness of the CaF ₂ single crystal at 25 ° C. is C ₁₁ = 164 GPa, C ₁₂ = 43.7 GPa, and C ₄₄ = 34.6 GPa.

(3) Load condition The inner side of the optical member 10 is set to 30 atm or 90 atm, so that the optical member 10 receives the radial pressure P (FIG. 2) evenly in the circumferential direction of the inner peripheral surface 10i. did.

(4) Crystal Plane Arrangement The crystal plane arrangement of the optical member 10 will be described in more detail with reference to FIG. The upper side of FIG. 3A shows the arrangement of the unit lattice (cubic lattice) UL of the CaF ₂ single crystal with respect to the optical member 10 and the principal axis x, principal axis y, and principal axis z of the unit lattice UL with respect to the optical member 10. The lower side of the figure shows the arrangement of the optical member 10 with respect to the unit cell UL with reference to the unit cell UL of CaF ₂ single crystal. The upper and lower sides in the figure correspond to each other, and the relationship between the direction of the central axis A of the optical member 10 and the directions of the main axis x, the main axis y, and the main axis z of the unit cell UL is the same in the upper and lower sides of FIG. It is. For convenience, the central axis A of the optical member 10 is drawn as passing through the origins of the main axes x, y, and z on the upper side of FIG. In the lower part of the figure, only the outline of the central axis A of the optical member 10 and the reduced end face 10e is drawn in order to represent the arrangement of the optical member 10.

In the crystal plane arrangement of the optical member 10 that is the object of this analysis, as shown in FIG. 3A, the main axis z of the unit cell UL coincides with the central axis A of the optical member 10. In addition, the main axis x of the unit cell UL exists in a plane orthogonal to the central axis A in parallel with a pair of opposing surfaces of the outer peripheral surface 10o, and the diagonal lines DL1 and DL2 projected on the same plane. It exists at an angle of 45 ° with the projection line. Similarly, in the plane orthogonal to the central axis A, the main axis y exists in parallel with another set of opposing surfaces of the outer peripheral surface 10o, and the projected lines of the diagonal lines DL1 and DL2 projected on the same plane Each exists with an angle of 45 °. Therefore, in the crystal plane arrangement of this analysis, of the three {100} planes of the CaF ₂ single crystal orthogonal to each other, a plane defined by the main axis x and the main axis y (hereinafter referred to as “xy plane” as appropriate) The (001) plane that exists in parallel with the center axis A exists perpendicular to the central axis A. In addition, a (100) plane that exists in parallel with a plane defined by the main axis y and the main axis z (hereinafter referred to as “yz plane” as appropriate) exists in parallel with a pair of opposing surfaces of the outer peripheral surface 10o. The (010) plane existing in parallel with the plane defined by the main axis x and the main axis z (hereinafter referred to as “xz plane” as appropriate) is in parallel with another set of facing surfaces of the outer peripheral surface 10o. Existing.

Next, the results of simulation performed under the above conditions will be described. The present inventor has found that the maximum shear that occurs on the three {100} planes of the CaF ₂ single crystal forming the optical member 10, that is, the (100) plane, the (010) plane, and the (001) plane, under the above analysis conditions. The magnitude of the stress and the position where the shear stress occurs were estimated.

  As shown in the table of FIG. 4, when the internal pressure is 30 atm, the maximum shear stress generated on the (001) plane is 8.95 MPa, and the maximum shear stress generated on the (100) plane is 0.46 MPa on the (010) plane. The maximum shear stress is 0.46 MPa. Further, the position where the shear stress occurs is as shown in FIG. 5A shows the position of the shear stress generated on the (001) plane, FIG. 5B shows the position of the shear stress generated on the (100) plane, and FIG. 5C shows the shear stress generated on the (010) plane. As a guide, a shear stress of about 8 to 10 MPa is obtained in a region represented by red or amber, and a shear stress of about 6 to 8 MPa is produced in a region represented by orange or blue. In a region represented by yellow or light blue, a shear stress of about 4 to 6 MPa is generated. In the green region, a smaller shear stress is generated or no shear stress is generated. Although FIG. 5 shows a color, when displayed in black and white, the area represented by red, dark blue, and blue is a particularly dark area, and the area represented by light blue is a slightly light area. Is displayed as a particularly light-colored area. From the table of FIG. 4 and FIG. 5, when gas is sealed in the optical member 10 and the internal pressure is set to 30 atm, shear stress is generated in the (001) plane at four locations in the peripheral wall of the optical member 10, and the maximum The value is estimated to be 8.95 MPa on the inner peripheral surface 10i.

  As shown in the table of FIG. 4, when the internal pressure is 90 atm, the maximum shear stress generated on the (001) plane is 26.85 MPa, the maximum shear stress generated on the (100) plane is 1.39 MPa, and the (010) plane. The maximum shear stress generated in 1.39 MPa. Further, the position where the shear stress occurs is as shown in FIG. 6A shows the position of the shear stress generated on the (001) plane, FIG. 6B shows the position of the shear stress generated on the (100) plane, and FIG. 6C shows the shear stress generated on the (010) plane. As a guideline, a shear stress of about 24 to 32 MPa is obtained in a region represented by red or amber, and a shear stress of about 16 to 24 MPa is represented in a region represented by orange or blue. In a region represented by yellow or light blue, a shear stress of about 8 to 16 MPa is generated. In the green region, a smaller shear stress is generated or no shear stress is generated. Although FIG. 6 shows a color, when displayed in black and white, the red, dark blue, and blue areas are particularly dark areas, and the light blue area is a slightly light area. Is displayed as a particularly light-colored area. From the table of FIG. 4 and FIG. 6, when gas is sealed inside the optical member 10 and the internal pressure is 90 atm, shear stress is generated on the (001) plane at four locations in the peripheral wall of the optical member 10, and its maximum value Is estimated to be 26.85 MPa on the inner peripheral surface 10i.

According to the above analysis results, in the optical member 10, as shown in FIG. 7, shear stress of {100} plane is intensively generated at four locations in the peripheral wall. This direction depends on the crystal plane arrangement of the CaF ₂ single crystal, and a plane that is parallel to the (010) plane and includes a plane including the central axis A is rotated by 45 ° in one direction around the central axis A. A position intersecting the peripheral wall of the optical member 10 and a plane parallel to the (100) plane and rotated by 45 ° in the same direction around the central axis with respect to the plane including the central axis A intersects the peripheral wall of the optical member 10 It is a position to do. Here, in the optical member 10 of Example 1, the (010) plane is parallel to a pair of opposing surfaces of the outer peripheral surface 10o, is a plane parallel to the (010) plane, and includes the central axis A. The surface is equal to a surface defined by a main axis x and a central axis A parallel to a pair of opposing surfaces of the outer peripheral surface 10o. Therefore, a plane parallel to the (010) plane and rotated by 45 ° in one direction around the central axis A including the central axis A is a plane including either one of the diagonal lines DL1 and DL2. Similarly, in the optical member 10, the (100) plane is parallel to another set of opposing surfaces of the outer peripheral surface 10o, and the plane that is parallel to the (100) plane and includes the central axis A is It is equal to a surface defined by a main axis y and a central axis A parallel to another set of opposing surfaces of the outer peripheral surface 10o. Therefore, a plane parallel to the (100) plane and rotated by 45 ° in the same direction around the central axis A, including the central axis A, is a plane including either one of the diagonal lines DL1 and DL2. Therefore, the position where the shear stress is concentrated in the optical member 10 is a position where the diagonal line DL1 and the peripheral wall of the optical member 10 intersect, and a position where the diagonal line DL2 and the peripheral wall of the optical member 10 intersect (a pair of the outer surfaces 10o). Corner).

  Further, according to FIGS. 5 and 6, the shear stress generated on the (001) plane at the above position is a plane parallel to the (010) plane and including the central axis A in one direction around the central axis A. The surface rotated by 45 ° intersects the inner peripheral surface 10i of the optical member 10 and the surface parallel to the (100) plane and including the central axis A is rotated by 45 ° in the same direction around the central axis. It is the largest at a position where the surface intersects the inner peripheral surface 10i of the optical member 10 (or a position where the diagonal line DL1 and diagonal line DL2 intersect the inner peripheral surface 10i). The portion where the greatest shear stress is generated is hereinafter referred to as “maximum shear stress portion” as appropriate. From the table of FIG. 4, the shear stress generated in the maximum shear stress portion is 8.95 MPa when the internal pressure is 30 atm and 26.85 MPa when the internal pressure is 90 atm. On the other hand, the magnitude of the shear stress gradually decreases as the distance from the maximum shear stress portion increases in the circumferential direction and as the distance from the radial direction increases (FIGS. 5 and 6).

Here, as shown in FIG. 8, the critical decomposition shear stress (CRSS) of the {100} plane, {110} plane, and {111} plane of the CaF ₂ single crystal is {100} in the temperature range of about 600 ° C. or less. } It is known that the surface value is the smallest. Further, the difference between the critical decomposition shear stress of the {100} plane and the critical decomposition shear stress of the {110} plane and the {111} plane is a temperature below the temperature (about 250 ° C.) at which plastic deformation occurs in the CaF ₂ single crystal. It is known to be particularly large in the region. Therefore, in the temperature range of 600 ° C. or lower, particularly in the temperature range of 250 ° C. or lower, when stress is generated in the optical member 10 formed of CaF ₂ single crystal, the breakdown that occurs in the optical member 10 is mainly {100 } It is considered that the shear stress generated on the surface reaches the critical decomposition shear stress and slip occurs inside the CaF ₂ single crystal.

In the optical member 10 of the present embodiment, as described above, the magnitude of the shear stress generated in the maximum shear stress portion reaches 26.85 MPa when the internal pressure is 90 atm. This value greatly exceeds the CRSS (about 13 MPa) of the CaF ₂ single crystal at 25 ° C., which can be read from FIG. 8, and it is considered that the CaF ₂ single crystal slips at the maximum shear stress portion. However, the optical member 10 of Example 1 has a sufficient thickness on the outer side in the radial direction of the maximum shear stress portion. As shown in FIGS. 6 and 7, the optical member 10 has a certain distance in the radial direction from the maximum shear stress portion. In a position from the distant position to the outer peripheral surface 10o, no shear stress is generated on the {100} surface. Therefore, no slip occurs in the CaF ₂ single crystal at this portion located outside the maximum shear stress portion. Therefore, even in the maximum shear stress portion, slip does not occur in the portion located on the outer side, so that the occurrence of slip is suppressed, and as a result, no slip occurs on the peripheral wall of the optical member 10 of Example 1.

  As described above, in the optical member 10 of Example 1, the position where the {100} plane shear stress on the peripheral wall is generated, that is, the plane parallel to the (010) plane and including the central axis A is around the central axis A. The surface rotated 45 ° in one direction at a position intersecting the peripheral wall of the optical member 10 and a surface parallel to the (100) plane and including the central axis A is 45 ° in the same direction around the central axis A. The positions where the rotated surface intersects the peripheral wall of the optical member 10 are located at the diagonal portions of the outer peripheral surface 10o, respectively, and the radial thickness of the peripheral wall at these positions is larger than the thickness at other positions. It has become. Therefore, the optical member 10 has a high internal pressure and a peripheral wall at a position where a large {100} plane shear stress is generated even when a {100} plane shear stress exceeding CRSS occurs on or in the vicinity of the inner peripheral surface 10i. Due to the large thickness, the occurrence of slip is suppressed. Therefore, the optical member 10 of Example 1 has high strength against the internal pressure.

<Comparative Examples 1 and 2>
Next, the strength against the gas pressure (internal pressure) accommodated in the optical member 11 (FIG. 3B) of the comparative example 1 (FIG. 3B) and the optical member 12 of the comparative example 2 (FIG. 3C), that is, pressure resistance or In order to investigate the durability, the following simulation (finite element analysis) was performed.

The optical members 11 and 12 of Comparative Examples 1 and 2 are each formed of a narrowly defined CaF ₂ single crystal made of one crystal, and have the same shape as the optical member 10 of Example 1. On the other hand, in the optical member 11 of Comparative Example 1, the {110} plane of the CaF ₂ single crystal exists in a direction orthogonal to the central axis A. In the optical member 12 of Comparative Example 2, the CaF ₂ single crystal The {111} plane is present in a direction perpendicular to the central axis A. The crystal plane arrangement of the optical members 11 and 12 will be described in detail later.

<Analysis conditions>
In the simulation, for the optical members 11 and 12 described above, when an internal pressure is generated in the optical members 11 and 12, the maximum generated on the {100} plane of the CaF ₂ single crystal forming the optical members 11 and 12. The value of shear stress and the position where shear stress occurred were analyzed. Among the analysis conditions of this simulation, (1) position constraint conditions, (2) material properties, and (3) load conditions are the same as the simulation performed for the optical member 10 of Example 1, and thus the description thereof is omitted.

(4) Crystal Plane Arrangement The crystal plane arrangement of the optical members 11 and 12 will be described in more detail with reference to FIG. 3B and 3C, the optical members 11 and 12, the unit lattice (cubic lattice) UL of the CaF ₂ single crystal, and the principal axis x, the principal axis y of the unit lattice UL, as in FIG. The arrangement | positioning relationship of the principal axis z is shown. In the lower diagram, only the outlines of the central axis A and the reduced end faces 11e and 12e of the optical members 11 and 12 are drawn in order to represent the arrangement of the optical members 11 and 12. In the optical member 11 of Comparative Example 1, the {110} plane exists in a direction orthogonal to the central axis A as shown in FIG. The unit cell UL in FIG. 3B is arranged at a position obtained by rotating the unit cell UL in FIG. 3A around the main axis x by 45 ° from the positive direction of the main axis z toward the positive direction of the main axis y. Has been. Therefore, the principal axis x of the unit cell UL exists in a plane orthogonal to the central axis A and is inclined by 45 ° with respect to the projection line projecting the diagonal line DL1 into the same plane, and the principal axes y and z are respectively yz. In the plane, they are inclined with respect to the central axis A by 135 ° and 45 °. In the optical member 11 of Comparative Example 1, the (001) plane, the (100) plane, and the (010) plane are the xy plane, the yz plane, and the xz plane, respectively, in the state where the principal axes x, y, and z exist as described above. Exists in parallel.

  In the optical member 12 of Comparative Example 2, the {111} plane exists in a direction orthogonal to the central axis A as shown in FIG. 3C, the unit cell UL in FIG. 3A is rotated 54.7359 ° from the positive direction of the main axis z to the positive direction of the main axis y about the main axis x. Then, it rotates 45 ° from the positive direction of the main axis y to the positive direction of the main axis x about the main axis z after rotation, and then counterclockwise when the optical member 12 is viewed from above about the central axis A Obtained by rotating 45 °. Accordingly, the main axes x, y, and z of the unit cell UL are inclined with respect to the central axis A by a predetermined angle. In the optical member 12 of Comparative Example 2, the (001) plane, the (100) plane, and the (010) plane are the xy plane, the yz plane, and the xz plane in the state where the principal axes x, y, and z exist as described above. Exists in parallel.

Next, the results of simulation performed under the above conditions will be described. Under the above analysis conditions, the present inventor generates three {100} planes of the CaF ₂ single crystal forming the optical members 11 and 12, that is, (100) plane, (010) plane, and (001) plane. The magnitude of the maximum shear stress and the position where the shear stress occurs were estimated.

  As shown in the table of FIG. 4, in the optical member 11 of Comparative Example 1, when the internal pressure is 30 atm, the shear stress generated on the (001) plane is 6.5 MPa, and the shear stress generated on the (100) plane is 8. 94 MPa, the shear stress generated on the (010) plane is 6.5 MPa, and when the internal pressure is 90 atm, the shear stress generated on the (001) plane is 19.49 MPa, the shear stress generated on the (100) plane is 26.82 MPa, The shear stress generated in the (010) plane is 19.49 MPa. Further, the position where the shear stress is generated is as shown in FIG. 9 (when the internal pressure is 30 atm) and FIG. 10 (when the internal pressure is 90 atm). 9 (a) and 10 (a) show the position of the shear stress generated on the (001) plane, and FIGS. 9 (b) and 10 (b) show the position of the shear stress generated on the (100) plane. (C) and FIG. 10 (c) show the positions of the shear stress generated on the (010) plane, respectively. The reading of FIG. 9 is the same as FIG. 5 and the reading of FIG. 10 is the same as FIG.

  According to the above analysis results, in the optical member 11, large shear stress is generated on the {100} plane at six locations in the peripheral wall (regions R1, R2, R3, R4, FIGS. 9, 10, and 11). R5 and R6 In the black and white drawings, the regions R1 to R4 are difficult to read the difference in color density from the region where no shear stress is generated, but are displayed in orange, light blue, and blue in the color display, rather than the surroundings It can be seen that there is a large shear stress). Further, in the optical member 11, large shear stress is generated on the {100} plane even at a position where the diagonal lines DL1 and DL2 do not intersect, that is, a position where the thickness of the peripheral wall is relatively thin (regions R5 and R6). The position is the surface of the outer peripheral wall 11o in the center part of each of the two opposite sides of the outer peripheral wall 11o, as shown in FIGS. 9 (b) and 10 (b). Moreover, the magnitude | size is 26.82 MPa from the table | surface of FIG. 4, and is far exceeding CRSS (about 13 MPa) in 25 degreeC.

  As described above, in the optical member 11 of Comparative Example 1, when the internal pressure is set to 90 atm, a {100} plane shear stress larger than CRSS is generated on the outer peripheral surface 11o in a relatively small thickness portion of the peripheral wall. Therefore, unlike the optical member 10 of Example 1, the optical member 11 of Comparative Example 1 cannot withstand an internal pressure of 90 atm and breaks. Further, in the optical member 11 of Comparative Example 1, large shear stress is generated on the {100} plane at six locations in the peripheral wall. Therefore, if the peripheral wall is formed thick at a position where the shear stress of the {100} plane is increased, the shape of the optical member 11 becomes complicated, which is not only disadvantageous in terms of optical characteristics but also inconvenience in processing.

  Further, as shown in the table of FIG. 4, in the optical member 12 of Comparative Example 2, when the internal pressure is 30 atm, the shear stress generated on the (001) plane is 9.44 MPa, and the shear stress generated on the (100) plane is When the shear stress generated on the (010) plane is 9.08 MPa and the internal pressure is 90 atm, the shear stress generated on the (001) plane is 28.31 MPa and the shear stress generated on the (100) plane is 27.MPa. The shear stress generated at 23 MPa and the (010) plane is 27.43 MPa. Further, the position where the shear stress is generated is as shown in FIG. 12 (when the internal pressure is 30 atm) and FIG. 13 (when the internal pressure is 90 atm). 12 (a) and 13 (a) show the position of the shear stress generated on the (001) plane, and FIGS. 12 (b) and 13 (b) show the position of the shear stress generated on the (100) plane. FIGS. 13C and 13C show the positions of shear stress generated on the (010) plane, respectively. The reading method of FIG. 12 is the same as that of FIG. 5 and the reading method of FIG.

  According to the above analysis result, in the optical member 12, large shear stress is generated on the {100} plane at six locations in the peripheral wall (FIG. 14). Further, in the optical member 12, a large shear stress is generated on the {100} plane even at a position that does not intersect the diagonal lines DL1 and DL2, that is, at a position where the thickness of the peripheral wall is relatively thin. The positions are four places on the surface of the outer peripheral wall 12o in the vicinity of the corner C from FIGS. 12 (b), 12 (c), 13 (b), and 13 (c). Moreover, the magnitude | size is about 27.23 MPa-27.43 MPa from the table | surface of FIG. 4, and is far exceeding CRSS (about 13 MPa) in 25 degreeC.

  As described above, in the optical member 12 of Comparative Example 2, when the internal pressure is 90 atm, a {100} plane shear stress larger than CRSS is generated on the outer peripheral surface 12o in a relatively thin portion of the peripheral wall. Therefore, unlike the optical member 10 of Example 1, the optical member 12 of Comparative Example 2 cannot withstand an internal pressure of 90 atm and breaks. Further, in the optical member 12 of Comparative Example 1, large shear stress is generated on the {100} plane at six locations in the peripheral wall. Therefore, if the peripheral wall is formed thick at a position where the shear stress of the {100} plane is increased, the shape of the optical member 12 becomes complicated, which is not only disadvantageous in terms of optical characteristics but also inconvenience in processing.

Next, the manufacturing method of the optical member 10 of this embodiment is demonstrated. The CaF ₂ single crystal used as the material of the optical member 10 can be manufactured using, for example, a single crystal manufacturing method described in Japanese Patent No. 4569872, Japanese Patent Application Laid-Open No. 2006-327837, and the like. The crystal orientation of the obtained CaF ₂ single crystal ingot is measured using an X-ray crystal orientation measuring device or the like.

After specifying the existence direction of three {100} planes orthogonal to each other in the CaF ₂ single crystal ingot by the above measurement, so that any one of the three {100} planes is in a direction orthogonal to the central axis A. Cutting is performed to obtain a prism having a substantially square cross-sectional shape. At this time, the directions of two diagonal lines perpendicular to the prism and the directions of two {100} planes that are not orthogonal to the central axis A among the three {100} planes are 45 ° around the central axis A, respectively. It cuts out so that it may have the inclination of. That is, cutting is performed so that two pairs of opposing faces of the prism and two {100} planes that are not orthogonal to the central axis A among the three {100} planes are parallel to each other. . Thereafter, the central part of the prism is further machined out, and the corner of the prism is rounded with a predetermined curvature to obtain a cylindrical member as shown in FIG.

  Finally, the optical member 10 of this embodiment can be obtained by performing an optical polishing process on the inner peripheral surface, the outer peripheral surface, and the end surface of the cylindrical member obtained by machining. For the optical polishing treatment in the present embodiment, a general polishing method applied to an optical component can be used as it is, and for example, polishing can be performed using a polishing pad or polishing sand.

  Next, the effects of this embodiment will be summarized.

  As described above, in the optical member 10 of the present embodiment, the four diagonal portions of the optical member 10 coincide with the four locations where the value of the maximum shear stress generated on the {100} plane increases in the peripheral wall. That is, in the optical member 10 of the present embodiment, the thickness of the peripheral wall is large at a position where the value of the maximum shear stress generated on the {100} plane increases in the peripheral wall. Therefore, according to the optical member 10 of this embodiment, an optical member having a simple shape and high strength against internal pressure can be obtained.

  In addition, since the optical member 10 of the present embodiment is a square tube having a substantially square cross-sectional shape, it is easy to cut out, and handling such as storage and transportation after manufacture is also easy. Moreover, since roundness is given to the corner | angular part C, it is prevented that a defect | deletion, such as a chip | tip, arises at the time of a preservation | save and conveyance.

  In the above embodiment, the diagonal lines DL1 and DL2 are planes parallel to the (010) plane and including the central axis A, and planes parallel to the (100) plane and including the central axis A. Although it is inclined 45 ° around the central axis A with respect to the surface, it is not limited to this. The inclinations of the diagonal lines DL1 and DL2 around the central axis A with respect to the plane parallel to the (010) plane and including the central axis A and the plane parallel to the (100) plane and including the central axis A are It may be in the range of about 45 ° ± 5 °. Even with such an arrangement, a plane that is a plane parallel to the (010) plane and rotated by 45 ° around the central axis A in a predetermined direction around the plane including the central axis A intersects the peripheral wall of the optical member 10; And a plane parallel to the (100) plane and rotated by 45 ° around the central axis A in the same predetermined direction around the plane including the central axis A intersects the peripheral wall of the optical member 10. If the peripheral wall is thicker than other portions, the effects of the present invention can be obtained.

In the above embodiment, the (001) plane of the {100} planes is present orthogonal to the central axis A. However, in the CaF ₂ single crystal, since the (100) plane, the (010) plane, and the (001) plane are equivalent to each other, the crystal plane of the CaF ₂ single crystal is centered on the (100) plane or the (010) plane. You may arrange | position so that the axis | shaft A may be orthogonally crossed.

  In the above embodiment, the optical member 10 has a cylindrical shape including the inner peripheral surface 10i having a circular cross-sectional shape and the outer peripheral surface 10o having a substantially square cross-sectional shape, but is not limited thereto. A cylindrical shape whose peripheral wall intersects with a plane that is parallel to any one of the {100} planes and that is rotated by 45 ° around the central axis A in a predetermined direction around the central axis A And other points at a point that intersects a plane that is parallel to the other {100} surface and that includes the central axis A and that is rotated by 45 ° around the central axis A in the same predetermined direction. As long as the shape is thick, any shape may be used. As an example, there are a shape in which the thickness of four locations in the circumferential direction in the cylinder is locally increased, a cylindrical shape having a substantially octagonal outer periphery in which the thickness of these locations is larger than the other locations, and the like. Further, the cross-sectional shape of the hollow portion is not limited to a circle, and may be a polygon such as an octagon. In addition, it is desirable that the thickness at a location where a large thickness is required is about 2 to 16 times the thickness at a location where the thickness is the smallest.

  In the above embodiment, the corner C is rounded with a predetermined curvature, but the corner C may not be rounded. Alternatively, the corner portion C may be chamfered.

In the above-described embodiment, a lid (not shown) is attached to the openings 10a on both ends 10e of the optical member 10 to seal the gas, and the optical member 10 is used alone as a light source member. The method of using the member 10 is not limited to this. For example, the optical member 10 may be used as a part of a larger optical member obtained by joining the optical member 10 and another CaF ₂ single crystal member by pressure bonding or the like. The larger optical member obtained in this way is, for example, two optical members 10 sharing the central axis A, and sandwiched between the two optical members 10 in the direction of the central axis A, and coaxial with the central axis A. And an optical member having a spherical shell portion having a rotation axis. Further, for example, the optical member 10 may be an optical member in which a plate-like lid portion is crimped to one end and an annular flange portion is crimped to the other end.

  In the above embodiment, the gas is not limited to a gas for emitting plasma light, but may be any gas for emitting light in a wavelength region ranging from the vacuum ultraviolet region to the infrared region, such as excimer light.

  As long as the characteristics of the present invention are maintained, the present invention is not limited to the above embodiments, and other forms conceivable within the scope of the technical idea of the present invention are also included in the scope of the present invention. .

According to the calcium fluoride optical member of the present invention, it is possible to easily obtain a CaF ₂ single crystal optical member that can be suitably used without breaking even under high temperature and high pressure environments. Therefore, it becomes possible to provide a calcium fluoride optical member having sufficient strength in various fields such as a semiconductor manufacturing apparatus.

10, 11, 12 Optical members 10i, 11i, 12i Inner peripheral surfaces 10o, 11o, 12o Outer peripheral surface A Central axes DL1, DL2 Diagonal line UL Unit lattice

Claims (11)

A calcium fluoride optical member used in a gas-sealing container that seals gas inside ,
It is formed of single crystal calcium fluoride and has a cylindrical shape,
The central axis of the cylinder and one of the three {100} crystal planes of the single crystal calcium fluoride orthogonal to each other are orthogonal to each other,
A radial thickness of the peripheral wall of the cylinder is 45 ° in one direction around the central axis with a plane parallel to any one of the three {100} crystal faces and including the central axis of the cylinder. Rotate the surface where the rotated surface and the peripheral wall intersect, and the plane parallel to the remaining one of the three {100} crystal planes and including the central axis of the cylinder by 45 ° in the one direction around the central axis Calcium fluoride optical member that is larger than the other part at the place where the finished surface and the peripheral wall intersect.   The calcium fluoride optical member according to claim 1, wherein a cross section of the inner peripheral surface of the cylinder perpendicular to the central axis is circular.   The calcium fluoride optical member according to claim 1 or 2, wherein a cross section of the outer peripheral surface of the cylinder perpendicular to the central axis is substantially square.   One of the two opposing sides of the substantially square cross section is parallel to the direction in which any one of the three {100} crystal faces exists, and the other of the opposing sides and the three The calcium fluoride optical member according to claim 3, wherein a direction in which the remaining {100} crystal plane is present is parallel. The calcium fluoride optical member according to any one of claims 1 to 4, which is used as a light source member. A gas-sealing container comprising the calcium fluoride member according to any one of claims 1 to 5. An optical member having a substantially rectangular tube shape , which is used for a gas-sealing container and is formed of a single crystal calcium fluoride and has a cylindrical space in which gas is sealed .
Another set of the substantially square cylinders facing each other is parallel to a first surface of three orthogonal {100} surfaces of the single crystal calcium fluoride. An optical member whose outer surface is parallel to the second surface of the {100} surfaces, and whose bottom surface or upper surface of the substantially square tube is parallel to the third surface of the {100} surfaces. A method for producing a calcium fluoride optical member used in a gas-sealing container that seals gas inside ,
Identifying the direction in which three {100} crystal planes of single crystal calcium fluoride are orthogonal to each other;
Scraping the single crystal calcium fluoride into a cylindrical shape,
In the cutting, the central axis of the cylinder and any one of the three {100} crystal faces are orthogonal to each other, and the radial thickness of the peripheral wall of the cylinder is the three {100} crystals. A surface that is parallel to any one of the other surfaces and includes the central axis of the cylinder, rotated by 45 ° in one direction around the central axis, and a portion where the peripheral wall intersects, and the three {100} crystals A plane parallel to the remaining one of the planes and including the central axis of the cylinder is made to be larger than the other part at a point where the surface rotated by 45 ° in the one direction around the central axis and the peripheral wall intersect. A method for producing a calcium fluoride optical member.   The manufacturing method according to claim 8, wherein a cross section of the inner peripheral surface of the cylinder perpendicular to the central axis is circular.   The manufacturing method according to claim 8 or 9, wherein a cross section of the outer peripheral surface of the cylinder perpendicular to the central axis is substantially square.   One of the two opposing sides of the substantially square cross section is parallel to the direction in which any one of the three {100} crystal faces exists, and the other of the opposing sides and the three The manufacturing method according to claim 10, wherein the direction in which the remaining {100} crystal plane is present is parallel.

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