JP2009152538A - Optical element for gas laser, and gas laser device using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical element for a gas laser such as a Brewster window, a beam expander prism or the like increasing polarization purity, and suppressing degradation by strong ultraviolet (in particular, ArF) laser beam irradiation; and to provide a gas laser device using the same. <P>SOLUTION: In this optical element for a gas laser, at least either of an incident plane and an emission plane is parallel to the (111) crystal face of a CaF<SB>2</SB>crystal; and a laser beam having entered from the incident plane permeates a surface including the [111] axis and a first azimuthal axis between the [111] axis and the first azimuthal axis in a track obtained by rotating the [001] axis around the [111] axis, a surface including the [111] axis and a second azimuthal axis between the [111] axis and the second azimuthal axis obtained by rotating the [010] axis around the [111] axis or a surface including the [111] axis and a third azimuthal axis between the [111] axis and the third azimuthal axis in a track obtained by rotating the [100] axis around the [111] axis, and emitted from the emission plane. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical element for a gas laser and a gas laser apparatus using the same, and more particularly to an optical element for an ultraviolet gas laser used in a semiconductor exposure apparatus such as an excimer laser or a fluorine molecular laser and a gas laser apparatus using the optical element. .

(Light source for exposure)
As semiconductor integrated circuits are miniaturized and highly integrated, improvement in resolving power is demanded in semiconductor exposure apparatuses. For this reason, the wavelength of light emitted from the exposure light source is being shortened, and a gas laser device is used as the exposure light source instead of the conventional mercury lamp. As a current gas laser apparatus for exposure, a KrF excimer laser apparatus that emits deep ultraviolet light having a wavelength of 248 nm and an ArF excimer laser apparatus that emits vacuum ultraviolet light having a wavelength of 193 nm are used. As a next-generation exposure technique, an immersion technique for shortening the apparent wavelength of the exposure light source by filling the space between the exposure lens and the wafer with a liquid and changing the refractive index is being applied to ArF excimer laser exposure. In ArF excimer laser immersion, a wavelength of 134 nm is obtained when pure water is used as the immersion liquid. Further, as an exposure light source for the next generation, there is a possibility that F ₂ laser immersion exposure using an F ₂ (fluorine molecule) laser device that emits vacuum ultraviolet light having a wavelength of 157 nm may be employed. The F ₂ laser immersion is said to have a wavelength of 115 nm.

(Exposure optics and chromatic aberration)
A projection optical system is adopted as an optical system of many semiconductor exposure apparatuses. In the projection optical system, chromatic aberration correction is performed by combining optical elements such as lenses having different refractive indexes. Currently, there are no optical materials other than synthetic quartz and CaF ₂ suitable for use as the lens material of the projection optical system in the wavelength (ultraviolet) range of 248 nm to 157 nm of the laser wavelength that is the light source for exposure. For this reason, as the projection lens for the KrF excimer laser, an all-refractive type monochromatic lens composed only of synthetic quartz is adopted, and as the projection lens for the ArF excimer laser, all projection lenses composed of synthetic quartz and CaF ₂ are adopted. A refractive type partial achromatic lens is used. However, since the natural oscillation spectral line width of KrF excimer laser and ArF excimer laser is as wide as about 350 to 400 pm, when these projection lenses are used, chromatic aberration is generated and the resolution is lowered. Therefore, before the chromatic aberration can be ignored, it is necessary to narrow the spectral line width of the laser light emitted from these gas laser devices. For this reason, in these gas laser devices, a band-narrowing module having a band-narrowing element (such as an etalon or a grating) is provided in the laser resonator, thereby realizing a narrow band of the spectral line width.

(Immersion lithography and polarized illumination)
As described above, in the case of ArF excimer laser immersion lithography, when H ₂ O is used as the medium, the refractive index becomes 1.44, so the lens numerical aperture NA proportional to the refractive index is in principle the conventional aperture. The number can be increased by 1.44 times the number. As the NA increases, the influence of the polarization purity of the laser beam as the light source increases. In the case of TE polarized light whose polarization direction is parallel to the direction of the mask pattern, there is no influence, but in the case of TM polarized light in which it is orthogonal, the contrast of the image is lowered. This is because, in the latter case, the electric field vector at the focal point on the wafer is in a different direction, and as the angle of incidence on the wafer increases, the intensity becomes weaker than the former, where the electric field vector is the same. It is. This effect becomes stronger when NA approaches or exceeds 1.0, and ArF excimer laser immersion corresponds to this case. Therefore, it is necessary to control a desired polarization state in the illumination system of the exposure apparatus as described above. For the control of this polarized illumination, it is required that the polarization state of the laser input to the illumination system of the exposure apparatus is linearly polarized light. The polarization purity is a ratio of linearly polarized light and non-linearly polarized light, and the polarization of laser is required to maintain high polarization purity.

(Conventional technology to increase polarization purity)
As techniques for increasing the polarization purity of laser light, there are techniques described in Patent Document 1 and Patent Document 2 so far.

  The one described in Patent Document 1 is such that laser light is transmitted perpendicularly to the cleavage plane (111) of the calcium fluoride crystal of an optical element used for a laser such as a beam expander prism or a front mirror, so that the inside of the optical element is This is a method for preventing deterioration of polarization purity due to birefringence received when light passes through.

  Patent Document 2 discloses that when the light passes through the optical element so that the optical axis of the laser beam is transmitted perpendicularly to the (100) plane of the calcium fluoride crystal of the optical element used in the laser. This is a method for preventing deterioration of polarization purity due to intrinsic birefringence.

  However, the above prior art has the following problems.

  In the thing of patent document 1, the description of the concrete means for making an optical axis pass perpendicularly to the (111) plane of a window as an optical element perpendicularly, and making the surface into a Brewster angle is described. In order to achieve both, the surface of the calcium fluoride crystal cut into a window is not a crystal orientation surface with a hard surface to some extent, and high-precision polishing with a small surface roughness cannot be performed. When polishing with a small surface roughness is not possible, scratches called latent scratches remain in an area within submicron from the polished surface. This latent scratch has a problem that the laser beam is absorbed by the laser irradiation and the surface of the crystal is damaged or a defect in which fluorine escapes occurs, so that it cannot actually be used as a window of the laser chamber.

  In the device described in Patent Document 2, the deterioration of polarization purity due to intrinsic birefringence is prevented by arranging the laser beam so that it passes perpendicularly to the (100) plane of the optical element, but when stress is applied. The stress birefringence generated in the [100] plane perpendicular to the (100) plane is the largest, and when used as a chamber window, the stress at the time of holding the window, the pressure due to a gas of several atmospheres in the chamber, and the laser There has been a problem that stress birefringence may occur due to exothermic stress caused by irradiation. Further, the cut surface is cut at an angle of 17.58 ° or 26.76 ° with the (111) surface, and this cut surface is used as both surfaces of the chamber window, so the following two problems have occurred. First, since the cut surface cannot be polished with high precision and small surface roughness, the threshold for surface damage due to laser irradiation is low. Secondly, when used as a laser chamber window, a gas pressure of about 4000 hPa is applied, so that there is a possibility of damage, for example, on the (111) plane that is prone to wall boundaries. Further, when the cut surface is cut at 17.58 ° from the (111) plane, the angle formed by the chamber window and the optical axis is 70 °, and the Fresnel reflection of P-polarized light and S-polarized light is 4.2%. The P-polarized light component is selected by transmitting through this window. However, since the Fresnel reflection of P-polarized light is large, there is a problem that the output of the laser cannot be secured.

Therefore, as in Patent Document 4, at least one of the optical elements for an ultraviolet gas laser such as a window made of calcium fluoride crystal having two planes and ultraviolet rays incident from one plane 2 and emitted from the other planes. The optical element for an ultraviolet gas laser whose plane is parallel to the (110) crystal plane of the calcium fluoride crystal prevents deterioration of polarization purity due to intrinsic birefringence and stress birefringence, and makes the cut surface smooth and laser irradiation. A technique for preventing the occurrence of cracks and defects is disclosed.
Japanese Patent Laid-Open No. 11-177173 US Patent Application Publication No. 2003/219056 JP 2002-353545 A JP 2006-73921 A

  However, in the technique shown in Patent Document 4, while preventing deterioration in polarization purity due to intrinsic birefringence and stress birefringence, the cut surface of the calcium fluoride crystal faces (110), thereby reducing the surface roughness to some extent. By performing high-precision polishing, surface damage of the calcium fluoride crystal due to laser irradiation was prevented to some extent.

  The present invention has been made in view of such problems of the prior art, and its purpose is to cut calcium fluoride crystals at the hardest crystal plane (111) and to reduce the surface roughness of the crystal plane. By performing high-precision polishing, latent scratches are reduced and surface damage of the calcium fluoride crystal due to laser irradiation is prevented. Such as Brewster windows and beam expander prisms using calcium fluoride crystals that reduce the Fresnel reflectivity of P-polarized light, increase the purity of polarization, and suppress deterioration due to irradiation with strong ultraviolet (particularly ArF) laser light. An optical element for a gas laser and a gas laser device using the same are provided.

  Therefore, the optical element for gas laser of the present invention has an incident plane and an emission plane, and in the optical element for an ultraviolet gas laser made of calcium fluoride crystal, the ultraviolet ray is incident from the incident plane and is emitted from the emission plane, Either one of the emission planes is parallel to the (111) crystal plane of the calcium fluoride crystal, and the laser beam incident from the incident plane has the [111] axis and the [001] axis centered on the [111] axis. A plane between the first azimuth axis in the rotated locus and including the [111] axis and the first azimuth axis, and the [111] axis and the [111] axis as the center. The plane including the [111] axis and the second azimuth axis, or the [111] axis and the [111] axis between the second azimuth axis in the locus of rotating the axis Rotate the [100] axis around Between a third azimuthal axis in the locus was, and, [111] and the shaft, passes through the surface, including the said third azimuthal axis, characterized in that it is emitted from the exit plane.

  The incident angle of the laser beam on the incident plane is in the range of 24.9 ° to 68.73 °.

  The rotation angle of the first azimuth axis from the [001] axis, the rotation angle of the second azimuth axis from the [010] axis, or the rotation angle of the third azimuth axis from the [100] axis is: , Respectively, 34 degrees or more and 36 degrees or less or -34 degrees or more and -36 degrees or less.

  In addition, the calcium fluoride crystal is cut on both sides of the (111) plane, and the laser beam incident on the crystal passes through the plane including the [111] axis and the [001] axis, with the [111] axis as the central axis. An arrangement rotated 30 ° ± 10 ° counterclockwise, and an arrangement passing through a plane including the [111] axis and the [010] axis, rotated 30 ° ± 10 ° counterclockwise with the [111] axis as the central axis. Or an arrangement that passes through a plane including the [111] axis and the [100] axis and is rotated 30 ° ± 10 ° counterclockwise about the [111] axis as a central axis. .

Furthermore, a gas laser apparatus using the optical element for gas laser of the present invention includes a laser chamber, an optical resonator installed on one side of the laser chamber and the opposite side thereof, a laser gas sealed in the laser chamber, Means for exciting the laser gas, and two windows provided in the laser chamber for emitting light generated from the excited laser gas to the outside of the laser chamber, the window being on the optical axis of the optical resonator In the gas laser apparatus arranged along the window, each of the windows includes the optical element for the gas laser.

  The window is counterclockwise about the [111] axis as a central axis from the arrangement in which the laser light incident in the crystal passes through a plane including the [111] axis and the [001] axis when viewed from the inside of the laser chamber. An arrangement rotated by 30 ° ± 10 °, an arrangement passing through a plane including the [111] axis and the [010] axis, and an arrangement rotated by 30 ° ± 10 ° counterclockwise around the [111] axis as the central axis, Alternatively, the arrangement is such that the arrangement passing through the plane including the [111] axis and the [100] axis is rotated 30 ° ± 10 ° counterclockwise with the [111] axis as the central axis.

  Furthermore, a gas laser device using the optical element for a gas laser of the present invention includes a laser chamber, an optical resonator disposed on one side of the laser chamber and the opposite side thereof, a laser gas sealed in the laser chamber, Means for exciting the laser gas, two windows provided in the laser chamber for emitting light generated from the excited laser gas to the outside of the laser chamber, and a beam splitter for dividing the laser light, In the gas laser apparatus arranged along the optical axis of the optical resonator, the beam splitter is composed of the optical element for gas laser.

  Furthermore, a gas laser apparatus using the optical element for gas laser of the present invention includes a laser chamber, an optical resonator installed on one side of the laser chamber and the opposite side thereof, a laser gas sealed in the laser chamber, In the gas laser apparatus, comprising: means for exciting the laser gas; two windows provided in the laser chamber for emitting light generated from the excited laser gas to the outside of the laser chamber; and a beam expanding optical system. The wedge substrate of the optical system is composed of the optical element for gas laser.

  The optical element for gas laser of the present invention makes it possible to perform high-precision polishing with a small surface roughness by setting the crystal surface to the (111) plane, and prevents surface damage by preventing light absorption by laser irradiation due to latent scratches. Can do. Then, by arranging the crystal so as to reduce Fresnel reflection due to P-polarized light, the polarization purity can be increased. Furthermore, by arranging the crystal so that the relationship between the laser optical axis and the crystal axis is such that the deterioration of the inside of the crystal due to high-intensity ultraviolet laser light is suppressed, the output laser light over time according to the number of laser operation shots It is possible to suppress the deterioration of the polarization purity.

  Hereinafter, an optical element for an ultraviolet gas laser and an ultraviolet gas laser apparatus according to an embodiment of the present invention will be described.

FIG. 1 shows the crystal lattice of CaF ₂ . In the present embodiment, the CaF ₂ crystal is cut along the (111) plane in accordance with the crystal orientation. The CaF ₂ crystal is composed of a face-centered cubic lattice as shown in FIG.

As shown in FIG. 2, when the angles θ and φ of the light traveling direction L with respect to the axes [001] and [100] of the CaF ₂ crystal are defined, the directions of φ = 45 ° and θ = 54.74 ° in FIG. Is the [111] axial direction. Since the surface of the (111) plane is the hardest than the surface of the other crystal axis, it is possible to perform polishing with a small surface roughness and few latent scratches.

FIG. 3 shows a window 1 using CaF ₂ (calcium fluoride) according to the present embodiment. FIG. 3A is a top view of the window 1, and FIG. 3B is a cross-sectional view of the window 1.

FIG. 3A is a view of the CaF ₂ crystal as viewed from directly above the [111] axis, and shows each plane orientation axis of the CaF ₂ crystal radially. Since the CaF ₂ crystal has a face-centered cubic lattice as shown in FIG. 1, the axis of crystal orientation is three-fold symmetric when the [111] axis is the axis of symmetry. Therefore, when viewed from directly above the [111] axis of the CaF ₂ crystal window, when the [001] axis is the reference axis, the clockwise angle is positive, and the counterclockwise angle is negative, the [001] axis is The angle formed by the [011] axis is 60 degrees, the angle formed by the [001] axis and the [010] axis is 120 degrees, the angle formed by the [001] axis and the [110] axis is 180 degrees, and the [001] axis and the [001] axis The angle formed with the [101] axis is −60 degrees, and the angle formed between the [001] axis and the [100] axis is −120 degrees.

Here, with the [111] axis as the center, the [001] axis is defined as the rotation designated azimuth axis with the azimuth axis rotated clockwise by the angle γ as the first azimuth axis, and the angle γ is defined as the rotation designated azimuth angle. . However, since the CaF ₂ crystal has a face-centered cubic lattice as described above, the reference axis may be the [011] axis or the [101] axis instead of the [001] axis. In this case, the rotation designated azimuth axes are set as a second azimuth axis or a third azimuth axis, respectively.

FIG. 3B is a view of the CaF ₂ crystal seen in a cross section including the rotation designated azimuth axis and the [111] axis.

The window 1 made of CaF ₂ crystal is polished by the surfaces 2 and 2 ′ that are parallel to the (111) plane. For example, in the present embodiment, the laser beam is P-polarized with respect to the CaF ₂ crystal substrate at an incident angle α of approximately Brewster angle 56.34 ° within the plane including the rotation designated azimuth axis and the [111] axis. Incident in the center. Then, on the surface 2, the light is refracted at a refraction angle β of 33.65 ° according to Snell's law. At this time, in the plane CaF ₂ internal refraction optical axis comprises a [111] axis and the rotation specified azimuthal axis of CaF ₂ crystal, and [111] between the angle of the axis and the rotation specified azimuthal axis (0 ° < The CaF ₂ crystal is arranged so as to transmit β <54.74 °. Then, the light passes through the CaF ₂ crystal, and the laser light is refracted according to Snell's law again at the surface 2 ′ in the same manner as the surface 2. Parallel to the plane including the axis, the window exits with P-polarized linearly polarized light, and light propagates again into the gas.

  Here, the rotation designated azimuth angle γ is in the range of −10 ° to −50 ° or + 10 ° to + 50 °, thereby suppressing the deterioration of the polarization purity of the output laser light with time due to the number of laser operation shots. be able to. However, as will be described later, the incident angle of the incident laser beam is incident at an oblique incidence (an angle greater than 0) in order to suppress the surface reflection of the window.

  The rotation designated azimuth angle γ is more preferably in the range of −30 ° to −40 ° or + 30 ° to + 40 °. Further, it is more preferable that the designated rotation azimuth angle γ is in the range of −34 ° to −36 ° or + 34 ° to + 36 °.

  Next, the relationship between the Brewster angle and the polarization will be described. In general, the chamber window used in the gas laser resonator is often arranged at a Brewster angle with respect to the optical axis. This is because, by setting the Brewster angle, Fresnel reflection of the P-polarized component of the light incident on the window becomes zero on the window surface, and the internal absorption of the crystal is very small and almost 100% is transmitted. This is because there is no loss of laser light and the output energy does not decrease.

Since the laser light is output after reciprocating several to dozens of times in the resonator, the S-polarized component is attenuated by Fresnel reflection while passing through the Brewster window, which is a so-called polarizing element, several times. The P-polarized light component is transmitted without being attenuated, and is amplified by passing through the laser medium. As a result, the laser beam is output as linearly polarized light in the P-polarized direction.

In a narrow-band laser, in order to narrow the spectral line width, the beam is expanded by a prism and is incident on a grating which is a wavelength dispersion element. In order to increase the expansion ratio of the beam, the enlargement prism increases the incident angle of the prism and uses a plurality of expansion prisms to increase the total expansion ratio. This is because the spectral line width of the laser and the magnification ratio are generally in an inversely proportional relationship. The slope of the magnifying prism is coated with an antireflection film for P-polarized light so that the reflection loss is reduced even at a high incident angle (greater than the Brewster angle). Since this antireflection film is a film having a high reflectivity with respect to S-polarized light, as a result, the P-polarized component survives by laser oscillation, and the polarization purity of the output laser light becomes very high. The window of the laser chamber is tilted so that the reflection loss of the P-polarized component selected by this prism is reduced. When the Brewster angle is set, the reflection loss of P-polarized light at the window becomes 0% as described above, so that a sufficient laser output can be obtained. In an ArF excimer laser (wavelength 193.368 nm), the refractive index n of calcium fluoride is 1.501958 at 20 ° C., so the Brewster angle is 56.336 °. In the F ₂ laser (wavelength 157.63 nm), since the refractive index n of calcium fluoride is 1.559261 at 20 ° C., the Brewster angle is 57.3 °.

  Next, a change in polarization due to birefringence will be described. In general, light propagating in a crystal is a linear combination of two linearly polarized waves orthogonal to each other, and the polarization state and the polarization direction are determined by the magnitude of the phase velocity and amplitude of each wave. When birefringence occurs in the crystal, the phase velocity of the light beam propagating in the crystal shifts depending on the polarization direction. As a result, the light beam that was linearly polarized light passes through the birefringent material, so that the phase of two waves that are orthogonal to each other shifts and is no longer linearly polarized light (substantially becomes elliptically polarized light). For this reason, when birefringence occurs in the crystal, the light incident as P-polarized light is transmitted through the inside of the crystal, thereby deteriorating the purity of P-polarized light and reducing the light intensity of the P-polarized component. In order to increase the polarization purity and suppress the deterioration, it is necessary to arrange so that the relationship between the laser optical axis and the crystal axis is such that the deterioration of the inside of the crystal to be a high-intensity ultraviolet laser beam is suppressed.

Here, the distribution of the magnitude of birefringence depending on the crystal orientation of CaF ₂ will be described. In birefringence in crystals, intrinsic birefringence (intrinsic birefringence), which originally exists in ideal crystals without disturbance, and stress birefringence (stress birefringence) caused by external mechanical and thermal forces. There are two. Recently, it has been found that intrinsic birefringence occurs even with calcium fluoride which is an equiaxed crystal. Intrinsic birefringence becomes more influential as the wavelength of light approaches the atomic spacing that makes up the crystal. Therefore, when it is used in the short wavelength region of ArF excimer laser or F ₂ laser, the influence of intrinsic birefringence becomes large and cannot be ignored. Both intrinsic birefringence and stress birefringence have a difference in birefringence depending on the laser optical axis and crystal orientation, and are calculated.

FIG. 4 shows the distribution of the magnitude of intrinsic birefringence of CaF ₂ depending on the crystal orientation. If the angles θ and φ of the light traveling direction L with respect to the axes [001] and [100] of the CaF ₂ crystal are defined as shown in FIG. 2, the intrinsic birefringence becomes as shown in FIGS. 4 (a) and 4 (b). The solid line in FIG. 4A shows the case where the angle θ with respect to the axis [001] in the traveling direction L is changed between 0 ° and 90 ° while φ is kept at 45 °. The dotted line is the case where the angle θ with respect to the axis [001] in the traveling direction L is changed between 0 ° and 90 ° while keeping φ at 0 °, and the solid line in FIG. This is a case where the angle φ with respect to the axis [100] in the traveling direction L is changed between 0 ° and 90 ° while maintaining 90 °. As is clear from FIGS. 4A and 4B, the intrinsic birefringence becomes zero in the directions of crystal orientations [111], [100], [010], and [001], and conversely, [110] , [011], and [101] in the direction of the crystal orientation are maximum.

  FIG. 5 is a graph showing the relationship between the number of laser oscillation shots and the polarization purity of the output laser beam.

  Specifically, this graph shows a rotation direction angle γ = −6 ° (near the [001] axis in which a CaF 2 crystal having a surface parallel to the (111) plane as a surface and the intrinsic birefringence is minimized. ), The rotational azimuth angle γ = −60 ° (near the [101] axis) at which the intrinsic birefringence is maximized, and the rotational azimuth angle γ = −34 ° near the middle of the [001] axis and the [101] axis. The Brewster window arranged at 35 ° is mounted in the laser chamber of the amplification stage of the two-stage laser system described later, and the number of laser oscillation shots and the output when the endurance operation is performed at an ArF laser output of 40 W and a repetition frequency of 4000 Hz. The relationship of the polarization purity of a laser beam is shown.

  When the rotation azimuth angle γ = −6 ° (incident from the direction of about [001]), the initial polarization purity of the output laser beam was 99.7%, and the polarization purity became 97% after about 10 billion shots. .

Further, when the rotation designated azimuth angle γ = −34 ° and −36 °, the initial polarization purity of the output laser light is about 99.5%, the number of oscillation shots of the laser is 30 billion shots, and the polarization purity is about 97. .5%.

  Further, when the rotation designated azimuth angle γ = −60 ° (incident from the direction of about [101]), the polarization purity of the output laser light was about 96%.

  Thus, the polarization purity can be increased by aligning the rotation designated azimuth axis obtained by rotating the [001] axis with the [111] axis as the center and the laser optical axis.

  Further, the rotation designated azimuth angle is set to the both axes (for example, [001] axis and [010] axis, [001] axis and [011] axis, [010] axis and [011] axis, [010] axis and [110] axis. ], Or intermediate region of [100] axis and [010] axis, [100] axis and [101] axis) γ = −10 ° to −50 ° or + 10 ° to + 50 °, preferably γ = −30 ° From the durability test, it is possible to suppress deterioration of polarization purity over time by adjusting the laser optical axis at -40 ° or + 30 ° to + 40 °, more preferably γ = −34 ° to −36 ° or + 34 ° to + 36 °. Indicated.

Next, the relationship between the angle of incidence on the CaF ₂ crystal surface and the reflectance of P-polarized light will be described. FIG. 6 shows the relationship between the angle of incidence on the CaF ₂ crystal surface and the reflectance of P-polarized light.

The P-polarized light reflectivity R _p and the S-polarized light reflectivity R _s when light enters the transparent medium having a refractive index n from the air at an incident angle φ can be obtained by the Fresnel equation (1).

ここで、sinφ = n・sinχ（Snell's law：スネルの法則）である。

Here, sinφ = n · sinχ (Snell's law).

As shown in FIG. 7, for example, assuming that the refractive index of the CaF ₂ crystal is 1.501958 at a wavelength of 193 nm, the Fresnel reflectivity of the P-polarized component light is 4.02% at an incident angle of 0 degree (normal incidence). The incident angle at which the P-polarized Fresnel reflection is 3% is 24.90 degrees. Thereafter, the Fresnel reflectivity decreases monotonously until the incident angle reaches the Brewster angle (56.34 degrees). At this Brewster angle, the reflectance of P-polarized light is 0%. Subsequently, when the incident angle becomes larger than the Brewster angle, the Fresnel reflection increases monotonously. The incident angle at which the P-polarized Fresnel reflection is 3% is 68.73 degrees.

  For example, when a CaF2 crystal window that reflects Fresnel on both sides of the window is mounted in the laser chamber, the Brewster angle (56.34 °) at which the reflectance of the P-polarized light becomes zero is the smallest loss of the laser resonator. ).

Assuming that the Fresnel reflection per plane of the P-polarized laser window within the allowable range of the resonator loss that can maintain the output of this laser is 3%, the incident angle to the CaF ₂ window is 24.9 ° to 68.73 °. The interval is preferable.

FIG. 6 shows the relationship between the optical axis and the axis of the CaF ₂ crystal when the incident angles to the CaF ₂ window are 24.9 ° and 68.73 °. The CaF ₂ crystal 1 is polished by the surfaces 2 and 2 ′ that are parallel to the (111) plane. Since the surface of the (111) plane is the hardest than the surface of the other crystal axis, it is possible to perform polishing with a small surface roughness and few latent scratches.

FIG. 7A shows a case where the incident angle is 24.90 °. When a P-polarized laser beam is incident on the optical element of the CaF ₂ crystal at an incident angle α of 24.90 °, the light on the surface 2 follows the Snell's law at a refraction angle β = 16.28 ° inside the CaF ₂ crystal. Refract. At this time, the CaF ₂ crystal is arranged so that the refractive optical axis inside the CaF ₂ is transmitted in a plane including the [111] axis of the CaF ₂ crystal and the designated rotational azimuth axis. Then, the laser beam passes through the CaF ₂ crystal, is refracted again on the surface 2 ′ in accordance with Snell's law similarly to the surface 2, and propagates again into the gas.

FIG. 7B shows a case where the incident angle is 68.73 °. When a P-polarized laser beam is incident on the optical element of the CaF ₂ crystal at an incident angle α of 68.73 °, the light on the surface 2 follows the Snell's law and the refraction angle β = 38.24 ° inside the CaF ₂ crystal. Refract. At this time, the CaF ₂ crystal is arranged so that the refractive optical axis inside the CaF ₂ is transmitted in a plane including the [111] axis of the CaF ₂ crystal and the designated rotational azimuth axis. Then, the laser beam passes through the CaF ₂ crystal, is refracted again on the surface 2 ′ in accordance with Snell's law similarly to the surface 2, and propagates again into the gas.

Before the window 1 is attached, X-ray diffraction analysis may be performed and the crystal orientation may be measured in advance. First mark the side of the window 1 in the [001], [100] or [010] axial direction, rotate it relative to that mark, and add a second mark to the rotation designated azimuth axis, If it is attached according to the second mark, it is efficient.

As described above, the CaF ₂ crystal (111) face was a surface of the window, CaF ₂ in an angular range of 68.73 ° from the incident angle 24.90 ° of the CaF ₂ crystal as Fresnel reflection of the P-polarized decreases By making the light incident on the crystal and rotating the CaF ₂ crystal about the [111] axis, laser output light with high polarization purity can be obtained. Further, by setting the CaF ₂ crystal while limiting the rotation designated azimuth angle, deterioration inside the crystal due to the high-intensity ultraviolet laser beam can be suppressed. As a result, deterioration of the polarization purity of the output laser light with time can be suppressed.

The case where the optical element for an ultraviolet gas laser according to the present embodiment is used as a window has been described above, but this can also be used at other parts of the laser apparatus. In order to explain the example, FIG. 8 shows a schematic configuration mainly of the optical system of the two-stage laser system and an arrangement example of the optical element for the ultraviolet gas laser according to the present invention therein.

The two-stage laser system includes an oscillating laser 10 and an amplifying laser 20 that amplifies the laser beam (seed light) oscillated from the oscillating laser 10, and has a high bandwidth of 40 W or more particularly in a narrow band. This is expected for an ArF excimer laser device or F ₂ laser device for exposure that requires output.

  The oscillation laser 10 includes a laser chamber 11 in which a laser gas is sealed, a narrow-band module 14 constituting a resonator, and a partial reflection mirror 15 as an output mirror, and further includes a laser gas excitation system (not shown) A control system, a cooling system, a gas exchange system, and the like are included.

  As described above, two windows 12 and 13 are attached to the laser chamber 11 on the optical axis. The band narrowing module 14 includes one or a plurality of beam expanding prisms 16 (two in the drawing) constituting the beam expanding optical system and a grating 17 (or etalon) as a band narrowing element.

  The amplification laser 20 also includes a laser chamber 21 in which a laser gas is sealed, and partial reflection mirrors 24 and 25 that constitute a resonator, and further includes a laser gas excitation system and a control system (not shown), and a cooling system. Gas exchange systems and the like.

  Two windows 22 and 23 are attached to the laser chamber 21 on the optical axis. In FIG. 8, the laser light oscillated from the oscillation laser 10 is reflected by the mirrors 18 and 19 and is incident on the amplification laser 20. Since the laser window is disposed in the resonator of the oscillation stage and amplification stage laser, a large number of laser beams reciprocate.

Therefore, by polishing the surface of the CaF ₂ crystal with the (111) plane and rotating the CaF ₂ crystal about the [111] axis, it is possible to obtain laser output light with high polarization purity. By disposing the CaF ₂ crystal while limiting the rotation designated azimuth angle, deterioration inside the crystal due to high-intensity ultraviolet laser light can be suppressed. As a result, deterioration of the polarization purity of the output laser light with time can be suppressed.

  As described above, the windows 12, 13, 22, and 23 attached to the laser chambers 11 and 21 are preferably formed of the optical element for ultraviolet gas laser according to the present invention.

The partial reflection mirror 15 constituting the resonator of the oscillation laser 10 and the partial reflection mirrors 24 and 25 of the amplification laser 20 are cut along the (111) plane to minimize birefringence, and CaF ₂ It is desirable that the optical axis of the laser light transmitted through the crystal be perpendicular to the (111) plane.

  The same effect can be obtained by applying the present invention to an MOPA (Master Oscillator Power Amplifier) system from which the partial reflection mirrors 24 and 25 of the amplification laser 20 are removed.

Further, the present invention can be applied to a beam splitter for sampling the beam of this laser system. When the surface of the beam splitter is coated with a single-sided AR coating or a single-sided partial reflection coating, or is sampled using Fresnel reflection, an uncoated CaF ₂ crystal substrate is used.

  In this example, the first beam splitter 31 for the beam sample of the oscillation stage laser power monitor 30 and the second beam splitter 41 for the beam sample of the monitor module 40 for measuring the energy and spectrum of the laser after amplification are applied. is doing.

  Furthermore, the present invention can be applied to the third beam splitter 51 used in the optical pulse stretcher 50. A single-sided AR coating and a single-sided partial reflection coating (about R = 60%) are applied.

  In the case of these beam splitters, since the incident angle α is normally 45 °, the refraction angle β in the CaF 2 crystal is 28.09 °.

FIG. 9 shows an example in which the beam expansion optical systems 91 and 91 ′ by combining two wedge substrates are arranged in the resonator of the amplification stage laser. FIG. 9A shows an example in which the beam expanding optical system 91 ′ is arranged only on the output side, and FIG. 9B shows an example in which the beam expanding optical systems 91 and 91 ′ are arranged on the rear side and the output side.

When the beam expansion optical systems 91 and 91 ′ are arranged in the resonator of the amplification stage laser, the light reciprocates many times by laser oscillation, so that the surface of the CaF ₂ crystal of the beam expansion optical systems 91 and 91 ′ By polishing the (111) plane and rotating the CaF ₂ crystal about the [111] axis, it is possible to obtain laser output light with high polarization purity. By disposing the CaF ₂ crystal while limiting the rotation designated azimuth angle, deterioration inside the crystal due to high-intensity ultraviolet laser light can be suppressed. As a result, deterioration of the polarization purity of the output laser light with time can be suppressed.

FIG. 10 shows an arrangement example in the case where two beam substrates 92 and 93 are used in each beam expanding optical system 91 and 91 ′. The second wedge substrate 93 is vertically inverted with respect to the first wedge substrate 92 and arranged in a “C” shape so that the beam incident angles are the same. By arranging in this way, the laser optical axis after emission of the beam expansion optical systems 91 and 91 ′ is made parallel to the laser optical axis before incidence of the beam expansion optical systems 91 and 91 ′ (deflection angle β = 0).
°) can. This principle will be described with reference to FIG. FIG. 10 shows a laser beam path when the laser beam is incident on the two wedge substrates 92 and 93. The angle of the beam optical path on the second wedge substrate 93 is .theta.5, .theta.6, .theta.7, .theta.8, as shown, and the beam deflection angle of the emitted light from the first wedge substrate 92 is .beta.1. If the beam deflection angle of the light emitted from the wedge substrate 93 is β2,
β1 = θ1 −θ2 + θ3 −θ4 (11)
β2 = θ1−θ5 + θ6−θ7 + θ8 (12)
It becomes. Now, if the second wedge substrate 93 has the same shape as the first wedge substrate 92 and is turned upside down to have the same incident angle (θ5 = θ1),
θ5 = θ1 (13)
θ6 = θ2 (14)
θ7 = θ3 (15)
θ8 = θ4 (16)
α1 = α2 (17)
Holds. Substituting these equations (13) to (17) into equation (12),
β2 = 0 (18)
It becomes.

That is, under the above conditions, the deflection angle can be set to 0, and the optical axis of the emitted laser light can be made parallel to the laser optical axis in the chamber 3. Such a beam expander using a combination of two wedge substrates is less dependent on the emission angle of the laser beam due to wavelength change, and is used as a beam expander for expanding the amplification stage laser resonator and the amplified beam. used.

  The incidence angles of the wedge substrates 92 and 93 are larger than 56.34 ° in the case of the Brewster angle at which the P-polarized reflectance is 0 (in the case of ArF laser wavelength 193.368 nm. In this case, It is necessary to attach an antireflection film on the surface so that the P-polarized reflectance can be ignored at the incident angle.

  For example, when a beam expansion optical system with a beam expansion ratio of 2.0 times is designed, the incident angle is 68.7 ° and the wedge angle is 4.4 °. Further, the incident angle on the back surface of the wedge substrate is 60.0 °. Since the P-polarized reflectance of Fresnel reflection at an incident angle of 60.0 ° is 0.2%, it is not necessary to attach an antireflection film to this surface, but since the first surface is 68.7 °, the antireflection film It is necessary to attach.

In FIG. 10A, the orientation of the surface on the incident side of the wedge substrate is made parallel to the (111) plane, and the laser light incident at an incident angle of 68.7 ° is reflected in the CaF ₂ crystal according to Snell's law. Refracts at 38.34 ° and refracts on the exit side surface. The wedge substrate is placed so that the optical axis is within the plane including the plane orientation [111] axis and the designated rotational orientation axis, and between the angles formed by these crystal axes. Then, the light is emitted at a refraction angle of 60.0 degrees on the exit surface of the wedge substrate.

Furthermore, the second wedge substrate is similarly arranged so that the relationship between the optical axis and the crystal axis is the same. Thus, by polishing the surface of the CaF ₂ crystal with the (111) plane and rotating the CaF ₂ crystal about the [111] axis, it is possible to obtain the output light of the laser with high polarization purity. By disposing the CaF ₂ crystal while limiting the rotation designated azimuth angle, deterioration inside the crystal due to high-intensity ultraviolet laser light can be suppressed. As a result, deterioration of the polarization purity of the output laser light with time can be suppressed. Further, since the incident surface is polished so as to be parallel to the (111) plane, a surface with low surface roughness and less latent scratches can be obtained, so that the damage threshold due to laser irradiation of the antireflection film is improved.

In FIG. 10B, the surface orientation on the exit side of the wedge substrate is manufactured in parallel with the (111) plane, and the optical axis passing through the inside of CaF ₂ includes the plane orientation [111] axis and the specified rotation azimuth axis. In
The wedge substrate is installed so as to be between the angles formed by both axes.

  Here, when the energy density of the incident-side surface is high, the arrangement of FIG. 10A becomes the (111) surface in the arrangement of FIG. 10A compared to FIG. Resistance is improved.

  Next, another embodiment will be described.

FIG. 11 is a sectional view showing the window 1 before rotation using CaF ₂ (calcium fluoride), and FIG. 12 is a top view showing the window 1 after rotation.

FIG. 11 is a view of the CaF ₂ crystal as seen in a cross section including the [001] axis, the [110] axis, and the [111] axis. The window 1 made of CaF ₂ crystal is polished by the first surface 3a and the second surface 3b that are parallel to the (111) plane. For example, in the present embodiment, the laser beam is centered on the window 1 with respect to the CaF ₂ crystal substrate at an incident angle α = 55.7 ° within a plane including the [001] axis, the [110] axis, and the [111] axis. Is incident on. Then, on the first surface 2, light is refracted at a refraction angle β = 33.4 ° according to Snell's law. In this case, [001] axis of the CaF ₂ internal refraction optical axis L is CaF ₂ crystal, [110] axis and the [111]
The CaF ₂ crystal is arranged so as to transmit through the angle between the [111] axis and the [001] axis (0 ° <γ <54.7 °) within the plane including the axis. Then, after passing through the CaF ₂ crystal and following the Snell's law again on the second surface 3b as in the first surface 3a, the laser beam has the [001] axis, the [110] axis and the [111] axis. The light exits from the window 1 at an exit angle α = 55.7 ° within the plane including the same.

  In this embodiment, from this state, the window 1 is installed at a position rotated 30 ° counterclockwise about the [111] axis as the central axis.

FIG. 12 is a view of the CaF ₂ crystal viewed from directly above the [111] axis, and shows the respective plane orientation axes of the CaF ₂ crystal in a radial manner. Since the CaF ₂ crystal has a face-centered cubic lattice as shown in FIG. 1, the axis of crystal orientation is three-fold symmetric when the [111] axis is the axis of symmetry. Therefore, when viewed from directly above the [111] axis of the CaF ₂ crystal window, when the [001] axis is the reference axis, the clockwise angle is negative, and the counterclockwise angle is positive, the [001] axis is The angle formed by the [011] axis is -60 degrees, the angle formed by the [001] axis and the [010] axis is -120 degrees, the angle formed by the [001] axis and the [110] axis is 180 degrees, and the [001] axis. And the [101] axis is 60 degrees, and the angle between the [001] axis and the [100] axis is 120 degrees.

  As shown in FIG. 12, the window 1 is counterclockwise about the [111] axis as a central axis with respect to the arrangement in which the laser light incident on the crystal passes through a plane including the [111] axis and the [001] axis. Installed at a position rotated 30 °.

  Next, the measurement window 102 is installed at the light incident angle used in the chamber, and the [111] axis is rotated as the central axis, the crystal orientation in the light propagation direction is changed, and the change in the polarization state is observed. The results are shown.

FIG. 13 is a diagram showing a polarization state observation experimental system. The laser beam used is a linearly polarized narrowband ArF laser 101 (4 kHz, 10 mJ), and the measurement window 102 is installed at an incident angle α = 55.7 ° so that it can be simulated in a chamber window. The polarization direction of the incident laser was also incident in a direction parallel to the paper surface as shown in FIG. 13 together with the actual apparatus. The laser beam that passed through the measurement window 102 was put into the polarization degree measuring instrument 103 and its linear polarization purity was measured. The polarization measuring instrument 103 uses two folding windows 104a and 104b so that the degree of polarization of the reflected laser beam does not change by folding the optical path. Passing through the lotion prism 105, the sensor 106 measures the output. Then, the lotion prism 105 was rotated to measure the output, and the linear polarization purity was measured. As shown in FIG. 14, the measurement window 102 uses a (111) plane cut, and rotates it at 10 ° intervals around the [111] axis as the rotation center, over the range of 0 to 360 °. Changes in linear polarization purity were measured.

  FIG. 15 is a graph showing the measurement result of the polarization purity with respect to the rotation angle. θ = 0 indicates that the optical axis is in the [001] axis direction. The positive direction of the rotation angle θ is when the measurement window 102 is rotated counterclockwise.

  From the graph shown in FIG. 15, the angle with the best polarization purity is not the [001] axial direction, [100] axial direction and [010] axial direction where the intrinsic birefringence is minimized, but counterclockwise from the respective positions. It can be seen that θ = 30 °.

16 and 17 show an example in which the window of CaF ₂ (calcium fluoride) according to this embodiment is used in the chamber 21 of the two-stage laser system shown in FIG. 16 is a view showing the chamber 21, FIG. 17A is a view of the first window 22 as viewed from the arrow A inside the chamber, and FIG. 17B is a view showing the second window 23 from the arrow B inside the chamber. FIG.

  As described above, the two first windows 22 and the second windows 23 are attached to the chamber 21 on the optical axis L. Laser light oscillated from an oscillation laser (not shown) is configured to enter the chamber 21. A large number of laser beams reciprocate in the first and second windows 22 and 23 by the first partial reflection mirror 24 and the second partial reflection mirror 25.

In the present embodiment, the first window 22 and the second window 23 as shown in FIG. 11 in which both sides of the CaF ₂ crystal are cut at the (111) plane are used as the windows installed to be inclined with respect to the laser optical axis L. Use. In this embodiment, as shown in FIGS. 17A and 17B, the first window 22 and the second window 23 are viewed from the inside of the chamber 21 (see arrows A and B in FIG. 16). From the arrangement in which each of the laser beams incident on the crystal passes through a plane including the [111] axis and the [001] axis, both of them have the same angle as the central axis and θ = 30 counterclockwise at the same angle. ° Install in a rotated position.

  When determining the angle, it is preferable to perform X-ray diffraction analysis in advance and measure the crystal orientation [001] axis, [010] axis, and [100] axis. It is efficient to mark the side surfaces of the window in the [001] axis, [010] axis, and [100] axis direction, and rotate the window by rotating the angle θ = 30 ° counterclockwise.

  In the embodiment shown in FIGS. 11 to 17, the rotation is performed by setting the angle θ = 30 ° in the counterclockwise direction, but it is not necessarily 30 ° just, and the range is about 30 ° ± 10 °. If it is within, it is allowed.

  Therefore, the window 1 of the present embodiment uses a window in which calcium fluoride crystals are cut on both sides (111) with respect to the chamber window installed to be inclined with respect to the laser optical axis. An arrangement in which the laser light incident on the crystal passes through a plane including the [111] axis and the [001] axis and the window is rotated 30 ° ± 10 ° counterclockwise about the [111] axis as a central axis The window is rotated 30 ° ± 10 ° counterclockwise around the [111] axis as the central axis from the arrangement passing through the plane including the [111] axis and the [010] axis, or the [111] axis and the [ The window is rotated 30 ° ± 10 ° counterclockwise with the [111] axis as the central axis from the arrangement passing through the plane including the 100] axis.

The optical element for ultraviolet gas laser and the ultraviolet gas laser apparatus of the present invention have been described based on the embodiments. However, the present invention is not limited to these embodiments, and various modifications can be made. For example, a CaF ₂ substrate is used not only as a laser chamber window but also as a polarizing element, and a polarizing element is provided in an optical resonator of an amplification stage laser so as to have a relationship between a laser optical axis and a crystal orientation as in the present invention. You may arrange.

It is a diagram showing a crystal lattice of CaF _2. Axis of CaF ₂ crystal [001] is a diagram showing the definition of the angle θ and φ of the traveling direction L of the light with respect to [100]. Is a top view and a sectional view of a window with a CaF ₂ according to the present invention. It is a diagram showing a distribution of the intrinsic birefringence of CaF ₂ by the crystal orientation. It is a figure which shows the relationship between the polarization purity of an output laser beam, and the number of oscillation shots of a laser. It is a diagram showing the relationship between the angle and the P-polarized light reflectance of incident on CaF ₂ crystal surface. Is a diagram showing the relationship between the axis and the optical axis of the CaF ₂ crystal when the incident angle to the CaF ₂ windows and 24.9 ° and 68.73 °. It is sectional drawing in the case of applying the optical element for ultraviolet gas lasers of this invention to a laser system. It is a figure which shows the example which has arrange | positioned the beam expansion optical system by the combination of two wedge board | substrates in the resonator of the amplification stage laser. It is a figure which shows the laser beam path when a laser beam injects into two wedge board | substrates. It is sectional drawing which shows the window before rotation. It is a top view which shows the window after rotation. It is a figure which shows a polarization state observation experimental system. It is a figure which shows the window for measurement. It is a graph which shows the measurement result of the polarization purity with respect to a rotation angle. Shows a window with a CaF ₂ according to the present invention. It is the figure which looked at the window of FIG. 3 from the inside of a chamber.

Explanation of symbols

1 ... Window 2, 2 '... Surface (cut surface)
3 ... Laser light 4 ... Emission light 10 ... Oscillation laser 11 ... Laser chamber 12, 13 ... Window 14 ... Narrow band module 15 ... Output mirror (partial reflection mirror)
16 ... Beam expanding prism 17 ... Grating 18, 19 ... Mirror 1
DESCRIPTION OF SYMBOLS 20 ... Amplifying laser 21 ... Laser chamber 22, 23 ... Window 24, 25 ... Partial reflection mirror 30 ... Oscillation stage laser power monitor 31 ... First beam splitter 40 ... Monitor module 41 ... Second beam splitter 50 ... Optical pulse stretcher 51 ... Third beam splitters 91 and 91 '... Beam expansion optical systems 92 and 93 ... Wedge substrate

Claims (8)

In an optical element for an ultraviolet gas laser comprising an incident plane and an emission plane, and ultraviolet rays are incident from the incident plane and made of calcium fluoride crystals emitted from the emission plane.
At least one of the incident plane and the emission plane is parallel to the (111) crystal plane of the calcium fluoride crystal,
Between the [111] axis and the first azimuth axis in the trajectory obtained by rotating the [001] axis around the [111] axis, and the [111] axis from the incident plane, A plane including the first azimuth axis;
Between the [111] axis and the second azimuth axis in the locus obtained by rotating the [010] axis around the [111] axis, and the [111] axis and the second azimuth axis Including surface,
Or
Between the [111] axis and the third azimuth axis in the locus obtained by rotating the [100] axis about the [111] axis, and the [111] axis and the third azimuth axis Including surface,
An optical element for a gas laser, characterized by being emitted from an emission plane.   2. The optical element for a gas laser according to claim 1, wherein an incident angle of the laser beam to an incident plane is in an angle range of 24.9 ° to 68.73 °.   The rotation angle of the first azimuth axis from the [001] axis, the rotation angle of the second azimuth axis from the [010] axis, or the rotation angle of the third azimuth axis from the [100] axis, respectively, 3. The optical element for a gas laser according to claim 1, wherein the optical element is for a gas laser of 34 degrees or more and 36 degrees or less or −34 degrees or more and −36 degrees or less. Cut both sides of the calcium fluoride crystal on the (111) plane,
An arrangement in which the laser light incident on the crystal passes through a plane including the [111] axis and the [001] axis and is rotated counterclockwise by 30 ° ± 10 ° about the [111] axis,
An arrangement rotated through 30 ° ± 10 ° counterclockwise from the arrangement passing through the plane including the [111] axis and the [010] axis, with the [111] axis as the central axis,
Or
2. The arrangement according to claim 1, wherein the arrangement is such that the arrangement passing through the plane including the [111] axis and the [100] axis is rotated 30 ° ± 10 ° counterclockwise about the [111] axis as the central axis. Item 3. The optical element for gas laser according to Item 2.   A laser chamber; an optical resonator disposed on one side of the laser chamber; and an opposite side of the laser chamber; a laser gas sealed in the laser chamber; a means for exciting the laser gas; and light generated from the excited laser gas. A gas laser device having two windows provided in the laser chamber for emitting to the outside of the laser chamber, wherein the windows are arranged along an optical axis of the optical resonator. A gas laser device using the optical element for gas laser according to any one of claims 1 to 4, characterized by comprising an optical element for operation. The window is viewed from inside the laser chamber,
An arrangement in which the laser light incident on the crystal passes through a plane including the [111] axis and the [001] axis and is rotated counterclockwise by 30 ° ± 10 ° about the [111] axis,
An arrangement rotated through 30 ° ± 10 ° counterclockwise from the arrangement passing through the plane including the [111] axis and the [010] axis, with the [111] axis as the central axis,
Or
6. The arrangement according to claim 5, wherein the arrangement is such that the arrangement passing through the plane including the [111] axis and the [100] axis is rotated 30 ° ± 10 ° counterclockwise about the [111] axis as the central axis. Gas laser device.   A laser chamber; an optical resonator disposed on one side of the laser chamber; and an opposite side of the laser chamber; a laser gas sealed in the laser chamber; a means for exciting the laser gas; and light generated from the excited laser gas. A gas laser having two windows provided in the laser chamber for emission to the outside of the laser chamber and a beam splitter for dividing the laser light, the window being disposed along the optical axis of the optical resonator 4. The gas laser apparatus using the gas laser optical element according to claim 1, wherein the beam splitter includes the gas laser optical element.   A laser chamber; an optical resonator disposed on one side of the laser chamber; and an opposite side of the laser chamber; a laser gas sealed in the laser chamber; a means for exciting the laser gas; and light generated from the excited laser gas. In the gas laser apparatus having two windows provided in the laser chamber for emitting to the outside of the laser chamber and a beam expanding optical system, a wedge substrate of the beam expanding optical system is composed of the gas laser optical element. A gas laser device using the optical element for gas laser according to any one of claims 1 to 3.

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