JP4822285B2 - Optical element for gas laser and gas laser apparatus using the same - Google Patents

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, the polarization purity is prevented from deteriorating due to intrinsic birefringence and stress birefringence, and the cut surface of the calcium fluoride crystal is (110) -faced so that the surface roughness is small 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. An optical element for a gas laser such as a Brewster window or a beam expander prism using a calcium fluoride crystal that reduces Fresnel reflectivity of P-polarized light and prevents polarization purity deterioration due to intrinsic birefringence, and a gas laser using the same Is to provide a device.

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 is within the plane including the [111] axis and the [001] axis. In the plane including the [111] axis and the [010] axis, between the [111] axis and the [010] axis, or between the [111] axis and the [100] axis. In the plane to include, it passes between the [111] axis and the [100] axis, is emitted from the emission plane, and the incident angle of the laser beam to the incident plane is 24.9 ° to 68.73 °. The fluoridation is within an angular range The calcium crystal is rotated around the [111] axis and is placed in a region where intrinsic birefringence is minimized .

  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.

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, 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 reducing the Fresnel reflection due to the P-polarized light and disposing the intrinsic birefringence to a minimum, it is possible to prevent the polarization purity of the output laser light from deteriorating.

  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 cross section of the window 1 using CaF ₂ (calcium fluoride) according to the present embodiment.

The window 1 is incident on the optical element at an angle greater than 0, the optical axis refracted in the CaF ₂ crystal is in a plane including the [111] axis and the [110] axis, and [111] ] And CaF ₂ crystals are arranged so as to transmit through the angle between the [001] axis and the [001] axis.

The window 1 made of CaF ₂ crystal is polished on the surfaces 2 and 2 ′ in a plane parallel to the (111) plane. Then, for example, when a P-polarized laser beam is incident on the CaF ₂ crystal substrate at an incident angle α of Brewster angle of 56.34 °, the light on the surface 2 has a refraction angle of 33.65 ° according to Snell's law. Refracts at β. At this time, in CaF ₂ inside the refractive optical axis in a plane containing the [111] axis and the [110] axis of the CaF ₂ crystal, and, [111] axis and the [001] between the angle between the 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 beam is refracted again by the surface 2 ′ according to Snell's law similarly to the surface 2, and the light propagates again into the gas.

  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, it is necessary to arrange or minimize the influence of birefringence.

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 differ in the magnitude of birefringence depending on the crystal orientation, and are obtained by calculation.

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.

The CaF ₂ crystal is composed of a face-centered cubic lattice as shown in FIG. The direction of φ = 45 ° and θ = 90 ° in FIG. 1 is the [110] axis direction, and the direction of φ = 45 ° and θ = 54.74 ° is the [111] axis direction. In the present invention, intrinsic birefringence is reduced by passing the laser optical axis through φ = 45 ° and 0 <θ <54.74 °. In other words, by disposing the CaF ₂ crystal in the region of the one-dot chain line in FIG. 4A, deterioration of the polarization purity due to intrinsic birefringence is prevented as much as possible.

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. 5 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. 5, for example, when 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. 6A 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 refractive optical axis of the inner this CaF ₂ is to transmit in a plane containing the [111] axis and the [001] axis of the CaF ₂ crystal, CaF ₂ crystal is disposed. 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. 6B 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 refractive optical axis of the inner this CaF ₂ is to transmit in a plane containing the [111] axis and the [001] axis of the CaF ₂ crystal, CaF ₂ crystal is disposed. 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.

When the window 1 of the present invention is attached to the laser chamber, the optical axis of the laser light in the CaF ₂ crystal constituting the window 1 is formed in both crystal directions within the plane including the [111] axis and the [001] axis. The mounting position of the window 1 must be rotationally adjusted in the plane so as to pass substantially between the corners. This is shown in FIG. FIG. 8 shows a case where the window 1 is rotated around the [111] axis as shown in FIG. 7 while light of 193 nm is incident on the actual laser window at an incident angle of 56.34 ° (Brewster angle). The result of having measured with the birefringence measuring apparatus of measurement wavelength 193nm is shown. As apparent from FIG. 8, the intrinsic birefringence is minimized three times in one rotation (in the figure, 0 °, about 120 °, and about 240 °).

  For this reason, when the window 1 is attached to the laser chamber, the window 1 is rotated around the plane orientation [111] axis so that the intrinsic birefringence is minimized (p-polarized light intensity after transmission is maximized). preferable.

 Alternatively, before attaching the window 1, X-ray diffraction analysis may be performed and the crystal orientation may be measured in advance. It is efficient to mark the side of the window 1 in the [001], [100], or [010] axial direction and attach it according to the mark.

FIG. 9 shows relative values of intrinsic birefringence measured when the window 1 is rotated about the [111] axis when the incident angle is 24.9 °, 45 ° and 68.73 ° to the CaF ₂ crystal window 1. Showing change.

  In the range of the incident angle from 24.9 ° to 68.73 ° when the Fresnel reflection loss of P-polarized light is 3% or less, the intrinsic birefringence is around 0 °, 120 °, and 240 ° at which the intrinsic birefringence is minimized. A region where is minimized is observed.

  9A, 9B, and 9C, as the incident angle increases, the minimum value of intrinsic birefringence decreases and the maximum value increases.

As described above, the CaF ₂ crystal (111) plane was used as 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 CaF ₂ crystal incident on the crystal, rotating the CaF ₂ crystal about the [111] axis, and setting the CaF ₂ crystal in a region where the intrinsic birefringence is minimized, it is possible to obtain laser output light with high polarization purity. it can.

  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. 10 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 is particularly high in a narrow band of 40 W or more. 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. 10, the laser light oscillated from the oscillation laser 10 is reflected by 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, the polarization purity can be prevented from deteriorating by polishing the surface of the CaF ₂ crystal with the (111) plane and aligning the crystal orientation so as to reduce the intrinsic birefringence.

As described above, it is desirable to configure the windows 12, 13, 22, and 23 attached to the laser chambers 11 and 21 with 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. 11 shows an example in which the beam expanding optical systems 91 and 91 ′ by combining two wedge substrates are arranged in the resonator of the amplification stage laser. FIG. 11A shows an example in which the beam expanding optical system 91 ′ is arranged only on the output side, and FIG. 11B 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 intrinsic birefringence of the beam expansion optical systems 91 and 91 ′ is taken into consideration. If it is not installed, the polarization purity deteriorates. Thus, as in the present invention, the polarization purity can be prevented from deteriorating by arranging the CaF ₂ crystal axis so that the intrinsic birefringence is reduced.

FIG. 12 is 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 do. This principle will be described with reference to FIG. FIG. 12 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. 12A, 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 in the plane including the plane orientation [111] axis and the [001] 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. By arranging in this way, the intrinsic birefringence can be reduced, so that the polarization purity of the laser does not deteriorate. 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. 12B, the surface orientation on the exit side of the wedge substrate is made parallel to the (111) plane, and the optical axes passing through the CaF ₂ are the plane orientation [111] axis and the plane orientation [001] axis. The wedge substrate is placed so as to be between the angles formed by the two axes on the included surface.

  Here, when the energy density of the incident side surface is high, the arrangement of FIG. 12A becomes the (111) surface in the arrangement of FIG. 12A compared to FIG. Resistance is improved.

  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.

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]. It is a cross-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 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 a figure for demonstrating the adjustment method when attaching the window of FIG. 3 to a laser chamber.

It shows a relative value change in each of the intrinsic birefringence measured when the window in the case of 56.34 ° incident angle CaF ₂ crystal window rotated at [111] axis. 24.9 ° incident angle CaF ₂ crystal window, 45 °, a diagram showing a window [111] Relative value change in each of the intrinsic birefringence measured when rotating the axis in the case of 68.73 ° is there. 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.

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 (4)

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,
In the plane including the [111] axis and the [001] axis, the laser beam incident from the incident plane is between the [111] axis and the [001] axis,
In a plane including the [111] axis and the [010] axis, between the [111] axis and the [010] axis,
Or in a plane including the [111] axis and the [100] axis, between the [111] axis and the [100] axis,
Passed through the injection plane ,
The incident angle of the laser beam on the incident plane is in the range of 24.9 ° to 68.73 °,
The optical element for gas laser, wherein the calcium fluoride crystal is rotated around the [111] axis and is disposed in a region where intrinsic birefringence is minimized . 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. The gas laser device using the optical element for gas laser according to claim 1 , comprising an optical element for operation. 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 in the apparatus, the beam splitter is a gas laser apparatus using an optical element for a gas laser according to claim 1, characterized in that comprises an optical element for the gas laser. 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 claim 1 .

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