Fluoride crystal, window material using fluoride crystal, laser oscillator, optical element, and optical device

Abstract

Provided is a fluoride crystal containing, on a mass basis, 2-420 ppm of Mg and at least 300 ppm and 55% (5.5 × 105 ppm or less) of Sr.

Description

Fluoride crystal, and window material, laser oscillator, optical element, and optical device using the fluoride crystal

[0001] The present invention relates to fluoride crystals, and window materials, laser oscillators, optical elements, and optical devices that use fluoride crystals. This invention claims priority to Japanese Patent Application No. 2023-220262, filed on December 27, 2023, and the contents of that application are incorporated by reference into this application in designated states where incorporation by reference of documents is permitted.

[0002] Patent Document 1 discloses an apparatus and method for producing fluoride crystals. Fluorite is an example of a fluoride crystal. Fluorite is widely used as an optical material, but has the drawback of being easily broken in terms of mechanical strength.

[0003] Japanese Patent Application Laid-Open No. 2003-342098

[0004] In one aspect of the present invention, Mg is 2 ppm or more and 420 ppm or less, and Sr is 300 ppm or more and 55% (5.5 × 10 ５ It is a fluoride crystal containing less than 1 ppm of fluoride.

[0005] Another aspect of the present invention is a window material using the above-mentioned fluoride crystal.

[0006] Another aspect of the present invention is a laser oscillator using the above-mentioned fluoride crystal.

[0007] Another aspect of the present invention is an optical element using the above-mentioned fluoride crystal.

[0008] Another aspect of the present invention is an optical device including the optical element described above.

[0009] FIG. 1 is a schematic diagram showing an example of the configuration of a laser oscillator using a fluoride crystal according to the present embodiment; FIG. 2 is a schematic diagram showing another example of the configuration of a laser oscillator using a fluoride crystal according to the present embodiment; FIG. 3 is a perspective view showing an example of an optical device according to the present embodiment used as an imaging device; FIG. 4 is a block diagram showing an example of the configuration of a microscope according to the present embodiment; and FIG. 5 is a block diagram showing an example of the configuration of an exposure device according to the present embodiment.

[0010] An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. The present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be practiced with appropriate modifications within the scope of its gist.

[0011] In this specification, unless otherwise specified, the content of the element to be added is expressed as the content in a cationic state in ppm by mass. ppm by mass is synonymous with "mg / kg."

[0012] Fluorite (calcium fluoride) is an isotropic material with a wide wavelength transmission range, making it widely used as an optical material. However, from the perspective of mechanical strength, it has a problem of being prone to cracking, i.e., plastic deformation (slip). Mechanical strength is generally improved by doping with other elements (solid solution strengthening), but most doping elements deteriorate the laser durability of fluorite (greater reduction in transmittance during laser irradiation). In addition, there is a limit to the doping concentration for each element (solid solubility limit). Ca colloid reduces transmittance at wavelengths of 200 nm or less, generates crystal defects at wavelengths around 380 nm, and aggregates these crystal defects at wavelengths around 560 nm.

[0013] In this embodiment, various studies were conducted using the load at which plastic deformation (slip) occurred as an index for evaluating mechanical strength. In addition, various studies were conducted using the maximum value of the decrease in transmittance near a wavelength of 560 nm (peak near 560 nm) and the ratio of the maximum value of the decrease in transmittance near a wavelength of 560 nm to the maximum value of the decrease in transmittance near a wavelength of 380 nm (peak near 560 nm / peak near 380 nm) as indexes for durability.

[0014] <Fluoride Crystal> The fluoride crystal according to this embodiment contains, on a mass basis, Mg of 2 ppm or more and 420 ppm or less, Sr of 300 ppm or more and 55% (5.5 × 10 ５The lower limit of the Mg content may be 4 ppm, 7 ppm, or 10 ppm. The upper limit of the Mg content may be 200 ppm, 150 ppm, or 100 ppm. The lower limit of the Sr content may be 400 ppm, 800 ppm, or 1000 ppm. The upper limit of the Sr content is 50% (5.0 × 10 ５ ppm), or 40% (4.0 × 10 ５ ppm), or 20% (2.0 × 10 ５ ppm), or 10% (1.0 × 10 ５ It may also be expressed as ppm.

[0015] The ratio of the Mg content to the Sr content (Mg / Sr) in the fluoride crystal according to this embodiment is, on a mass basis, greater than 0 and equal to or less than 0.5. The lower limit of the Mg / Sr ratio may be 0.000001, 0.00001, or 0.0001. The upper limit of the Mg / Sr ratio may be 0.3, 0.1, or 0.022.

[0016] The fluoride crystal according to this embodiment is doped with Mg and Sr. By adding Mg and Sr to the fluoride base material in the above-described contents and ratios, the strength and laser durability of the resulting fluoride crystal can be improved. Furthermore, Mg and Sr are added in the form of halides, oxides, carbonates, nitrates, or sulfates, and preferably in the form of fluorides.

[0017] The base material of the fluoride crystal according to this embodiment is CaF ２ , BaF ２ etc., preferably CaF ２ CaF ２ Fluorite has a wide transmission wavelength range from ultraviolet to infrared, and is therefore suitable for use in optical components.

[0018] The fluoride crystal according to this embodiment is preferably a single crystal, which has fewer grain boundaries than a polycrystal and therefore has high ultraviolet transmittance and optical homogeneity, making it more suitable for use in optical components.

[0019] <Physical Properties of Fluoride Crystal> Evaluations of the mechanical strength (high temperature pressure test) and laser durability (X-ray irradiation test) of the fluoride crystal according to this embodiment will be described below.

[0020] Mechanical Strength The fluoride crystal according to this embodiment is heated to 200°C and pressed for 4 hours using a weight. The high temperature creates conditions that make the fluoride crystal more susceptible to slipping, and further pressure is applied to the fluoride crystal.

[0021] After high-temperature pressing, the sample is cooled to room temperature, birefringence is measured, and the presence or absence of slippage is evaluated based on the presence or absence of streaky birefringence. The same test is repeated by increasing the thickness of the weight until slippage occurs, and the ease of slippage is evaluated based on the magnitude of the load (weight thickness) at which slippage occurs, and this is used as an index of mechanical strength.

[0022] The fluoride crystal used in the high-temperature pressure test has a diameter of 50 mm and a thickness of 2 mm. Since the appearance of slips depends on the orientation, two faces are polished, and the plane orientation is the 111 plane (with an orientation deviation of 3 degrees or less from the 111 plane, preferably 0 degrees).

[0023] The weight used in the high-temperature pressure test was a cylindrical weight made of tungsten, with a base diameter of 30 mm.

[0024] The fluoride crystal according to this embodiment is formed by tungsten (density 19.3 g / cm 3 ) after high-temperature pressing. ３ When a weight made of fluoride is used, slippage does not occur if the weight has a thickness of 25 mm or less. The upper limit of the weight thickness at which slippage does not occur may be 50 mm, 60 mm, or 100 mm. As described above, the fluoride crystal according to this embodiment is a material that is less susceptible to slippage and has high mechanical strength.

[0025] X-ray irradiation test: The fluoride crystal according to this embodiment is irradiated with X-rays for 5 hours. By irradiating with X-rays, defects in the fluoride crystal are generated at an accelerated rate, allowing the durability of the fluoride crystal to be tested efficiently in a short period of time. The radiation source is a 4 kW Rh radiation source, and the dose is 15 kGy / min. The transmittance is then measured, and a transmittance spectrum is obtained at wavelengths of 200 nm to 800 nm. A difference spectrum is calculated from the difference between the transmittance spectrum obtained before and after X-ray irradiation. In other words, the obtained difference represents the amount of transmittance reduction at each wavelength. The decrease in transmittance near a wavelength of 560 nm is due to absorption by Ca colloid, and the decrease in transmittance near a wavelength of 380 nm is due to absorption by F-center, a precursor of Ca colloid. The maximum amount of transmittance reduction near a wavelength of 560 nm (peak near 560 nm) was used as an indicator of durability. In addition, the ratio of the maximum transmittance decrease near a wavelength of 560 nm (peak near 560 nm) to the maximum transmittance decrease near a wavelength of 380 nm (peak near 380 nm) was used as another indicator of durability. The smaller the value, the more durable the glass material, which is less likely to generate Ca colloids. Near a wavelength of 560 nm refers to, for example, wavelengths of 540 nm to 600 nm, and near a wavelength of 380 nm refers to, for example, wavelengths of 370 nm to 450 nm. Note that the position of the peak near 380 nm tends to shift toward the longer wavelength side as the amount of Sr doping increases.

[0026] The fluoride crystal used in the X-ray irradiation test has a diameter of 30 mm and a thickness of 5 mm. Two surfaces may be polished to have a 111 plane orientation.

[0027] In the fluoride crystal according to this embodiment, the maximum value of the transmittance decrease (peak near 560 nm) at wavelengths of 540 nm or more and 600 nm or less is 1.2% or less. The upper limit of the maximum value of the transmittance decrease at this wavelength may be 1.0%, 0.5%, or 0.3%. The lower limit of the maximum value of the transmittance decrease at this wavelength is, for example, 0%. Note that the maximum value of the transmittance decrease at wavelengths of 540 nm or more and 600 nm or less indicates the transmittance decrease (absorbance) at the wavelength at which the transmittance decrease (absorbance) is greatest in the wavelength range of 540 nm or more and 600 nm or less.

[0028] In the fluoride crystal according to this embodiment, the ratio of the maximum amount of decrease in transmittance at wavelengths of 540 nm or more and 600 nm or less (peak near 560 nm) to the maximum amount of decrease in transmittance at wavelengths of 370 nm or more and 450 nm or less (peak near 380 nm) is 0.4 or less. The upper limit of this ratio may be 0.1. Furthermore, this ratio may be 0. As such, the fluoride crystal according to this embodiment has a small ratio of the peak near 560 nm / peak near 380 nm, and is therefore a highly durable glass material that is less likely to generate Ca colloids.

[0029] <Uses of Fluoride Crystals> As described above, the fluoride crystals according to this embodiment have excellent mechanical strength and laser durability, and therefore can be suitably used in optical elements, optical devices, and the like. An example of an optical element is a laser window material. The laser window material can be used in a laser oscillator. Other examples of optical elements include lenses, prisms, mirrors, various window materials, beam splitters, polarizing beam splitters, and diffractive optical elements. Examples of optical systems that use the optical elements include interchangeable camera lenses, objective lenses, projection lenses, and radiation-resistant optical systems. These optical systems can be suitably applied to various optical devices, such as imaging devices such as interchangeable-lens cameras and non-interchangeable-lens cameras, microscope devices such as fluorescence microscopes and, in particular, polarizing microscopes, exposure devices for semiconductors and flat-panel displays, and astronomical optical devices.

[0030] <Laser Window Material> FIGS. 1A and 1B are schematic diagrams showing a laser oscillator as an example of a laser window material according to this embodiment. The laser oscillator 1 emits laser light without any particular limitation, and may be an excimer laser, a solid-state laser, a fiber laser, a gas laser, or the like. The laser oscillator 1 is composed of resonators 12 and 14 and a laser medium 16, and includes a window material 18 made of a fluoride crystal according to this embodiment. The laser medium 16 is disposed between the resonators 12 and 14, which are arranged parallel to each other. The resonator 12 is designed to totally reflect the laser light L, while the resonator 14 is designed to transmit a portion of the laser light L. The laser medium 16 is excited by excitation light (not shown), and the generated laser light L is amplified by traveling back and forth between the resonators 12 and 14 and the laser medium 16. A portion of the laser light L emitted from the laser medium 16 is extracted through the resonator 14. The window material 18 is provided between the resonators 12 and 14 as shown in Fig. 1A, and may be located at both ends of the laser medium, or may be provided in a housing 20 surrounding the resonators 12 and 14 and the laser medium 16 as shown in Fig. 1B. Furthermore, the laser window material according to this embodiment can be used not only in laser oscillators but also in any part of an optical device through which a laser passes (not shown). The fluoride crystal according to this embodiment is suitable for use as a laser window material because it is less likely to produce Ca colloids due to ultraviolet light irradiation, which causes a decrease in laser output, or slip due to stress load, which causes a deterioration in polarization.

[0031] <Imaging Device> FIG. 2 is a perspective view showing an example of an imaging device using the optical device according to this embodiment. The imaging device 2 is a so-called digital single-lens reflex camera (interchangeable lens camera), and the photographing lens 103 (optical system) includes an optical element using the fluoride crystal according to this embodiment as its base material. A lens barrel 202 is detachably attached to a lens mount (not shown) of a camera body 201. Light passing through a lens 203 of the lens barrel 202 is focused on a sensor chip (solid-state imaging element) 204 of a multi-chip module 206 disposed on the rear side of the camera body 201. This sensor chip 204 is a bare chip such as a so-called CMOS image sensor, and the multi-chip module 206 is, for example, a COG (chip-on-glass) type module in which the sensor chip 204 is bare-chip mounted on a glass substrate 205. Note that when the fluoride crystal according to this embodiment is used in an imaging device, it is possible to avoid intentionally doping the fluoride crystal with an element that causes fluorescence.

[0032] <Microscope> Fig. 3 is a block diagram showing an example of the configuration of a polarizing microscope 3 according to this embodiment. The polarizing microscope 3 includes an illumination optical system 301 and an imaging optical system 302. The illumination optical system 301 includes a collector lens 303, a relay lens 304, a condenser lens 305, etc., while the imaging optical system includes a first objective lens 306, a second objective lens 307, etc. The illumination optical system 301 or the imaging optical system 302 includes at least one optical element whose base material is the fluoride crystal according to this embodiment. In addition, a polarizing element 308 is provided between the relay lens 304 and the condenser lens 305, and a polarizing element 309 is provided between the first objective lens 306 and the second objective lens 307. The following description will focus on the optical system of the polarizing microscope 3.

[0033] The illumination optical system 301 irradiates the sample S with natural light as illumination light, and the imaging optical system 302 forms an image of the observation light emitted from the sample S. The natural light emitted from the light source 310 passes through a collector lens 303 and a relay lens 304 and is incident on a polarizing element 308. The polarizing element 308 polarizes the natural light and converts it into polarized light. The polarized light is collected by a condenser lens 305 and irradiated onto the sample S.

[0034] In this way, the illumination optical system 301 irradiates the sample S with illumination light, and the light emitted from the sample S becomes observation light and enters the first objective lens 306. The light that enters the first objective lens 306 is condensed by the first objective lens 306 and enters the polarizing element 309. The polarizing element 309 extracts light with a desired polarization from the incident light, and the light that passes through the polarizing element 309 enters the second objective lens 307.

[0035] The second objective lens 307 forms an image of the light that has passed through the polarizing element 309 at an imaging position F. An observer can observe the image formed at the imaging position F through an eyepiece, or can capture the image using a desired imaging device.

[0036] The fluoride crystal according to this embodiment is particularly suitable for use in polarizing microscopes because it is less susceptible to slips that cause deterioration of the degree of polarization.

[0037] 4 shows an example of an exposure apparatus 4 according to this embodiment, which includes a wafer stage 401 on which a wafer W (substrate) can be placed, an illumination optical system 402 that irradiates exposure light onto a reticle R (mask), a light source 403 that supplies exposure light to the illumination optical system 402, and a projection optical system 404 that projects the pattern of the reticle R onto the wafer W. The illumination optical system 402 includes a plurality of optical elements 405, and the projection optical system 404 includes a plurality of optical elements 406. These optical elements 405 and 406 are lenses, prisms, mirrors, filters, etc.

[0038] The exposure apparatus according to this embodiment may have an alignment optical system (not shown) that measures the relative positions of the reticle R and the wafer W. The reticle R may be a transmissive reticle in which a desired pattern is drawn on a glass substrate, or may be a reflective reticle configured using micromirrors or the like. The exposure apparatus according to this embodiment may have a reticle exchange system (not shown) that exchanges the reticle R illuminated by the illumination optical system 402.

[0039] The exposure apparatus according to this embodiment includes at least one optical element that uses the fluoride crystal according to this embodiment as a base material. In particular, it is preferable that at least one of the optical element 405 included in the illumination optical system 402 and the optical element 406 included in the projection optical system 404 uses the fluoride crystal according to this embodiment as a base material. The fluoride crystal according to this embodiment is less likely to generate Ca colloids even when irradiated with a high-energy laser, and is therefore suitable for use in exposure apparatuses.

[0040] Examples of the present invention and comparative examples will be described below, but the present invention is not limited to these.

[0041] <Production of Fluoride Crystals> CaF ２ The raw material is MgF ２ Raw materials and SrF ２ The raw materials were mixed and melted by the VGF (Vertical Gradient Freezing) method. Single crystals were grown by unidirectional solidification starting from a seed crystal by adjusting the heater output. The resulting single crystal material was annealed at a temperature between 1000°C and the melting point to remove strain.

[0042] The method for producing the fluoride crystal according to this embodiment is not particularly limited, and not only the VGF method but also known crystal growth methods such as the VB (Vertical Bridgman) method and the Cz (Czochralski) method can be used.

[0043] Test pieces (TP) of the desired shape were taken from the annealed single crystal. In this case, the TP for the mechanical strength test was Φ50-t2mm, and the test TP for laser durability evaluation was Φ30-t5mm, with two polished faces of Φ50mm and Φ30mm. The plane orientation of each polished face was 111. Since the appearance of slips is orientation dependent, TP polished on the 111 plane was used for the mechanical strength test.

[0044] <Mechanical Strength Evaluation> A fluoride crystal (Φ50-t2mmTP) according to this embodiment was placed on a tungsten jig, with only 1 mm of the entire periphery grounded. A Φ30mm tungsten weight (manufactured by Plansee) was placed in the center of this TP, which was then placed in a tabletop electric furnace and held at 200°C for 4 hours, followed by a cooling test. After cooling to room temperature, the crystal was removed and its birefringence was measured (WPA-100-L, manufactured by Photonic Lattice). The presence or absence of slips was evaluated based on the presence or absence of streaky birefringence. The same test was repeated, increasing the thickness of the weight until slips occurred, and the minimum thickness at which slips occurred was used as an index of mechanical strength. The thicker this thickness, the more durable the glass material, which is less likely to slip, and this was compared with the additive concentration. Note that Comparative Example 1 has a composition equivalent to that of existing fluorite. The results of the mechanical strength evaluation are shown in Tables 1 and 2.

[0045]

[0046]

[0047] From Tables 1 and 2, it was found that the higher the concentrations of Mg and Sr, the less likely slip occurs.

[0048] <Laser Durability Evaluation> The fluoride crystal (Φ30-t5mmTP) according to this embodiment was placed in a RIGAKU XRF device (ZSX Primus II) and subjected to 5 hours of X-ray irradiation. The radiation source was a 4 kW Rh radiation source at a dose of 15 kGy / min. After 5 hours of irradiation, transmittance measurements (Agilent Cary 5000) were performed to obtain a transmittance spectrum at wavelengths of 200 nm to 800 nm. A difference spectrum was calculated from the difference between the transmittance spectrum obtained before and after X-ray irradiation. In other words, the difference represents the amount of transmittance reduction at each wavelength. The decrease in transmittance near a wavelength of 560 nm is due to absorption by Ca colloid, and the decrease in transmittance near a wavelength of 380 nm is due to absorption by F-center, a precursor to Ca colloid. The maximum amount of transmittance reduction near a wavelength of 560 nm (peak near 560 nm) was used as an indicator of durability. In addition, the ratio of the maximum transmittance decrease near a wavelength of 560 nm to the maximum transmittance decrease near a wavelength of 380 nm was used as another indicator of durability. The smaller the value, the more durable the glass material is, with less Ca colloid generation. Note that Comparative Example 4 has a composition equivalent to that of existing fluorite. The results of the laser durability evaluation are shown in Tables 3 and 4.

[0049]

[0050]

[0051] From Tables 3 and 4, it was found that the higher the concentrations of Mg and Sr, the less likely Ca colloids were to be generated and the higher the durability against laser.

[0052] From the above, we found that Mg and Sr do not deteriorate laser durability, but rather improve it. We experimentally demonstrated that co-doping with these elements produces fluoride crystals with excellent mechanical strength.

[0053] 1...laser oscillator, 12...resonator, 14...resonator, 16...laser medium, 18...window material, 20...casing, L...laser light, 2...imaging device, 201...camera body, 202...lens barrel, 203...lens, 204...sensor chip, 205...glass substrate, 206...multi-chip module, 3...polarizing microscope, 301...illumination optical system, 302...imaging optical system, 3030...collector lens, 304...relay lens, 305...condenser lens, 306...first objective lens, 306, 307...second objective lens, 308...polarizing element, 308, 309...polarizing elements, 310...light source, S...sample, F...imaging position, 4...exposure device, 401...wafer stage, 402...illumination optical system, 403...light source, 404...projection optical system, 405, 406...optical element, W...wafer, R...reticle

Claims

1. A fluoride crystal containing Mg in an amount of 2 ppm or more and 420 ppm or less, and Sr in an amount of 300 ppm or more and 55% (5.5 × 10 ５ ppm) or less, based on mass.

2. The fluoride crystal according to claim 1, wherein, on a mass basis, the ratio of the Mg content to the Sr content (Mg / Sr) is greater than 0 and not more than 0.

5.

3. The base material of the fluoride crystal is CaF ２ The fluoride crystal according to claim 1 or 2, which is such.

4. The fluoride crystal according to any one of claims 1 to 3, wherein the fluoride crystal is a single crystal.

5. The fluoride crystal according to any one of claims 1 to 4, which does not exhibit Slip under heating at 200 °C and pressurization for 4 hours.

6. The fluoride crystal according to claim 5, wherein no Slip occurs when the pressurization is performed using a tungsten weight having a diameter of 30 mm and a thickness of 25 mm or less on a fluoride crystal having a diameter of 50 mm and a thickness of 2 mm.

7. The fluoride crystal according to any one of claims 1 to 6, wherein the maximum value of the transmittance decrease at a wavelength of 540 nm or more and 600 nm or less after X-ray irradiation for 5 hours is 1.2% or less.

8. The fluoride crystal according to any one of claims 1 to 7, wherein the ratio of the maximum value of the transmittance decrease at a wavelength of 540 nm or more and 600 nm or less to the maximum value of the transmittance decrease at a wavelength of 370 nm or more and 450 nm or less after X-ray irradiation for 5 hours is 0.4 or less.

9. The fluoride crystal according to claim 7 or 8, wherein the X-ray irradiation is performed using a 4 kW Rh radiation source at a dose rate of 15 kGy / min.

10. A window material using the fluoride crystal according to any one of claims 1 to 9.

11. A laser oscillator using the fluoride crystal according to any one of claims 1 to 9.

12. An optical element using the fluoride crystal according to any one of claims 1 to 9.

13. An optical device including the optical element according to claim 12.

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