JP2007071831A - Optical material evaluation method and apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resist ultraviolet laser damage of lenses, prisms, light guide rods, optical windows, polarizing plates, and other optical component materials used in an apparatus using laser light in the vacuum ultraviolet region such as calcium fluoride. The evaluation is performed safely and easily at a low cost, non-destructively.
SOLUTION: A visible light laser such as an argon ion laser is incident on an optical material to be examined, and light scattered in a lateral direction (preferably at a 90 ° position) with respect to the incident axis of the visible light laser. Observe with a CCD camera. The amount of scatterers in the optical material is calculated from the observation result. By comparing this data with the separately acquired scatterer amount and UV laser damage resistance data, the UV laser damage resistance of the subject is predicted. Since a visible light laser with relatively low energy is used instead of an ultraviolet laser that destroys the sample, the subject can be inspected nondestructively, and the measurement itself can be performed at low cost and safely.
[Selection] Figure 3

Description

  The present invention relates to a method and apparatus for non-destructive, low cost and safe evaluation of ultraviolet laser damage resistance of optical materials.

There is an increasing demand for optical materials for strong ultraviolet laser applications having a wavelength of 400 nm or less. For example, with the increase in capacity and performance of semiconductor integrated circuits, the wavelength of light sources used in exposure apparatuses has been shortened, and ArF excimer lasers (wavelength 193 nm) are currently used. It has been studied further utilized to various applications of short F such ₂ excimer laser (wavelength 157 nm) wavelengths. Fluoride of alkali metal and alkaline earth metal (sodium fluoride, potassium fluoride, lithium fluoride, etc.) for lenses and window materials of devices with short wavelengths and high energy lasers in the vacuum ultraviolet region. Demand for single crystal materials, high-quality quartz glass, etc. is increasing, such as calcium bromide, barium bromide, magnesium bromide, and strontium bromide.

  These optical materials are required to have excellent durability that does not change (decrease) in optical characteristics such as transmittance with long-term irradiation with strong ultraviolet laser light. However, current optical materials vary from high quality materials with sufficient laser resistance to low quality materials that cannot withstand the intended ultraviolet laser.

  Therefore, conventionally, a material used for such an optical application is irradiated with a stronger ultraviolet laser for a long time, and its durability is inspected to determine whether it can be used. For example, in the evaluation of laser damage resistance, the energy that destroys the material by focusing and irradiating an ultraviolet laser inside the material is used as an index of damage resistance. In addition, for lenses and window materials for semiconductor exposure apparatuses, inspections such as measurement of transmittance change before and after irradiation with a strong ultraviolet laser and for a long period of irradiation have been performed.

  Since the inspection using a strong ultraviolet laser as described above is a destructive inspection, an optical material used for an actual lens or window material cannot be used as a test sample, and a test sample judged to have an equivalent quality (for example, It was necessary to prepare a test sample cut out from the immediate vicinity in the same crystal ingot.

  However, even samples that are judged to have the same quality cut out from the side of the same ingot do not always have the same laser resistance, and such samples are considered to have no problem. However, it may deteriorate more rapidly than judged from the sample during actual use.

  Furthermore, a high-intensity ultraviolet laser light source is very expensive and is expensive to maintain. In addition, the high-intensity ultraviolet laser beam has a high risk, and requires a sufficient safety device and equipment, resulting in a large-scale device configuration, and there is a problem that the operation of inspection and measurement becomes complicated.

  Accordingly, an object of the present invention is to provide a method for nondestructively evaluating the ultraviolet laser damage resistance of an optical material itself. Furthermore, it is also an object of the present invention to provide a method for simply evaluating the ultraviolet laser damage resistance at low cost without using a high-cost and dangerous high-intensity ultraviolet laser light source.

  The present inventors have intensively studied to achieve the above object. As a result, it was found that minute defects in the optical material have a great influence on the ultraviolet laser resistance. In addition, the present inventors have also found that the amount of the minute defect can be measured from the scattered light caused by the incidence of the visible light laser using a visible light laser that does not damage the optical material that is the subject, and the present invention has been completed. It was.

  That is, in the present invention, a visible light laser is incident on an optical material, light scattered in a direction transverse to the incident direction of the visible light laser is observed, and the scattered light in the optical material is observed from the scattered light. This is an optical material evaluation method for calculating the amount and predicting the ultraviolet laser damage resistance of the optical material from the calculated amount of scatterers.

Other inventions include
(1) means for holding an optical material;
(2) means for injecting a visible light laser while scanning the held optical material;
(3) Means for detecting scattered light of a visible light laser incident on an optical material, which is scattered in a direction transverse to the incident direction of the visible light laser;
(4) means for processing detected scattered light data and analyzing the amount of scatterers in the optical material;
(5) Means for predicting ultraviolet laser damage resistance from the amount of scatterers obtained by analysis,
It is an evaluation apparatus for ultraviolet laser damage resistance of an optical material.

  According to the method and apparatus of the present invention, by measuring the amount of minute defects inside an optical material as the amount of scatterers using a visible light laser, it is possible to non-destructively evaluate the correlated ultraviolet laser damage resistance. It becomes. Furthermore, there is no need to use expensive and dangerous high intensity ultraviolet laser light sources.

  Therefore, the evaluation of the ultraviolet laser damage resistance of the materials of lenses, prisms, light guide rods, optical windows, polarizing plates, and other optical components used in devices using laser light in the vacuum ultraviolet region is low cost, It is safe and simple, and is therefore extremely useful as an evaluation technique for various optical materials.

  In the present invention, the optical material for predicting the resistance to ultraviolet laser damage is not particularly limited as long as it is an optical material that requires such physical properties and transmits a visible light laser. For example, lenses and window materials of optical lithography equipment using ultraviolet light, particularly materials used as window materials, specifically lithium fluoride, sodium fluoride, potassium fluoride, rubidium fluoride, magnesium fluoride, fluoride Calcium, barium fluoride, strontium fluoride, aluminum fluoride, lithium barium fluoride, potassium magnesium fluoride, lithium aluminum fluoride, calcium strontium fluoride, magnesium magnesium fluoride, lithium strontium fluoride, cesium calcium fluoride, fluorine Examples thereof include alkali single crystals of lithium calcium aluminum fluoride, lithium strontium fluoride aluminum, lanthanoid fluorides, fluoride single crystals of alkaline earth metals, and high-quality quartz glass.

  According to the study by the present inventors, the ultraviolet laser damage resistance in such an optical material is closely related to the minute defects of the optical material. In the present invention, the amount of microdefects detected by the method described below is compared with data indicating the relationship between the separately evaluated ultraviolet laser damage resistance and the amount of microdefects. Resistance can be evaluated.

  In the present invention, minute defects in the optical material are detected using a visible light laser that has lower energy than the ultraviolet laser and hardly damages the optical material. More specifically, by utilizing the fact that such minute defects scatter the visible light laser, the visible light laser is incident on the optical material and the light scattered inside is detected. Here, in order to eliminate the influence of transmitted light and reflected light and perform more accurate scattered light measurement, measurement (detection) of the scattered light is performed from the lateral direction with respect to the incident direction of the visible light laser. Hereinafter, this method will be described in more detail.

  The visible light laser used in the present invention is not particularly limited. Argon ion laser (blue: 488 nm), helium-neon laser (red: 632.8), Nd: YAG second harmonic (532 nm), etc. Although it can be used without particular limitation, an argon ion laser with a short wavelength is used in consideration of the spatial resolution with respect to the size of the scatterer in the optical material to be measured, the sensitivity of the image receiving device, the cost of the light source, etc. It is preferable. The output of the visible light laser light source may be appropriately selected according to the density of defects in the condensing lens and the subject, but is generally in the range of 0.01 W to 200 W, more preferably 0.1 W to 100 W. Note that the intensity of the scatterer becomes stronger as the subject has more defects. Therefore, in such a case, the output of the visible light laser light source necessary for measurement with sufficient accuracy becomes lower.

  The magnification of the lens that collects the visible light laser incident on the optical material is not particularly limited, but is preferably 5 to 200 times and more preferably 10 to 100 times in order to increase the spatial resolution of the scattered image. Is preferred.

  The visible light laser incident on the optical material is scattered inside the optical material by minute defects existing on the optical path of the visible light laser. Specifically, the electric field component of the incident visible laser light displaces electrons in the optical material to create an electric dipole. The dipole emits a radiation wave having the same frequency as the incident wave in all directions. Considering the phase of the radiation wave and the incident wave, the sum is the light in the optical material. If the dipoles in the optical crystal are uniformly distributed, that is, if there are no microdefects, the light combined with the radiant waves is only forward scattered (light that travels straight inside the optical material). Scattered light is observed in directions other than the forward direction at the minute defect portion in which the electron density varies spatially and temporally. Since the scattered light is due to radiation from an electron dipole generated by incident light, the polarization of the incident visible light laser is preferably linearly polarized light or circularly polarized light parallel to the observation surface.

  The incident visible light laser can be incident on the optical material of the sample using a lens, and the scattered light from the condensed portion is observed.

  In the present invention, the amount (position, size, distribution state, etc.) of the scatterer (micro defect) is measured by detecting (observing) this scattered light. As shown in FIG. 3 showing the results of the examples described later, a close correlation is established between the amount of the scatterer and the ultraviolet laser damage resistance. Predict the UV laser damage resistance of optical materials. In other words, for several test samples with different qualities, the amount of scatterers was measured using the method of the present invention, UV laser damage resistance evaluation was actually performed on the same sample, and the evaluation obtained by both methods By performing calibration in association with values, it is possible to non-destructively evaluate and determine the ultraviolet laser damage resistance of an optical material.

  In the present invention, as described above, in order to eliminate the influence of the light that has been transmitted through the optical material without being scattered and the light reflected by the surface, the scattered light is measured more accurately. Measurement (observation) is performed from the lateral direction with respect to the incident direction of the visible light laser (in the following, when the three-dimensional space is indicated by XYZ, the incident axis of the visible light laser is the X axis) ). When measuring the scattered light, it may be in any position in the lateral direction, but it is easy to eliminate the influence of transmitted light and reflected light, and it is possible to obtain a strong scattering intensity, so that the incident axis (X Observation at a position that is 90 ° ± 20 ° with respect to the axis) is preferable, a position that is 90 ° ± 10 ° is more preferable, and observation at a position of approximately 90 ° indicates that the scattered light has a large S / N. Particularly preferred for the purpose of measuring by ratio. That is, it is most preferable to observe the scattered light on the YZ plane perpendicular to the X axis.

  The observation may be performed on the entire circumference (360 °) of the incident axis (X axis) of the visible light laser, but the angle is preferably narrow in order to measure with higher accuracy. For example, when the center position of observation is placed on the Y axis that is 90 ° with respect to the X axis, it is preferable to measure scattered light at an angle within ± 15 ° with respect to the Y axis, and ± 5 ° Is more preferable, and within ± 1 ° is most preferable. In order to obtain scattered light in such a range, a method of limiting the incident light by arranging a slit or the like in front of the detection means may be adopted.

  Furthermore, the scattered light detection position is a position where the angle formed by the axial direction of the scattered light to be detected and the surface of the subject (the surface from which the scattered light comes out) is as close to 90 ° as possible. It is preferable. In other words, it is preferable to adjust the position and orientation of the subject so that the above angle is obtained.

  The detection means used for measuring the scattered light is not particularly limited, but cameras capable of acquiring images such as a CCD camera are preferably used in that data processing relating to the obtained scattered light is easy. . In the case of a CCD camera, the resolution can be adjusted as appropriate by the lens used at the tip of the CCD camera.

  In conjunction with the scanning of the visible light laser, the scattered light generated by the minute defect is imaged on the light receiving element surface with a microscope adjusted to a photographing system such as a CCD camera and stored as digital image information.

  Usually, the incident area of the visible light laser is much smaller than the area of the incident surface of the optical material that is the subject, so the scattered light measured in this way is used to determine the region on the optical path of the visible light laser (primary Only information on scatterers (small defects) existing in the original), and information on other parts cannot be obtained. In order to obtain scatterer information in the entire area (three-dimensional) of the optical material that needs to be evaluated, for example, a method of moving the entire area necessary for scanning while keeping the incident axis parallel may be performed. In this case, depending on the accuracy required, the entire scanning area may not necessarily be scanned completely, and the scanning area may be formed in a fence shape by scanning at regular intervals. Such a fence-like scan is sufficient.

  Furthermore, even if scanning is performed for one section and the evaluation is performed assuming that the other sections are the same if there is no variation in the plane, the laser damage resistance can usually be predicted with sufficient accuracy. On the other hand, when the in-plane variation is large, a plurality of cross-sections are scanned to compare and evaluate the distribution of scatterers for each cross-section, and further scanning is performed as necessary based on the evaluation results. Can be done.

  Usually, although it depends on the method of manufacturing the optical material that is the subject, if the material is of a size that can be used for the lens or window material for the photolithography apparatus, the material has almost no variation depending on the site. It is sufficient that the scanning is performed on only one section. Of course, the above-mentioned “cross section” represents a surface scanned by a visible light laser, and does not indicate a physical cross section.

  Three-dimensional information about the scatterer in the optical material can be obtained from the position and speed scanned by the visible light laser and the measurement result of the scattered light obtained at each position and time. Of course, it is not always necessary to grasp the information as three-dimensional information. For example, the information may be two-dimensionalized as a plane parallel to the incident surface of the ultraviolet laser.

  The method for converting the data obtained by measurement into two-dimensional or three-dimensional data as described above is not particularly limited, and a known calculation means may be used. Generally, if a one-dimensional scattered image obtained by the CCD camera or the like is processed by an arithmetic device such as a computer, the position and size of the scatterer are obtained as a two-dimensional image along the visible light laser scanning section. An image reproducing the distribution state can be obtained. For this two-dimensional image, for example, image processing (binarization) / analysis (area ratio analysis) is performed to calculate the area ratio of the scatterer in the measurement surface of the test sample, that is, the amount taking into account the number and size. be able to. Further, if necessary, the above two-dimensional images may be stacked and processed as a three-dimensional image. Means for constructing a photographed scattered image into a two-dimensional image and reconstructing it into a scatterer image of a cross section of the test sample can be performed by software on a general personal computer. Means for obtaining the area ratio of the scatterer in the measurement cross section from the two-dimensional image of the scatterer in the test sample cross section can also be performed using commercially available image processing and image analysis software.

  Correlation between the amount of scatterers and the amount of scatterers obtained by separately measuring the amount of scatterers (volume value and area value) in the optical material as the specimen obtained as described above. By comparing with the data, the ultraviolet laser damage resistance of the optical material can be predicted. As the amount of the scatterer increases, the ultraviolet laser damage resistance deteriorates. Therefore, an optical material with a small amount (not ideal) of scatterers is a higher quality material in terms of resistance.

  Since the method of the present invention is a non-destructive inspection unlike the conventional method of actually irradiating an ultraviolet laser, a lens, a prism, an optical window, a polarization that uses the optical material evaluated in this way for an optical device. It can be used directly as a material or member such as a plate.

  Although the apparatus which performs the evaluation method of the above-mentioned this invention is not specifically limited, A typical aspect is demonstrated below with reference to FIG.

  FIG. 1 is a schematic diagram of a typical apparatus (hereinafter, the apparatus of the present invention) for carrying out the evaluation method of the present invention.

  An optical material 1 that is a subject is held by a sample holder 2. The sample holder 2 is designed so that it can be fixed without blocking the laser light transmitted through the subject 1 and the scattered light received by the image receiving device. The size of the optical material that can be held is not particularly limited, but it is preferable that it can hold a small test sample for measuring a minute region with high accuracy to a large test sample for obtaining a wide range of information. In this case, a method that can appropriately select the sample holder portion according to the size of the test sample is preferable. The sample holder 2 is preferably capable of adjusting the holding position and orientation of the subject, and may be provided with a mechanism for rotating the sample in various ways.

  The light source 3 for making the visible light laser incident on the optical material is as described above. Argon ion laser (blue: 488 nm), helium-neon laser (red: 632.8 nm), Nd: YAG 2 Double harmonics and the like are used. Among them, it is preferable to use an argon ion laser or the like having a short wavelength in consideration of the spatial resolution with respect to the size of the scatterer, the sensitivity of the image receiving device, and the like. Further, as the visible light laser light source 3, one having an output of about 0.01 W to 200 W, further about 0.1 W to 100 W is generally used as described above.

  In FIG. 1, the laser light is incident on the subject 1 after being reflected from the visible light source 3 by the reflecting mirrors 4 and 5 and then condensed by the condenser lens 6. The position and the number of mirrors used are not limited to this mode, and it is not necessary to use a reflection mirror at all, and it is possible to use three or more (reflect three or more times).

  The distance between the condenser lens 6 and the subject 1 is determined by the focal length of the condenser lens 6. Generally, a condensing lens having a focal length in the range of 2 to 500 mm may be used, and a condensing lens in the range of 5 to 100 mm is preferably used. Further, a mechanism capable of changing the distance within the above range may be provided in accordance with the size of the subject. The magnification of the condenser lens 6 is not particularly limited, but is preferably 5 to 200 times, and more preferably 10 to 100 times, in order to increase the spatial resolution of the scattered image.

  The beam diameter can be adjusted according to the size and type of the subject 1, and more preferably equipped with a collimator capable of adjusting the beam diameter (not shown). Furthermore, if necessary, a structure in which a plurality of visible light laser sources can be switched may be used.

  In the evaluation method of the present invention, the visible light laser light source 3 and / or the optical material 1 may be moved so that the visible light laser scans the inside of the optical material. In order to simplify the configuration of the apparatus, It is preferable to fix the position of the visible light laser light source and move the position of the optical material. That is, the subject 1 is scanned at a fixed position so that the subject 1 can be scanned in a state where the positions of the visible light laser light source 3 and scattered light detection means (and a reflection mirror used as necessary) are fixed. It is preferable that the specimen 1 is moved in the optical path of a visible light laser to scan the laser in the subject.

  For example, as shown in FIG. 1, when the optical axis (X axis) of the visible light laser and the optical axis (Y axis) of the scattered light to be detected are in a horizontal plane, the subject is in the vertical (Z axis) direction. Move to. In this case, depending on the size of the subject 1, the viewing angle of the CCD camera 7 serving as the detection means, the depth of focus of the focus lens 6, etc., all information in the X-axis direction may not be obtained at one time. In such a case, the subject 1 may be moved in the X-axis direction as necessary.

  On the other hand, when the optical axis of the visible light laser is horizontal and the optical axis of the scattered light to be detected is perpendicular (Z axis) to the laser optical axis, the subject is in the horizontal (Y axis) direction. Move. Furthermore, when the optical axis of the visible light laser is vertical (Z axis), the optical axis of the scattered light to be detected and the moving direction of the test sample intersect each other in the horizontal direction (one is the X axis and the other is the Y axis). (It will be an axis). In particular, an apparatus that moves the subject in the horizontal direction (XY plane direction) is preferable in consideration of accurate measurement even if the subject is large and heavy.

  In order to enable such movement, a structure in which the optical material 1 as the subject is fixed to the sample holder 2 and the sample holder 2 is fixed to the movable stage is preferable. In FIG. 1, this movable stage is an XYZ stage 8 that can move in three directions of XYZ, and can be moved with desired accuracy by computer control or the like. The movement accuracy is generally about 10 μm or less, although it depends on the intended measurement accuracy and the beam diameter of the visible light laser. Increasing the accuracy of the position of the visible light laser incident on the sample, that is, the minute positioning of the sample, is one of the important factors for improving the resolution and shortening the measurement time. In addition, it is also preferable to arrange these devices on the vibration isolator 9 because it is easy to obtain high measurement accuracy.

  As a device for movement, any device can be used without particular limitation as long as the subject 1 does not vibrate or the movement axis does not shift. Manual operation, an electric motor such as a stepping motor, a gear, a pulley, a belt, or the like may be used, or air pressure, hydraulic pressure, or the like may be used.

  When scanning the surface of the subject with a visible light laser, the speed of moving the optical material 1 as the subject using the movable stage as described above, that is, the speed of scanning the visible light laser is not particularly limited, What is necessary is just to select suitably from the magnitude | size of the test object 1, the precision desired with respect to an evaluation result, and the performance of the arithmetic unit for image processing. Usually, when scanning one step surface, it is preferable to move at a constant speed from the start to the end of scanning. Thereby, the movement amount can be obtained as a function of time.

  When scanning a plurality of cross-sections, the movable stage 8 may be moved in an axial direction different from the axis that is moved when scanning, and then scanning is performed again at that position. This movement is usually performed along an axis (Y-axis in FIG. 1) perpendicular to both the movement axis during scanning and the optical path axis of the visible light laser.

  In FIG. 1, a CCD camera 7, which is a means for detecting light (scattered light) incident on an optical material 1 that is a subject and scattered by a scatterer in the optical material, It is arranged at a position where it can detect light scattered in a direction of approximately 90 ° with respect to the optical axis (X axis). A plurality of detection means 7 may also be provided as necessary.

  The distance between the subject 1 and the scattered light detection means 7 is not particularly limited, but it is preferably in the range of 2 to 500 mm, taking into account the resolution and the intensity of the scattered light, and the range of 5 to 100 mm. Is more preferable. In addition, it is preferable to provide a mechanism that can be appropriately adjusted within the above range depending on the size of the subject 1 and the range in which the visible light laser is scanned (not shown).

  The scattered light data detected by the CCD camera 7 is combined with data relating to movement when the subject is moved using the movable stage 8 and processed by the method described above, and the scattered light in the subject is detected. Calculate and analyze the position, size, and distribution status. In FIG. 1, an arithmetic unit such as a computer 10 is used for the analysis, and data relating to scattered light obtained by a detecting means such as a CCD camera 7 can be directly input to the arithmetic unit as electronic data. Yes. The moving amount (moving speed) of the movable stage is also controlled by the same computer 10, and this computer 10 processes the data relating to the scattered light and the data relating to the moving amount of the movable stage.

  The amount of the scatterer in the optical material 1 that is the subject thus calculated and analyzed is compared with the correlation data between the ultraviolet laser damage resistance and the scatterer amount separately obtained as described above. Predict the UV laser damage resistance of the optical material. When comparing with the correlation data, it may be stored as electronic data in a storage device and compared with an arithmetic unit, but the correlation data is visualized as a figure or table, and the chart is used. You can confirm it.

  In FIG. 1, an arithmetic device for movement control of the movable stage 7, an arithmetic device for converting the detected scattered light into two or three dimensions, analysis results of the scatterer, ultraviolet laser damage resistance, and the amount of scatterer Although the calculation device for comparing the correlation data and the storage device storing the correlation data are all described in one computer 10, these are separate devices. It doesn't matter.

  When carrying out the evaluation method of the present invention, in order to prevent the influence of the reflection and scattering of the visible light laser on the surface of the subject, the subject has an incident surface of the visible light laser and a surface on the side receiving the scattered light. It is preferable that at least two surfaces orthogonal to each other, preferably three surfaces through which a visible light laser beam that has not been scattered pass, have an optical plane. The optical plane in the previous period is preferably subjected to high-precision polishing, and the surface roughness is preferably 10 nm or less, more preferably 1 nm or less in terms of Rq (RMS value).

  On the other hand, the apparatus of the present invention has a transparent matching oil bath so that it can be measured by immersing the object in the matching oil so that it can measure a test sample whose surface has not been polished and an irregularly shaped test sample that cannot be polished. And a sample holder in which the test sample can be fixed (these are not shown).

  EXAMPLES Hereinafter, although an Example is hung up and demonstrated, this invention is not limited to these Examples.

  Using the apparatus for evaluating the amount of scatterer using a visible light laser described below, the amount of scatterers of nine types of calcium fluoride single crystals a to i was evaluated under the following measurement conditions.

  The configuration of the apparatus is as shown in FIG. As the visible light laser light source 3, a 10 W argon ion laser (488 nm) is used, and the visible light laser is focused on the center of the calcium fluoride sample as the subject, and the optical axis (X axis) of the visible light laser is collected. On the other hand, the CCD camera 7 was arranged in the 90 ° direction to photograph the scattered light. The sample size is cut into a cube of 20 mm × 20 mm × 20 mm, and the surface roughness Rq (RMS) is measured on three surfaces: the incident surface of the visible light laser, the surface that receives the scattered light, and the surface through which the visible light laser passes. Optical polishing with a value of 0.5 nm or less was performed.

  The distance between the lens 6 for condensing the incident visible light laser and the sample was 50 mm. The distance between the sample and the lens of the CCD camera 7 was 30 mm. The sample was scanned in the Z-axis direction at a speed of 0.2 mm / min, and a 10 mm × 10 mm square scatterer image was taken. The data obtained by the CCD camera was sent to the computer 9 and binarized by image processing to obtain the area ratio (corresponding to the defect density) of the dot-like scatterers (dot-like minute defects), which was used as the scatterer amount.

  Next, the ultraviolet laser damage resistance of the calcium fluoride single crystals ai evaluated by the above method was evaluated. The ultraviolet laser damage resistance evaluation apparatus and measurement conditions are as follows.

The evaluation was performed using the ultraviolet laser damage resistance evaluation apparatus shown in FIG. A laser beam emitted from a second harmonic pulse laser (532 nm) 11 of Nd: YAG is applied to a CLBO (CsLiB ₆ O ₁₀ ) single crystal 12 which is a nonlinear optical crystal, and an ultraviolet laser of 266 nm which is a fourth harmonic. The light was generated, and the 266 nm ultraviolet laser light was condensed to a diameter of about 50 μm by the lens (100 mm) 21 through an attenuator and irradiated to the calcium fluoride sample 22. The sample was a 10 mm cube, and optical polishing with a surface roughness Rq (RMS value) of 0.5 nm or less was performed on two surfaces through which an ultraviolet laser beam was transmitted.

  Damage caused by ultraviolet laser is confirmed by plasma emission and observation by Nomarski microscope after irradiation (if ultraviolet laser damage occurs, plasma emission and irreversible damage marks occur on the surface). The incident energy intensity was measured with a power meter (not shown), and the laser intensity per unit area and time was calculated from the beam diameter and pulse width to measure the ultraviolet laser damage resistance as a damage threshold. As an index of UV laser damage resistance, commercially available synthetic quartz (there is almost no individual difference and UV laser damage resistance is stable) is used as a standard sample. Tolerance index.

  The amount of scatterers obtained by the method and apparatus of the present invention and the evaluation results of ultraviolet laser damage resistance are shown in Table 1, and the correlation between them is shown in FIG. 3 (graph). As is clear from FIG. 3, a close correlation was observed between the amount of optical material scatterers determined by the method and apparatus of the present invention and the resistance to ultraviolet laser damage. It can be seen that the ultraviolet laser damage resistance can be sufficiently evaluated.

It is the schematic of the apparatus for injecting a visible light laser into the optical material by this invention, and calculating the quantity of a scatterer from the scattered light. It is the schematic of the apparatus which measures the conventional ultraviolet laser damage tolerance using an ultraviolet laser light source. It is a graph which shows the correlation of the quantity of the scatterer measured with the method and apparatus of this invention, and the evaluation result of an ultraviolet laser damage tolerance.

Explanation of symbols

1. Subject (optical material)
2. 2. Sample holder Visible light source 4,5. Reflective mirror 6. 6. Condensing lens Scattered light detection means (CCD camera)
8). XYZ stage9. Anti-vibration table 10. Computer for device control / data processing / analysis 11. Nd: YAG pulse laser12. CLBO crystal 13. Prism 14. Beam damper 15. Mirror 16.1 / 2 wavelength plate 17. Polarizer 18. Beam splitter
19. Attenuator 20. Power meter A
21. Short focus lens 22. Sample 23. Power meter B

Claims (2)

  A visible light laser is incident on the optical material, the light scattered in the direction transverse to the incident direction of the visible light laser is observed, and the amount of scatterers in the optical material is calculated from the scattered light. A method for evaluating an optical material, wherein ultraviolet light damage resistance of the optical material is predicted from the amount of scatterers formed. (1) means for holding an optical material;
(2) means for injecting a visible light laser into the held optical material;
(3) means for moving the held optical material so that a visible light laser scans the inside thereof;
(4) Means for detecting scattered light of a visible light laser incident on an optical material, the light being scattered in a direction transverse to the incident direction of the visible light laser,
(5) means for processing the data of the detected scattered light to analyze the amount of scatterers in the optical material, and (6) means for predicting ultraviolet laser damage resistance from the amount of scatterers obtained by the analysis,
An evaluation apparatus for ultraviolet laser damage resistance of an optical material.

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