A method for testing radiation damage resistance of calcium fluoride crystal

Abstract

The application discloses a method for testing anti-radiation damage performance of calcium fluoride crystal. The method adopts X-ray radiation to irradiate calcium fluoride crystal, and judges the anti-radiation damage performance of the crystal according to an X-ray radiation dose for inducing color center absorption peak of the calcium fluoride crystal and a color center absorption coefficient of a characteristic absorption peak induced by radiation.

Description

TECHNICAL FIELD

[0001] The application relates to a fluorite crystal anti-radiation damage test method and belongs to the technical field of fluorite crystal detection. BACKGROUND

[0002] The transmission range of CaF2 single crystal covers the deep ultraviolet to the mid-infrared band, has super-low dispersion performance (Abbe coefficient: V d = 95.23), super-high refractive index uniformity and negative refractive index temperature coefficient (dn / dT = -10.6 x 10 -6 K -1 ), and is an irreplaceable optical material for an exposure system of a photolithography machine. With the development of semiconductor photolithography technology, a photolithography light source gradually develops from a mercury lamp light source to a deep ultraviolet laser light source with shorter radiation wavelength and larger single-photon energy, and ArF-193 nm is a typical representative. When an optical material is in a strong ultraviolet / deep ultraviolet laser radiation state for a long time, the material characteristics change, a photo-induced shrinkage effect occurs, mainly in the form of refractive index change and transmittance reduction, thereby affecting the stability of the whole system. Therefore, the determination of the anti-laser radiation performance of CaF2 crystal is a key means to judge whether the CaF2 crystal can be applied to ultraviolet photolithography.

[0003] At present, the anti-radiation performance test of the fluorite crystal is mainly performed by loading the fluorite crystal in a 193 nm laser light path for irradiation test. Commonly used 193 nm laser irradiation conditions are: energy density: 100 mJ / cm 2 ; frequency: 800 Hz. In the 193 nm laser irradiation test, the crystal often shows obvious laser damage phenomenon after several ten million laser pulses, which usually requires irradiation time of nearly 10 hours. With the improvement of the anti-radiation performance of the crystal, the irradiation test time will be further prolonged. The judgment of the damage of the crystal needs to unload the test crystal from the light path, and then judge whether the crystal is damaged. The undamaged crystal is put into the light path for testing. Some slight damage cannot be distinguished by naked eyes, and the slight color center cannot be observed due to the de-excitation phenomenon of the color center in the room temperature environment, which brings great difficulty to the judgment of the anti-radiation performance of the fluorite crystal. SUMMARY

[0004] In view of the above problems, the inventors have proposed the following technical scheme:

[0005] In a first aspect, the application provides a fluorite crystal anti-radiation damage test method. The method uses the X-ray irradiation dose of the X-ray irradiation for inducing the color center absorption peak of the fluorite crystal and the color center absorption coefficient of the characteristic absorption peak induced by the irradiation to judge the anti-radiation damage performance of the crystal.

[0006] Preferably, the method comprises: X-ray irradiating the calcium fluoride crystal, and performing a color center absorption spectrum test on the sample after X-ray irradiation to observe whether a color center absorption peak appears in the calcium fluoride crystal after X-ray irradiation; using the irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak as the test index for the calcium fluoride crystal with the color center absorption peak; continuing to perform X-ray irradiation on the calcium fluoride crystal without the color center absorption peak after X-ray irradiation until the color center absorption peak appears in the sample after X-ray irradiation in the color center absorption spectrum test, and then using the total irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak as the test index.

[0007] Preferably, the calcium fluoride crystals in the same batch with the color center absorption peak appearing at the same irradiation dose are used as the index for judging the anti-irradiation damage performance of the calcium fluoride crystal according to the color center absorption coefficient of the characteristic absorption peak induced by irradiation.

[0008] Preferably, the characteristic absorption peak is 225±5nm, 335±5nm, 400±5nm, or 580±5nm; or 335±5nm, 380±5nm, or 600±5nm.

[0009] Preferably, the characteristic absorption peak is 400±5nm / 380±5nm or 580±5nm / 600±5nm. More preferably, the characteristic absorption peak is the F center structure absorption peak, that is, the absorption peak at 400±5nm / 380±5nm. The F center is one of the color centers, which is formed by the electron captured by the fluorine vacancy in the calcium fluoride crystal lattice.

[0010] Preferably, the absorption coefficient is based on the Lamber-beer law wherein α is the absorption coefficient, the unit is cm -1 L is the thickness of the crystal, the unit is cm; I is the intensity of the outgoing light; I0is the intensity of the incident light.

[0011] Preferably, the irradiation dose rate of each X-ray irradiation is 1000-2000Gy / h, and the irradiation flux is 1000-2000Gy; preferably, the irradiation dose rate and the irradiation flux of each X-ray irradiation are the same.

[0012] Preferably, the detection wavelength band of the color center absorption spectrum test is 190-800nm, and the detection step length is 1-5nm. In this way, the accuracy of the color center absorption peak can be ensured.

[0013] In a second aspect, the application provides an application of the calcium fluoride crystal anti-irradiation damage test method in assisting in judging the anti-irradiation damage performance and service life of the calcium fluoride crystal under 193nm laser lithography. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 Picture of the crystal sample of Example 1;

[0015] Figure 2 Absorption spectra of the 1#, 2#, 3#, 4#, 5# samples of Example 1 before irradiation; the absorption spectra curves of each sample are highly overlapped;

[0016] Figure 3 Comparison of the 193 nm laser irradiation induced color centers (a) and the X-ray induced color centers (b) of the 1#, 2#, 3#, 4#, 5# samples of Example 1;

[0017] Figure 4 Color center absorption spectra of the 1#, 2#, 3# samples of Example 1 after the first X-ray irradiation;

[0018] Figure 5 Color center absorption spectra of the 1#, 2# samples of Example 1 after the second X-ray irradiation;

[0019] Figure 6 Color center absorption spectra of the 1# sample of Example 1 after the third X-ray irradiation;

[0020] Figure 7 Comparison of the 193 nm laser irradiation induced color centers and the characteristic absorption peak absorption coefficient of the color centers induced by the first X-ray irradiation of the 1#, 2#, 3#, 4#, 5# samples of Example 1. The left column chart of each sample is the pulse number, and the right column chart is the characteristic absorption peak absorption coefficient.

[0021] In the above figures, some curves are overlapped, which does not affect the disclosure of the present application. DETAILED DESCRIPTION

[0022] The present application is further illustrated by the following examples, which should be understood as merely illustrative of the present application, but not limiting the present application.

[0023] The inventors found that the calcium fluoride crystals with similar performance and basically consistent surface processing have greatly different anti-laser irradiation performance. For example, in the 193 nm laser irradiation experiment, the difference is that the transmittance of the calcium fluoride crystals decreases after different pulse numbers under the same laser irradiation condition, that is, irradiation damage occurs. The absorption spectrum analysis of the calcium fluoride crystals with irradiation damage shows that the irradiation damage is related to the formation of irradiation-induced color centers. This is mainly because the valence band electrons of the calcium fluoride crystal are excited to the conduction band through two-photon absorption under 193 nm laser irradiation, thereby generating self-activated defects in the calcium fluoride crystal. The self-activated defects form F-H pairs, and form color center structures such as F centers and H centers in the crystal to generate characteristic peak absorption, thereby causing the transmittance of the crystal to decrease. Specifically, the most initial induction condition for the irradiation-induced formation of color centers is two-photon absorption. The photon energy of the 193 nm laser is 6.4 eV, and the band gap of the calcium fluoride crystal is about 11.6 eV, so two-photon absorption is required to induce the formation of self-activated defects.

[0024] The inventors also unexpectedly found that by analyzing the anti-irradiation damage morphology of the calcium fluoride crystal, the color center absorption spectrum induced by the 193 nm laser irradiation of the calcium fluoride crystal is consistent with the color center absorption spectrum induced by the X-ray irradiation. It shows that the 193 nm laser irradiation and the X-ray irradiation have basically the same irradiation-induced color center principle. Moreover, the wavelength range of hard X-ray is about 0.008 nm-0.1 nm, and the photon energy is much larger than the band gap of the calcium fluoride crystal, so it has a larger photon energy and can more significantly accelerate the induction of color centers in the calcium fluoride crystal. The color centers induced by X-ray are stronger and more easily observed at room temperature by absorption spectrum. Therefore, the present application proposes to determine the anti-irradiation damage performance of the calcium fluoride crystal by X-ray irradiation test.

[0025] The present application proposes a method for testing the anti-irradiation damage of calcium fluoride crystals. The method uses the X-ray irradiation dose for irradiation-induced color center absorption peak of the calcium fluoride crystal and the color center absorption coefficient of the characteristic absorption peak induced by irradiation to determine the anti-irradiation damage performance of the crystal. This method uses the total X-ray irradiation dose and the color center absorption coefficient of the characteristic absorption peak to quantitatively and qualitatively characterize the anti-irradiation performance of the calcium fluoride crystal.

[0026] In some technical solutions, the method comprises: performing X-ray irradiation on the calcium fluoride crystal, and performing color center absorption spectrum testing on the sample after X-ray irradiation to observe whether a color center absorption peak appears in the calcium fluoride crystal after X-ray irradiation; using the irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak as the testing index for the calcium fluoride crystal with the color center absorption peak; continuing to perform X-ray irradiation on the calcium fluoride crystal without the color center absorption peak after X-ray irradiation until the color center absorption peak appears in the sample after X-ray irradiation in the color center absorption spectrum testing, and then using the total irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak as the testing index.

[0027] The method uses X-ray irradiation to qualitatively and quickly obtain the anti-irradiation performance of the calcium fluoride crystal under 193 nm laser irradiation. The method can simultaneously detect the irradiation damage performance of multiple calcium fluoride crystals. In actual application, in the X-ray irradiation test, the calcium fluoride crystal sample is preferably placed at a relatively consistent position on the sample table to ensure the same X-ray intensity and reduce test errors.

[0028] The calcium fluoride crystals of the same batch with the color center absorption peak appearing at the same irradiation dose are used as the index for judging the anti-irradiation damage performance of the calcium fluoride crystal according to the color center absorption coefficient of the characteristic absorption peak induced by irradiation. By comparing the absorption coefficients of the induced absorption peaks of different calcium fluoride crystals under the same X-ray irradiation condition, the anti-irradiation damage performance of the calcium fluoride crystal under 193 nm laser irradiation is qualitatively expressed. The smaller the color center absorption coefficient, the stronger the anti-irradiation performance.

[0029] The characteristic absorption peaks induced by X-ray irradiation mainly include absorption peaks at 225±5 nm, 335±5 nm, 400±5 nm, and 580±5 nm, or absorption peaks at 335±5 nm, 380±5 nm, and 600±5 nm. According to the color center absorption spectrum analysis, the typical characteristic absorption peaks are at 400±5 nm or 380±5 nm. As preferred, the absorption coefficient of the characteristic absorption peak at 380 nm or 400 nm is used to represent the X-ray irradiation damage. The absorption peak here is mainly the F center structure absorption peak. The F center occupies a dominant position in the irradiation-induced color center, has a symmetrical structure, strong local stability, and a more suitable absorption peak intensity for representing the strength of the irradiation-induced color center.

[0030] The absorption coefficient is based on the Lamber-beer law The absorption coefficient is obtained; wherein, α is the absorption coefficient, and the unit is cm -1 L is the thickness of the crystal, and the unit is cm; I is the intensity of the outgoing light; I0 is the intensity of the incident light.

[0031] In some embodiments, the characteristic absorption peak is a F-center absorption peak, i.e., an absorption peak at 400 ± 5 nm or 380 ± 5 nm. The detection wavelength range of the F-center absorption spectrum test is 190-800 nm, and the detection step is 1 nm. Preferably, the detection wavelength range of the F-center absorption spectrum test is 200-800 nm, and the detection step is 1 nm.

[0032] In some embodiments, the irradiation dose rate of each X-ray irradiation is 1000-2000 Gy / h, and the irradiation flux is 1000-2000 Gy. Preferably, the irradiation dose rate and the irradiation flux of each X-ray irradiation are the same. The irradiation flux and the irradiation dose of a single test are fixed, so that the total irradiation dose can be used to more scientifically compare the anti-irradiation performance of the calcium fluoride crystal. For example, the X-ray irradiation test is performed using a MultiRad 160 irradiation device, the irradiation dose rate of each X-ray irradiation is 1500 Gy / h, and each irradiation lasts for 1 h. When in use, the tube voltage of the radiation source is 10-225 kV; the ray coverage is 9-40 cm; and the distance from the ray source to the sample is 13-65 cm. In other embodiments, the cumulative irradiation dose accuracy is 0.01 Gy; and the dose rate accuracy is 0.0001 Gy / min.

[0033] In some embodiments, the surface of the calcium fluoride crystal is polished before X-ray irradiation. The polishing can be chemical polishing and / or mechanical polishing. The size of the calcium fluoride crystal is selected according to requirements, for example, the sample size is Φ25 × 4 mm.

[0034] Patent CN1768259A discloses a method for analyzing impurities and color centers in fluorides and a method for manufacturing single crystal culture materials. The method is used for analyzing impurities (color centers) in a material composed of calcium fluoride by comparing the light transmittance of the material before and after X-ray irradiation. In this method, the content of impurities such as oxides and moisture is determined by judging the light absorption coefficient of the color centers generated by X-ray irradiation of the fluoride crystal, and the addition conditions of the cleaning agent are determined accordingly. In fact, the generation of color centers by X-ray irradiation of the crystal is greatly affected by impurities, but the color centers are not formed by oxides, moisture, etc. The formation of color centers is mainly due to lattice defects during crystal growth, which capture electrons under the irradiation of high-energy rays. This process is strongly affected by impurities. In addition to oxides and moisture, the formation of color centers in fluorides is also affected by related impurity cations and the concentration of lattice defects in the crystal. Therefore, there is a certain error in determining the content of oxides and moisture by judging the light absorption coefficient of the color centers generated by X-ray irradiation. In the present application, the anti-irradiation performance of the crystal is determined by inducing the generation of color centers by X-ray irradiation, which can be used to assist in determining the anti-irradiation performance of the crystal under 193 nm laser irradiation, and quantitative values of single influencing factors are given.

[0035] In summary, the method of the present application uses X-rays to quickly detect the anti-radiation performance of calcium fluoride crystals, especially in the deep ultraviolet band, and uses the total radiation dose and the absorption coefficient of the X-ray radiation-induced color center to qualitatively and quantitatively characterize the anti-radiation performance of calcium fluoride crystals. This greatly simplifies the detection method of the anti-radiation performance of calcium fluoride crystals under 193nm laser irradiation, and can achieve simultaneous detection of multiple crystals, improving the test efficiency of the anti-radiation performance. In addition, the calcium fluoride crystal irradiation damage performance test method can also be used for 193nm laser lithography to assist in judging the anti-radiation damage performance and service life of calcium fluoride crystals under 193nm laser lithography.

[0036] The following exemplary describes the calcium fluoride crystal anti-radiation damage test method.

[0037] The calcium fluoride crystals are oriented and processed to prepare the samples to be tested. The surface of the sample to be tested is polished to a surface roughness of less than 1 nm. For example, different batches of calcium fluoride crystals are selected, the crystals are oriented and processed to prepare Φ25x4mm (111) face calcium fluoride crystal sample pieces, and the two surfaces of the sample pieces are precisely polished by a chemical mechanical method, with a roughness of less than 1 nm. Before testing, the absorption performance of the sample can be tested first, the purpose is to compare the color center absorption spectrum after irradiation to judge whether damage has occurred. The detection wavelength range is 200-800nm, and the detection step is 1nm.

[0038] The calcium fluoride crystals are subjected to a first X-ray irradiation. A MultiRad 160 X-ray instrument can be used for irradiation. The radiation dose rate and irradiation time can be adjusted according to actual needs.

[0039] The sample after the first X-ray irradiation is subjected to a color center absorption spectrum test. The test wavelength range is 200-800nm, and the detection wavelength is 1nm. It is observed whether the sample after the first X-ray irradiation has a color center absorption peak at the key wavelength (380nm / 400nm; 580nm / 600nm). If a color center absorption peak appears, then the irradiation dose of the calcium fluoride crystal and the absorption coefficient of the characteristic absorption peak (380nm / 400nm) generated by the first irradiation are used as test indicators. If no color center absorption peak appears, then a crystal that does not have a color center absorption peak is selected for a second X-ray irradiation.

[0040] The second X-ray irradiation is performed on the crystal without color center absorption peak. The MultiRad 160 X-ray instrument can be used for the irradiation. The irradiation dose rate and irradiation time can be adjusted according to actual needs. The color center absorption spectrum test is performed on the sample after the second X-ray irradiation. The test wavelength range is 200-800 nm, and the detection wavelength is 1 nm. Whether the sample after the second X-ray irradiation has a color center absorption peak in the key wavelength range (380 nm / 400 nm; 580 nm / 600 nm) is observed. If the color center absorption peak appears, the total irradiation dose of the calcium fluoride crystal and the absorption coefficient of the characteristic absorption peak (380 nm / 400 nm) generated by the second irradiation are used as test indexes. If the color center absorption peak does not appear, the crystal without color center absorption peak is selected for the third X-ray irradiation.

[0041] The third X-ray irradiation is performed on the crystal without color center absorption peak. The MultiRad 160 X-ray instrument can be used for the irradiation. The irradiation dose rate and irradiation time can be adjusted according to actual needs. The color center absorption spectrum test is performed on the sample after the third X-ray irradiation. The test wavelength range is 200-800 nm, and the detection wavelength is 1 nm. Whether the sample after the second X-ray irradiation has a color center absorption peak in the key wavelength range (380 nm / 400 nm; 580 nm / 600 nm) is observed. If the color center absorption peak appears, the total irradiation dose of the calcium fluoride crystal and the absorption coefficient of the characteristic absorption peak (380 nm / 400 nm) generated by the third irradiation are used as test indexes. If the color center absorption peak does not appear, the crystal without color center absorption peak is selected for the third X-ray irradiation.

[0042] In summary, the calcium fluoride crystal irradiation damage performance is detected by X-ray irradiation in the application. The total irradiation dose and the color center absorption coefficient of the characteristic absorption peak are used to qualitatively and quantitatively characterize the anti-irradiation performance of the calcium fluoride crystal. The greater the total irradiation dose, the stronger the anti-irradiation performance.

[0043] In order to further verify the method of testing the irradiation hardness of the calcium fluoride crystal by X-ray irradiation, the test results of the method are compared with the test results of the irradiation damage at 193 nm. The MLI-FBG excimer laser is used to adjust the energy density, frequency and pulse width to test the irradiation damage of different crystals at 193 nm. The color center is used as a standard to record the pulse number to characterize the anti-irradiation performance of the crystal. The color center absorption spectrum test is uniformly performed on the sample after the 193 nm laser irradiation. The test wavelength range is 200-800 nm, and the detection wavelength is 1 nm. It is found that the irradiation-induced color center principle of the 193 nm laser irradiation is consistent with that of the X-ray irradiation, and the color center induced by the X-ray is stronger and easier to be observed at room temperature through the absorption spectrum. Therefore, the anti-irradiation performance of the crystal under the 193 nm laser irradiation can be qualitatively and quickly obtained by the X-ray irradiation in the application.

[0044] As an example, the laser used in the 193 nm laser irradiation test is an MLI-FBG excimer laser, the gas medium is ArF, the energy stability is <1.5%, the energy density range is 80-120 mJ / cm 2 , the repetition frequency is 400-4000 Hz, and the pulse width is 5-10 ns.

[0045] The following further illustrates the embodiments to explain the present application in detail. It should also be understood that the following embodiments are only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present application are within the scope of protection of the present application. The specific process parameters in the following examples are only one example in the appropriate range, i.e. those skilled in the art can select within the appropriate range according to the description herein, and are not limited to the specific values in the following examples.

[0046] Example 1

[0047] Five different batches of calcium fluoride crystals were selected respectively, and the crystals were oriented and processed under the same conditions to prepare calcium fluoride crystal sample pieces with a size of Φ25x4mm and a <111> surface as shown in Figure 1 . Two pieces of the same crystal were prepared for each batch for 193 nm laser irradiation and X-ray irradiation tests. The sample pieces were precisely polished by a chemical mechanical method to ensure that the surface roughness was less than 1 nm.

[0048] The absorption performance of the sample pieces was detected before the test, and the detection wavelength band was 200-800 nm with a detection step of 1 nm. Figure 2 The absorption spectrum data of the sample of Example 1 before irradiation. Except for slight absorption at 306 nm and around 240 nm, there is basically no obvious absorption in the visible band.

[0049] An MLI-FBG excimer laser was used to adjust the energy density to 100 mJ / cm 2 , the frequency to 800 Hz, and the pulse width to 8 ns to perform 193 nm irradiation damage test on the five crystals. The color center was used as a reference to record the laser pulse number to represent the anti-irradiation performance of the crystal. The sample after 193 nm laser irradiation was tested for color center absorption spectrum, and the test wavelength band was 200-800 nm with a detection wavelength of 1 nm.

[0050] The MultiRad 160 X-ray instrument is used to adjust the radiation dose rate to 1500 Gy / h, and set the irradiation time to 1 h, to perform the first X-ray irradiation on five calcium fluoride crystals. The color center absorption spectrum test is performed on the sample after the first X-ray irradiation, the test wave band is 200-800 nm, and the detection wavelength is 1 nm. Whether the color center absorption peak appears in the key wave band (380 nm / 400 nm; 580 nm / 600 nm) is observed, and the crystal without color center absorption peak is selected for the second X-ray irradiation. After the first irradiation, 3#, 4# and 5# all appear color center absorption peaks. 1# and 2# do not appear color center absorption peaks.

[0051] The MultiRad 160 X-ray instrument is used to adjust the radiation dose rate to 1500 Gy / h, and set the irradiation time to 1 h, to perform the second X-ray irradiation on calcium fluoride crystals. The color center absorption spectrum test is performed on the sample after the second X-ray irradiation, the test wave band is 200-800 nm, and the detection wavelength is 1 nm. Whether the color center absorption peak appears in the key wave band (380 nm / 400 nm; 580 nm / 600 nm) is observed, and the crystal without color center absorption peak is selected for the third X-ray irradiation. After the second irradiation, 2# appears color center absorption peak, and 1# does not appear color center absorption peak.

[0052] The MultiRad 160 X-ray instrument is used to adjust the radiation dose rate to 1500 Gy / h, and set the irradiation time to 1 h, to perform the third X-ray irradiation on 1# calcium fluoride crystal. The color center absorption spectrum test is performed on the sample after the third X-ray irradiation, the test wave band is 200-800 nm, and the detection wavelength is 1 nm. Whether the color center absorption peak appears in the key wave band (380 nm / 400 nm; 580 nm / 600 nm) is observed, and the color center characteristic peak absorption coefficient is combined to comprehensively evaluate the anti-irradiation performance of calcium fluoride crystals.

[0053] Figure 3 The color center absorption curves of the five samples of Example 1 after 193 nm laser irradiation and X-ray irradiation are measured by the absorption spectrometer. The color center absorption peak intensity generated by 193 nm laser irradiation is obviously smaller than that generated by X-ray irradiation. Moreover, due to the weak color center absorption peak induced by 193 nm laser irradiation, the 193 nm laser irradiation experiment time is longer, and the color center has a de-excitation phenomenon at room temperature, so only the 5# and 4# crystals with serious color center can measure the color center absorption peak in the absorption spectrum test. The color center absorption peak position induced by X-ray irradiation is consistent with that induced by 193 nm laser, and the surface damage mechanism is consistent. Moreover, the color center absorption peak induced by X-ray irradiation is stronger, which is more convenient for comparing the color center absorption peak intensity of different crystals.

[0054] Figure 4The spectrum of the three crystals with the weakest color center absorption peak after the first X-ray irradiation. After magnifying the absorption spectrum, it can be found by comparison that the 3# crystal also preliminarily forms a weak color center absorption peak (not obvious) at about 400 nm, which can be judged to have been damaged. Therefore, the radiation resistance of 1# and 2# crystals is better than that of 3# crystal. This also shows that the color center absorption peak produced by X-ray irradiation is stronger and more suitable for comparing the intensity of color center absorption peaks of different crystals.

[0055] Figure 5 The color center absorption spectrum induced by the second X-ray irradiation under the same conditions after the first X-ray irradiation of the undamaged 1# and 2# crystals in the X-ray irradiation experiment. According to the spectrum, it can be found that the 2# crystal produces an obvious color center absorption peak after the second X-ray irradiation, while the 1# crystal still does not produce a color center absorption peak after the second X-ray irradiation. This shows that the radiation resistance of the 1# crystal is better than that of the 2# crystal.

[0056] Figure 6 The color center absorption spectrum induced by the third X-ray irradiation under the same conditions after the second X-ray irradiation of the undamaged 1# crystal in the second X-ray irradiation experiment. According to the spectrum, it can be found that the color center absorption peak is not produced after the third X-ray irradiation, which can be determined to have extremely strong radiation resistance. Through the X-ray irradiation experiment, it can be qualitatively concluded that the order of radiation resistance from strong to weak is: 1#, 2#, 3#, 4#, 5#.

[0057] Figure 7 The comparison of the radiation resistance of calcium fluoride crystals in the 193 nm laser irradiation experiment and the radiation resistance of the crystals in the X-ray irradiation experiment. In the 193 nm laser irradiation experiment, the laser pulse number that the calcium fluoride crystal can withstand when the color center appears is used as an evaluation index. In the 193 nm laser irradiation experiment, the 1# crystal and the 2# crystal still do not produce a color center absorption peak after 7200 pulses, which are considered to have good radiation resistance, and the irradiation experiment is not continued. Therefore, the radiation resistance of the five crystals in the 193 nm laser experiment is: 1# = 2# > 3# > 4# > 5#. In the X-ray irradiation experiment, the selected crystal samples are consistent with the samples used in the 193 nm laser irradiation experiment, and the absorption coefficient of the characteristic absorption peak (380 nm / 400 nm) produced by the first X-ray irradiation is used as an index. The X-ray irradiation hardness test results are: 1#: 4500 Gy + 0.228 cm -1 ; 2#: 3000 Gy + 0.331 cm -1 ; 3#: 1500 Gy + 0.264 cm -1 ; 4#: 1500 Gy + 0.2975 cm -1 ; 5#: 1500 Gy + 5.736 cm -1Since the color center absorption peak does not appear in the first X-ray irradiation experiment of the 1# and 2# crystals, the X-ray irradiation determination of the radiation resistance of the five crystals is: 1# = 2# > 3# > 4# > 5#. Therefore, the radiation resistance of the five calcium fluoride crystals obtained in the 193 nm laser irradiation experiment and the X-ray irradiation experiment is consistent.

[0058] Through data comparison, it can be found that the pulse number of the damage threshold representation method in the 193 nm laser is negatively correlated with the characteristic peak absorption coefficient of the damage threshold representation method in the X-ray irradiation experiment, and therefore, the X-ray can be used to replace the 193 nm laser for irradiation hardness testing. Compared with the 193 nm laser irradiation experiment results, the X-ray irradiation testing method can more quickly and accurately characterize the radiation resistance of the calcium fluoride crystal, and the testing results are more accurate and reliable.

Claims

1. A qualitative and quantitative test method for assisting in judging the radiation damage resistance performance of calcium fluoride crystal under 193 nm laser lithography, characterized in that, The method is based on the characteristics that the color center absorption spectrum induced by 193nm laser irradiation of calcium fluoride crystals is consistent with the color center absorption spectrum induced by X-ray irradiation, and the characteristics that the number of pulses representing the damage threshold of calcium fluoride crystals in 193nm laser is negatively correlated with the absorption coefficient of the characteristic absorption peak in the damage threshold representation method of X-ray irradiation test, X-ray irradiation of calcium fluoride crystals is used to induce color center absorption peaks of calcium fluoride crystals, and the X-ray irradiation dose for inducing color center absorption peaks and the color center absorption coefficient of the characteristic absorption peak are used to judge the anti-irradiation damage performance of calcium fluoride crystals under 193nm laser lithography. The method comprises: X-ray irradiation of calcium fluoride crystals, and color center absorption spectrum test of the sample after X-ray irradiation, to observe whether color center absorption peaks appear in the calcium fluoride crystals after X-ray irradiation; for the calcium fluoride crystals with color center absorption peaks, the irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak are used as test indexes; for the calcium fluoride crystals without color center absorption peaks after X-ray irradiation, continue X-ray irradiation until color center absorption peaks appear in the sample after X-ray irradiation, and then the total irradiation dose of X-ray irradiation and the color center absorption coefficient of the characteristic absorption peak are used as test indexes.

2. The method of claim 1, wherein, For the same batch of calcium fluoride crystals with color center absorption peaks at the same irradiation dose, the color center absorption coefficient of the characteristic absorption peak induced by irradiation is used as an index to judge the anti-irradiation damage performance of calcium fluoride crystals.

3. The method of claim 1, wherein, The characteristic absorption peak is 225 ± 5 nm, 335 ± 5 nm, 400 ± 5 nm, 580 ± 5 nm, 380 ± 5 nm or 600 ± 5 nm.

4. The method of claim 3, wherein, The characteristic absorption peak is 400 ± 5 nm, 380 ± 5 nm, 580 ± 5 nm or 600 ± 5 nm.

5. The method of claim 1, wherein, Absorption coefficient is based on Lamber-beer law Obtained; wherein, a is absorption coefficient, unit is cm -1 ; L is the thickness of the crystal, unit is cm; I is the intensity of the outgoing light; I0is the intensity of the incident light.

6. The method of claim 1, wherein, The irradiation dose rate of each X-ray irradiation is 1000-2000 Gy / h, and the irradiation flux is 1000-2000 Gy.

7. The method of claim 6, wherein, The irradiation dose rate and irradiation flux of each X-ray irradiation are the same.

8. The method of claim 1, wherein, The detection wavelength band of the color center absorption spectrum test is 190-800 nm, and the detection step is 1-5 nm.

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