CN114941170A - Method for improving 193nm laser irradiation hardness of calcium fluoride crystal - Google Patents

Abstract

The invention discloses a method for improving 193nm laser irradiation hardness of calcium fluoride crystals. The method improves the radiation hardness of the calcium fluoride crystal by one or more of the combination of controlling the content of impurity cations in the calcium fluoride raw material, wherein the impurity cations have low s orbitals and d orbitals overlapped with an F-heart wave function, avoiding the formation of F, H and M centers in the calcium fluoride crystal, reducing the fluorine vacancy concentration in the calcium fluoride crystal and reducing the oxygen concentration in the calcium fluoride crystal.

Description

A method for improving the hardness of calcium fluoride crystal irradiated by 193nm laser

technical field

The invention relates to a method for improving the 193nm laser irradiation hardness of calcium fluoride crystals, and belongs to the technical field of calcium fluoride crystals.

Background technique

The transmission range of CaF ₂ single crystal covers the deep ultraviolet to mid-infrared band, and has ultra-low dispersion properties (Abbé coefficient: V _d = 95.23), ultra-high refractive index uniformity and negative temperature coefficient of refractive index (dn/dT = -10.6×10 ^-6 K ^-1 ), which is an irreplaceable optical material in the lithography exposure system. With the development of semiconductor lithography technology, the lithography light source has gradually developed from a mercury light source to a deep ultraviolet laser light source with shorter radiation wavelength and greater single-photon energy, of which ArF-193nm is a typical representative. When the optical material is in the state of strong ultraviolet/deep ultraviolet laser radiation for a long time, the material properties will change, resulting in a photo-induced shrinkage effect, which is mainly manifested as a change in the refractive index and a decrease in transmittance, which affects the stability of the entire system. Therefore, the UV laser-induced damage of _CaF2 crystal is the key to its service performance.

Intrinsic defects of crystals and surface and subsurface defects caused by processing can affect the radiation damage resistance of materials. Subsurface defects generated by surface processing can lead to photothermal damage and optical field modulation enhancement damage in applications. In addition to surface and subsurface defects, there will inevitably be various intrinsic defects inside the CaF ₂ crystal, which can be divided into point defects, line defects, surface defects, and bulk defects according to the scale from small to large. Point defects are usually composed of color centers and impurity ions. Line defects in crystals mainly refer to dislocations. Common surface defects in CaF ₂ crystals are generally small-angle grain boundaries, twin grain boundaries, stacking faults, etc. Bulk defects are defects such as bubbles, cavities and inclusions on a larger scale. The radiation damage resistance of calcium fluoride crystals is a comprehensive reflection of the macroscopic service performance. During the interaction between the laser and the crystal, the absorption, scattering, and strain fields around the defects caused by the intrinsic defects of the CaF ₂ crystal may be reduced. Stability of optical properties. Therefore, from the perspective of the material itself, finding the key factors affecting the damage and then optimizing the crystal preparation process is an important way to improve the laser damage threshold of calcium fluoride and improve its comprehensive service performance.

SUMMARY OF THE INVENTION

The inventors found that there are key cation impurities in calcium fluoride crystals that affect the resistance to radiation damage at 193 nm, and these impurity ions usually have low-position s orbitals and d orbitals that overlap with the F-heart wave function. At the same time, the F vacancies and residual O in the calcium fluoride crystal will reduce its radiation damage resistance to a certain extent. In view of the above problems, the present invention provides a method for improving the hardness of calcium fluoride crystals by 193nm laser irradiation. With the impurity cation content of low s orbital and d orbital, and the control of O content and F vacancy concentration as low as possible, the obtained calcium fluoride crystal has significantly improved radiation damage resistance.

In view of this, the present invention provides a method for improving the hardness of calcium fluoride crystal irradiated by 193nm laser. The method avoids the formation of F center, H center and M center in the calcium fluoride crystal by controlling the content of impurity cations with low-position s orbital and d orbital overlapping with the F heart wave function in the calcium fluoride raw material, reducing calcium fluoride. The combination of one or more of the concentration of fluorine vacancies in the crystal and reducing the concentration of oxygen in the calcium fluoride crystal increases the radiation hardness of the calcium fluoride crystal.

Preferably, the method includes: purifying the calcium fluoride raw material so that the content of impurity cations in the calcium fluoride raw material with low-position s orbital and d orbital overlapping with the F heart wave function is below 1 ppm, and using purified fluorine. The calcium fluoride raw material and the oxygen scavenger are used as the initial raw materials, and the calcium fluoride crystal is grown by the crucible descending method or the temperature gradient method and annealed in a fluorine-containing atmosphere to obtain the calcium fluoride crystal with improved laser irradiation hardness.

Preferably, the impurity cations with low-position s orbital and d orbital overlapping with the F-heart wave function include Y, La, Ce, Gd, Tb, and Lu.

Preferably, the calcium fluoride raw material is purified by chemical precipitation method and/or zone melting method, so that the content of impurity cations in the calcium fluoride raw material with low-position s orbital and d orbital overlapping with the F-heart wave function is below 1 ppm.

Preferably, the calcium fluoride raw material is purified by chemical precipitation so that the content of impurity cations in the calcium fluoride raw material with low-position s orbital and d orbital overlapping with the F-heart wave function is between 1-10 ppm, and then the zone melting method is used to purify the calcium fluoride raw material. The calcium fluoride raw material is further purified so that the content of impurity cations in the calcium fluoride raw material with low-position s orbital and d orbital overlapping with the F-heart wave function is less than 1 ppm.

Preferably, the oxygen scavenger is lead fluoride and/or polytetrafluoroethylene.

Preferably, the content of the oxygen scavenger in the calcium fluoride raw material is 0.3-4 wt.%.

Preferably, the fluorine _- containing atmosphere is CF4 gas, or HF gas, or a mixed gas of _CF4 gas and inert gas, or a mixed gas of HF gas and inert gas, or a mixture of _CF4 gas, HF gas and inert gas. mixed composition.

Preferably, the volume fraction of CF ₄ gas in the mixed gas is 3-25%.

Preferably, the inert gas is Ar and/or N ₂ .

The method of the present invention utilizes means including reducing the content of key impurity cations, reducing the residual O content, and reducing the concentration of F vacancies in the crystal. When the total content of the key impurity cations in the calcium fluoride crystal is below 1 ppm, and the O content and the F vacancy concentration are as low as possible, the radiation damage resistance of the calcium fluoride crystal will be significantly improved.

Description of drawings

Fig. 1 is the crystal photograph of the calcium fluoride crystal sample prepared in Example 1 before and after irradiation (damage) at 193 nm;

Fig. 2 is the absorption spectrum of the calcium fluoride crystal sample prepared in Example 1 before and after 193nm irradiation;

Fig. 3 is the EPR spectrum of the calcium fluoride crystal sample irradiation of embodiment 1 and embodiment 3 preparation;

Fig. 4 is the double crystal rocking curve of the calcium fluoride crystal sample prepared by embodiment 1-5;

FIG. 5 shows the damage threshold of the calcium fluoride crystal samples prepared in Examples 1-5.

Detailed ways

The present invention is further described by the following embodiments, and it should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention. The following exemplifies the method for improving the hardness of calcium fluoride crystal irradiated by 193 nm laser in the present invention. It can also be referred to as a method for enhancing the antibody damage of calcium fluoride crystals under 193 nm laser irradiation.

Through the comprehensive analysis of absorption spectrum, electron paramagnanetic resonance (EPR), Raman spectrum (Raman), damage morphology and typical line defects of calcium fluoride crystal damaged samples, combined with simulation calculation, the influence of ArF was obtained. -Key influencing factors of damage under 193nm excimer laser irradiation. On this basis, the present invention regulates the above-mentioned key influencing factors, so as to improve the irradiation hardness of calcium fluoride crystals at 193 nm.

The absorption spectrum analysis of calcium fluoride crystals with different irradiation hardnesses shows that the samples with obvious bulk damage show several obvious color center absorption peaks, and the color centers correspond to the F center (fluorine vacancy captures electrons) and V _k center respectively. Or H heart (V _k heart is the dimer F ₂ ^- of interstitial fluorine and lattice fluorine in 100 direction; H heart is the dimer F ₂ ^- of interstitial fluorine and lattice fluorine in 111 direction), M heart (of F heart aggregates) and color center absorption peaks associated with O impurities. These color center absorption bands are mainly caused by the overlapping of the low-position s and d orbitals of the key impurity ions with the F-center wave function. The EPR results show that the single electron center in the samples with bulk damage is not only affected by the F nucleus, but also by the magnetic nucleus with the spin quantum number I=5/2. By introducing different types and contents of cationic impurities, on the premise of ensuring high optical quality, the key cationic impurities affecting the radiation resistance at 193nm were determined by the irradiation experiments at 193nm to be cations with low s orbital and d orbital impurities . Impurity cations with low s orbital and d orbital, including but not limited to Y, La, Ce, Gd, Tb, Lu plasma. The presence of these impurity cations will accelerate the formation of F, H and M hearts in calcium fluoride crystals.

It is found that the total content of impurity cations Y, La, Ce, Gd, Tb, Lu, etc. with low-position s orbital and d orbital in the calcium fluoride raw material after purification is preferably less than <1ppm. The lattice distortion caused by the above impurity content is in a controllable range, and defects such as crystal cracking will not be caused, and the crystal growth quality can be guaranteed. Moreover, it can also ensure that there is no absorption in the specific application band range. In practical applications, the requirements for other cationic impurities can be relaxed to effectively reduce costs.

Purification methods include, but are not limited to, chemical precipitation and/or zone melting. Both purification methods aim to reduce the critical cation content below 1 ppm to prevent the induction of color centers during irradiation. The calcium fluoride feedstock can be purified to <1 ppm total impurity cations using only chemical precipitation or zone melting. However, a single purification method is relatively limited. For example, chemical purification method, after two adsorptions by ion exchange resin, the purification effect is greatly reduced, and the zone melting method is difficult to achieve effective impurity removal for impurity ions with a segregation coefficient close to 1 . Therefore, it is preferable to use a combination of various purification methods to control the content of key impurity cations.

In some technical solutions, the raw material is purified by chemical precipitation method, and the raw material is further purified by zone melting method, and the content of impurity cations with low s orbital and d orbital is monitored. Preferably, chemical precipitation is used to purify high-purity calcium fluoride raw materials, and the content of cationic impurities such as Y, La, Ce, Gd, Tb, Lu, etc. is controlled at 1-10 ppm; Treatment, control the content of cationic impurities such as Y, La, Ce, Gd, Tb, Lu, etc. <1ppm. Combining two purification methods of chemical precipitation and zone melting to control the content of key impurity ions in the crystal. The experiment found that the cations with low-position s orbital and d orbital that can overlap with the F-heart wave function are mainly trivalent heavy metal cation impurities. Therefore, ion exchange resins can be considered for further purification. Cation exchange resin has stronger adsorption to cations with high price and large ionic radius, and is suitable for extracting key impurity ions in calcium fluoride raw materials.

As an example, the steps of chemical precipitation purification of calcium fluoride raw materials are:

The high-purity calcium fluoride raw material required for crystal growth is synthesized by the direct precipitation method. The raw materials for the reaction were analytically pure calcium nitrate and potassium fluoride. In order to remove the key impurity cations in the raw material, calcium nitrate and potassium fluoride solutions are first purified. 150g calcium nitrate was prepared into 0.15g/mL calcium nitrate solution, 1000mL sodium pyridine-2,6 dicarboxylate was added, the pH was adjusted between 1.6-2 by adding nitric acid, and then the mixed solution was added to 2-3mL/ It was pumped into a chromatographic column containing 500 mL of Bio-Rad AG1-X4 anion exchange resin at the rate of min, and the purified raw material was obtained after passing through the chromatographic column. The above purification steps were repeated three times to obtain purified starting materials. In this process, pyridine-2,6 dicarboxylic acid and impurity cations form an anion complex. When the mixed solution flows through the chromatographic column, the complex is adsorbed on the anion exchange resin, so as to achieve the purpose of purification. Potassium fluoride uses a similar procedure for impurity removal. 150 g of potassium fluoride was prepared into a 0.15 g/mL potassium fluoride solution, 1000 mL of sodium pyridine-2,6 dicarboxylate was added, and the pH was adjusted between 1.6-2 by adding nitric acid, and then the mixture was adjusted to 2- The rate of 3mL/min was pumped into a chromatographic column containing 500mL of Bio-Rad AG1-X4 anion exchange resin, and the primary purified raw material was obtained after passing through the chromatographic column. The above purification steps were repeated three times to obtain purified starting materials. The purified potassium fluoride is directly introduced into the purified calcium nitrate solution to react, and after the reaction is sufficient, it is aged for 3 hours, and the calcium fluoride gel is obtained by high-speed centrifugation, and then goes through the process of ultrasonic dispersion to remove the calcium fluoride gel K ⁺ ions and nitrate ions adsorbed by the gel, and repeat the centrifugation and washing process 3 times, after freeze-drying and grinding, high-purity calcium fluoride raw materials can be obtained.

Preferably, the calcium fluoride powder synthesized by the direct precipitation method is deoxidized and fluorinated in a high vacuum furnace to obtain a powder raw material with a purity better than 99.9999% (6N order). The reaction temperature is 500℃～900℃, the holding time is 6h～20h, and the fluorinating agent is CF ₄ . According to the difference in the boiling point of fluorides of different impurity elements, the impurity ions and oxygen-containing impurities in the raw materials can be further removed by physical sublimation by means of high-temperature melting to obtain higher-purity crystalline raw materials.

As an example, the step of purifying calcium fluoride raw material by zone melting method is: by analyzing the distribution of impurity elements in the crystal, the distribution law of some impurity ions in the crystal is obtained: the tail is higher than the head, and the center is higher than the boundary. In order to further improve the purity of the crystal, on the one hand, based on the segregation characteristics of impurities such as Ce, La, Gd in the CaF ₂ lattice, the growth interface (micro-convexity) and crystallization rate (atomic layer stacking) are precisely controlled, and the crystal growth process is fully utilized. The self-extracting ability of the crystal lattice in the crystal can realize the purification of the crystal; on the other hand, two times of growth and impurity removal, combined with multiple chemical purification (calcination, dissolution, precipitation) schemes, finally break through the impurity content of Ce, La, Gd and so on. There is a limit, so that the purity of the crystal raw material reaches the order of 7N.

In other technical solutions, the calcium fluoride raw material is first purified, and the content of key impurity cations is controlled at 1-10 ppm; then the calcium fluoride raw material is continuously purified by chemical precipitation, and the impurity cation content is controlled to be less than 1 ppm.

By analyzing the formation process of typical color centers such as F, H, and M centers, it is found that their formation is related to the F vacancies in the crystal. Therefore, controlling the concentration of F vacancies in the calcium fluoride crystal is another way to improve the irradiation hardness. means. During the growth of calcium fluoride crystals, the F-containing atmosphere is introduced to help reduce the concentration of F vacancies in the crystals. In addition, the absorption spectrum analysis of the damaged samples showed that the absorption peak at 480 nm is related to oxygen impurities. Considering that the incorporation of oxygen will lead to the increase of F vacancies in the crystal, it is difficult to avoid the calcium fluoride raw material and the crystal growth process. Therefore, reducing the residual oxygen in the crystal as much as possible is also an important means to improve the irradiation hardness.

Using purified calcium fluoride raw material and oxygen scavenger as initial raw materials, calcium fluoride crystals are grown in vacuum atmosphere by crucible descending method or temperature gradient method, and annealed in fluorine-containing atmosphere to obtain high laser irradiation hardness of calcium fluoride crystals.

Adding an appropriate amount of oxygen scavenger during the growth of calcium fluoride crystals can reduce the O content in the crystals. The oxygen scavenger can be lead fluoride or Teflon. The mass percentage of the oxygen scavenger in the calcium fluoride raw material ranges from 0.3 to 3 wt.%. As an example, the mass percentage of the oxygen scavenger in the calcium fluoride raw material is 2.5 wt.%.

_The fluorine _- containing annealing atmosphere is CF4 gas, or HF gas, or a mixed gas of _CF4 gas and inert gas, or a mixed gas of HF gas and inert gas, or a mixed gas of CF4 gas, HF gas and inert gas. In some technical solutions, the concentration of CF ₄ in the mixed gas ranges from 3% to 25%.

Taking the crucible descending method as an example, the steps of growing calcium fluoride crystals and annealing in a fluorine-containing atmosphere are:

Ingredients: Use the raw materials of 5N-grade pharmaceuticals after removing key cation impurities, and add 2.5wt.% PbF ₂ as an oxygen scavenger.

Mixing: Put the weighed raw materials into the mixing bucket, and stir on the mixer for 24 hours to make them evenly mixed.

Crystal growth and atmosphere annealing: after the furnace is installed, vacuumize to more than 5×10 ^-3 Pa, then heat up the material, first heat up to 200°C, and keep it for 10h to remove moisture and air in the raw material; 20～50°C/h Continue to heat up to 800°C and hold for 15h; then continue to heat up to 1400-1450°C, keep the material for 10 hours, and slowly lower the crucible to grow crystals after the material is finished. The descending speed is 0.1-0.5 mm/h, and after the crystal growth is completed, the temperature is lowered to room temperature at a rate of 20-50 °C/h. After standing for 5 hours, it was filled with a mixture of carbon tetrafluoride and argon with a volume fraction of 15% for annealing. The cooling rate of less than 20°C/h drops to room temperature.

Taking the temperature gradient method as an example, the steps of growing calcium fluoride crystals and annealing in a fluorine-containing atmosphere are:

Ingredients: Use the raw materials of 5N-grade pharmaceuticals after removing key impurities and cationic impurities, and add 2.5wt.% PbF ₂ as an oxygen scavenger.

Mixing: Put the weighed raw materials into the mixing bucket, and stir on the mixer for 24 hours to make them evenly mixed.

Crystal growth and atmosphere annealing: put the mixed raw materials into the crucible and put it into the temperature gradient range of the temperature gradient method crystal growth equipment equipped with a high vacuum closed furnace chamber. The specific location selection depends on the temperature distribution design of the crystal growth equipment. . Preferably, the position of the bottom of the lower charging bin is flush with the bottom edge of the heating element. Use a diffusion pump or a turbomolecular pump to evacuate the furnace chamber into a high vacuum state (better than 10 ^-3 Pa). During the growth process, the cavity is kept in high vacuum or filled with high-purity (better than 99.999%) argon and CF4 as protective atmosphere. Heat the crucible to a temperature 10-100°C higher than the melting point of the raw material at a heating rate of 20-80°C/h (subject to the temperature measured by a thermocouple placed at the bottom of the crucible), during which a constant temperature of 150-300°C for 5 ~30h to dry the moisture in the raw materials, and keep the temperature at 600~900℃ for 12h to play the role of oxygen scavenger. After the temperature of the chemical is kept constant for 5-30 hours, the temperature is lowered by 100-200°C at a cooling rate of 0.2-5°C/h, preferably, the temperature is lowered by 150°C at a cooling rate of 0.5-2°C/h to complete the crystal crystallization process. Finally, the temperature is lowered to normal temperature at a cooling rate of 10-30°C/h. After standing for 5 hours, fill with a mixture of carbon tetrafluoride and argon with a volume fraction of 15%, and heat up to 900 to 1100 °C at a heating rate of 20 to 50 °C/h, and then keep the temperature for 20 to 60 hours. The cooling rate of ℃/h was reduced to room temperature.

In fact, due to the Nurson diffusion mechanism, there is still a certain amount of oxygen diffusing under extremely low pressure. Therefore, it is difficult to ensure that the oxygen in the crystal is removed by vacuuming alone. Therefore, it is necessary to remove the oxygen in the crystal through a chemical reaction. In order to ensure the control of the crystal growth interface during the crystal growth process, vacuum growth and atmosphere annealing are used to remove oxygen. Introducing tetrafluoroethylene, its main deoxygenation mechanism is:

C ₂ F ₄ +2CaO+O ₂ →2CO ₂ ↑+2CaF ₂

In the prior art, the raw material is fluorinated by introducing a mixed gas of carbon tetrafluoride gas and argon gas before crystal growth, but the growth is still performed by vacuuming during crystal growth. This method can mainly remove the small molecule compounds adsorbed on the crystal surface, and the transmittance that can be improved is mainly the decrease in transmittance caused by the vibrational absorption of small molecules in the infrared band. However, the oxygen vacancy pairs that affect the irradiation hardness of calcium fluoride crystals will still appear during vacuum growth. Therefore, in the present invention, a fluorine-containing atmosphere such as a mixed gas of carbon tetrafluoride and argon is charged in the annealing process to improve the crystal growth process. Oxygen vacancy pairs that arise. At the same time, the fluorine-containing atmosphere here has another important purpose, which is to reduce the fluorine vacancies in the calcium fluoride crystal under the fluorine excess atmosphere.

From the perspective of crystal growth in the prior art, the added _PbF or polytetrafluoroethylene are used as oxygen scavengers, because during the crystal growth process, even in the case of vacuuming, there may still be a certain amount of oxygen. In addition, some impurity elements in calcium fluoride exist in the form of oxides. Therefore, from the perspective of preparation, adding an oxygen scavenger during the growth of fluoride crystals is a more common method. The present invention starts from improving the anti-irradiation damage performance of calcium fluoride crystals, based on the analysis of the results of laser irradiation damage experiments, and proposes three key factors affecting the anti-irradiation damage of calcium fluoride crystals: 5d orbital related Key impurity ion content, fluorine vacancies and lattice oxygen. The basis for controlling oxygen in the present invention is the analysis of damage mechanism. Here, lattice oxygen refers to the oxygen entering the lattice position in the crystal, which will form oxygen-vacancy pairs with vacancies in the lattice, and the vacancies capture electrons to form color centers, resulting in the formation of crystals. damage. The best way to suppress lattice oxygen is to mix the oxygen scavenger with the raw materials, because the calcium fluoride raw materials are mostly high-purity powders, because the powder has a larger specific surface area, and fluorine and hydrogen can also form hydrogen bonds. It has a greater adsorption effect on small molecular compounds in the air. Therefore, the small molecular compounds such as hydroxide and water molecules adsorbed on the surface of the raw material powder are difficult to be exhausted only by adding the deoxidizer powder to the deoxidizer tray, or should be mixed. In this way, the oxygen that may enter the crystal lattice is eliminated. The invention further improves the ratio of PbF ₂ oxygen scavenger and increases the mixing time to eliminate the existence of lattice oxygen as much as possible. In the crystal growth system, the Pb impurity exists only in the form of PbF ₂ or PbO. In the actual crystal growth process, the temperature control program can be set to volatilize and discharge these two substances. In the actual test, it was also confirmed that the Pb impurity was basically discharged and had no effect on the hardness of the 193nm laser irradiation. Moreover, the fluorine-containing atmosphere used in the present invention is not used to fluoride the raw materials, but the fluorine-containing atmosphere is used in the annealing process of the crystal, and the main purpose is to reduce or eliminate fluorine vacancies.

In summary, the three methods of reducing the content of key impurity cations, reducing the residual O content and reducing the concentration of F vacancies in the crystal can avoid the formation of color centers from the source and improve the radiation resistance of the crystal. In some technical solutions, through raw material treatment and growth process control, the total content of impurity ions such as Y, La, Ce, Gd, Tb, Lu, etc. is controlled to be less than 1ppm, 0.3-4wt% deoxidizer is added, and 3% In an atmosphere containing ~25% fluorine, the anti-irradiation damage performance of calcium fluoride crystals at 193 nm is significantly improved, and no damage occurs after 70 million pulses of irradiation.

的(111)面氟化钙晶体。晶体进行超精密抛光，粗糙度小于1nm。采用MLI-FBG准分子激光器，调节能量密度、频率和脉宽，对不同的晶体进行193nm下的辐照损伤测试。Orientation and processing of the growing crystal yields

(111) face calcium fluoride crystals. The crystal is ultra-precisely polished with a roughness of less than 1nm. The MLI-FBG excimer laser was used to adjust the energy density, frequency and pulse width, and different crystals were tested for radiation damage at 193 nm.

The following further examples are given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention, and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present invention belong to the present invention. scope of protection. The specific process parameters and the like in the following examples are only an example of a suitable range, that is, those skilled in the art can make selections within the suitable range through the description herein, and are not intended to be limited to the specific numerical values exemplified below.

In the following examples, calcium fluoride crystals with different impurity contents, different O contents and different F vacancy concentrations were prepared respectively. The crystals were grown by the crucible descent method. The impurity content is mainly adjusted by the different raw material purities. Different O contents are adjusted by the amount of deoxidizer. The F vacancy concentration is adjusted by the concentration of the fluorine-containing atmosphere.

Example 1

Calcium fluoride raw materials (calcium nitrate, potassium fluoride) were purchased commercially. Sodium pyridine-2,6 dicarboxylate and Bio-Rad AG1-X4 anion exchange resin are also commercially available. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, and Lu in the calcium fluoride raw material is 30.124 ppm.

The _CaF2 single crystal was grown by the crucible descent method. The selected crucible material is graphite crucible, and the bottom of the crucible is put into the CaF ₂ single crystal rod with the normal direction of the end face oriented by X-ray diffractometer as [111], and the crystal growth is carried out in a high vacuum atmosphere. Specifically:

Ingredients: Use 5N-grade pharmaceutical raw materials, mixed with 0.5wt.% PbF ₂ as an oxygen scavenger;

Mixing: put the weighed raw materials into the mixing bucket, and stir on the mixer for 24 hours to make it evenly mixed;

Crystal growth: After the furnace is installed, vacuumize to more than 5×10 ^-3 Pa, then heat up the material, first heat it up to 200°C, and keep it for 10 hours to remove the moisture and air in the raw material; 20～50°C/h continue to heat up to 800 ℃, heat preservation for 15 hours; then continue to heat up to 1400 ~ 1450 ℃, heat preservation for 10 hours, and slowly lower the crucible to grow crystals after the material is finished. The descending speed is 0.1-0.5 mm/h, and after the crystal growth is completed, the temperature is lowered to room temperature at a rate of 20-50 °C/h.

Example 2

The calcium fluoride raw material is purified by chemical precipitation method. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, and Lu in the purified calcium fluoride raw material was 8.623 ppm.

The high-purity calcium fluoride raw materials required for crystal growth are synthesized by the direct precipitation method. The reaction raw materials are analytically pure calcium nitrate and potassium fluoride. In order to remove the key impurity cations in the raw materials, the calcium nitrate and potassium fluoride solutions are first purified. . 150g calcium nitrate was prepared into 0.15g/mL calcium nitrate solution, 1000mL sodium pyridine-2,6 dicarboxylate was added, the pH was adjusted between 1.6-2 by adding nitric acid, and then the mixed solution was added to 2-3mL/ The pump was pumped into a chromatographic column containing 500 mL of Bio-Rad AG1-X4 anion exchange resin at a rate of min, and the purified raw material was obtained after passing through the chromatographic column. The above purification steps were repeated three times to obtain purified raw materials. In this process, pyridine-2,6 dicarboxylic acid and impurity cations form an anion complex. When the mixed solution flows through the chromatographic column, the complex is adsorbed on the anion exchange resin, so as to achieve the purpose of purification. Potassium fluoride uses similar steps to remove impurities: 150g potassium fluoride is prepared into a 0.15g/mL potassium fluoride solution, 1000mL sodium pyridine-2,6 dicarboxylate is added, and the pH is adjusted by adding nitric acid between 1.6-2 Then, the mixture was pumped into a chromatographic column containing 500 mL of Bio-Rad AG1-X4 anion exchange resin at a rate of 2-3 mL/min. After passing through the chromatographic column, a purified raw material was obtained, and the above purification steps were repeated. three times to obtain purified starting material.

The purified potassium fluoride is directly introduced into the purified calcium nitrate solution to react, and after the reaction is sufficient, it is aged for 3 hours, and the calcium fluoride gel is obtained by high-speed centrifugation, and then goes through the process of ultrasonic dispersion to remove the calcium fluoride gel K ⁺ ions and nitrate ions adsorbed by the gel, and repeat the centrifugation and washing process 3 times, after freeze-drying and grinding, high-purity calcium fluoride raw materials can be obtained. The prepared CaF ₂ powder can also be deoxidized and fluorinated in a high vacuum furnace to obtain powder raw materials with a purity of better than 99.9999% (6N order), the reaction temperature is 700 ° C, the holding time is 13h, and the fluorination The agent of choice is CF ₄ .

The crystal growth process was the same as that of Example 1. The difference lies in the pharmaceutical raw material after chemical precipitation is used to remove key cationic impurities.

Example 3

The calcium fluoride raw material was purified by chemical precipitation in the same manner as in Example 2. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by chemical precipitation is 8.623 ppm.

The calcium fluoride raw material purified by chemical precipitation method is further purified by zone melting method. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by the zone melting method is 0.801 ppm. The specific operation of the purification by zone melting method is to use the crucible descending method to carry out crystal growth. After the growth, take the same diameter (two-thirds of the crystal equal diameter part upwards) crystal as the raw material for secondary purification, and then the same after the secondary growth. The upper grade (two-thirds of the crystal isometrically taken) was taken as the raw material for the final crystal growth.

The crystal growth process was the same as that of Example 1. The difference lies in the use of chemical precipitation and zone melting to remove key cationic impurities in the pharmaceutical raw materials.

Example 4

The calcium fluoride raw material was purified by chemical precipitation in the same manner as in Example 2. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by chemical precipitation is 8.623 ppm.

The calcium fluoride raw material purified by the chemical precipitation method was further purified by the zone melting method using the same method as in Example 3. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by the zone melting method is 0.801 ppm.

The crystal growth process is basically the same as that of Example 1, except that the content of the oxygen scavenger is different. The _CaF2 single crystal was grown by the crucible descent method. The selected crucible material is graphite crucible, and the bottom of the crucible is put into the CaF ₂ single crystal rod with the normal direction of the end face oriented by X-ray diffractometer as [111], and the crystal growth is carried out in a high vacuum atmosphere. Specifically:

Ingredients: use chemical precipitation method and zone melting method to purify the raw material of medicine, mix with 2.5wt.% PbF ₂ as oxygen scavenger;

Mixing: put the weighed raw materials into the mixing bucket, and stir on the mixer for 24 hours to make it evenly mixed;

Crystal growth: After the furnace is installed, vacuumize to more than 5×10 ^-3 Pa, then heat up the material, first heat it up to 200°C, and keep it for 10 hours to remove the moisture and air in the raw material; 20～50°C/h continue to heat up to 800 ℃, heat preservation for 15 hours; then continue to heat up to 1400 ~ 1450 ℃, heat preservation for 10 hours, and slowly lower the crucible to grow crystals after the material is finished. The descending speed is 0.1-0.5 mm/h, and after the crystal growth is completed, the temperature is lowered to room temperature at a rate of 20-50 °C/h.

Example 5

The calcium fluoride raw material was purified by chemical precipitation in the same manner as in Example 2. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by chemical precipitation is 8.623 ppm.

The calcium fluoride raw material purified by the chemical precipitation method was further purified by the zone melting method using the same method as in Example 3. The total content of key impurity cations such as Y, La, Ce, Gd, Tb, Lu in the calcium fluoride raw material purified by the zone melting method is 0.801 ppm.

The crystal growth process is basically the same as that of Example 1, except that the content of the oxygen scavenger is different and the annealing is carried out in a fluorine-containing atmosphere. The single crystal growth method is the crucible descending method. The selected crucible material is graphite crucible, and the bottom of the crucible is put into the _CaF2 single crystal rod with the normal direction of the end face being [111] oriented by the X _- ray diffractometer, and the crystal grows in the mixed atmosphere of CF4 and Ar. _The volume fraction of CF4 in the mixed atmosphere was 40%. Specifically:

Ingredients: use chemical precipitation method and zone melting method to purify the raw material of medicine, mix with 2.5wt.% PbF ₂ as oxygen scavenger;

Mixing: put the weighed raw materials into the mixing bucket, and stir on the mixer for 24 hours to make it evenly mixed;

Crystal growth and atmosphere annealing: after the furnace is installed, vacuumize to more than 5×10 ^-3 Pa, then heat up the material, first heat up to 200°C, and keep it for 10h to remove moisture and air in the raw material; 20～50°C/h Continue to heat up to 800°C and hold for 15h; then continue to heat up to 1400-1450°C, keep the material for 10 hours, and slowly lower the crucible to grow crystals after the material is finished. The descending speed is 0.1-0.5 mm/h, and after the crystal growth is completed, the temperature is lowered to room temperature at a rate of 20-50 °C/h. After standing for 5 hours, it was filled with a mixture of carbon tetrafluoride and argon with a volume fraction of 15% for annealing. The cooling rate of less than 20°C/h drops to room temperature.

The calcium fluoride crystals prepared in Examples 1-5 are marked as samples 1#, 2#, 3#, 4#, and 5#, respectively.

Table 1 Oxygen content test results

The test results in Table 1 show that the incorporation of oxygen into the crystal can be reduced to a certain extent by increasing the amount of oxygen scavenger used.

的(111)面氟化钙晶体。辐照前为无色透明晶体，辐照后晶体表面损伤区域出现明显黑色损伤。Fig. 1 is the crystal photograph of the calcium fluoride crystal prepared in Example 1 before and after irradiation at 193 nm. The crystal used for irradiation is

(111) face calcium fluoride crystals. Before irradiation, it was a colorless and transparent crystal, and after irradiation, obvious black damage appeared in the damaged area of the crystal surface.

FIG. 2 is the absorption spectrum of the sample of Example 1 before and after being irradiated at 193 nm. There are no obvious absorption peaks in the crystal before irradiation, and the absorption spectrum of the crystal after irradiation mainly presents four absorption peaks, which are 326 nm (V _k center or H center); 378 nm (F center); 480 nm (oxygen-related); 600 nm (F These absorption peaks are mainly caused by the absorption energy levels formed by the hybridization of the d orbital of the impurity ion with the 1s orbital and 2p orbital of the F core.

FIG. 3 is the EPR spectra of the irradiated samples of Example 1 and Example 3. FIG. The EPR spectrum after irradiation of the crystal of Example 1 presents six peaks with nearly equal intensity, which are mainly caused by the hyperfine interaction between a single impurity cation with d orbital activity and the F center, indicating that the color center in the crystal of Example 1 mainly occurs. Dominated by impurity cations. However, the EPR spectrum of the crystal of Example 3 after irradiation presents seven spectral lines whose intensity is proportional to the binomial expansion coefficient of (1+x) ⁶ , which is mainly caused by the hyperfineness between the F atom and the F core in the calcium fluoride crystal. The interaction results, indicating that the color center in Example 3 is mainly caused by the vacancies present in the crystal.

Figure 4 is the twin rocking curve of the calcium fluoride crystals prepared in Examples 1-5. It can be seen from the figure that when the content of impurity ions in the crystal is large, it will affect the crystal proximity performance. Among them, the crystallization performance of Example 1 is the worst, and the half-peak width is 0.213°. When the impurity ion content in the crystal decreases, the crystal can maintain a Good crystallization properties, the half-peak width is about 0.010°.

FIG. 5 shows the damage thresholds of calcium fluoride crystals prepared in Examples 1-5. It can be found that the content of impurity cations with d orbital activity in the crystal is reduced by chemical precipitation and zone melting; and by increasing the content of oxygen scavenger and filling fluorine-containing atmosphere to reduce the number of vacancies in the crystal, both improve the crystal to a certain extent. radiation hardness. Among them, examples 1-4 all showed damage in the irradiation experiment, and example 5 did not show any damage after the irradiation of 70 million pulses, indicating that after the three combined processes, the crystal has a relatively high quality. Strong radiation hardness.

Claims (10)

1. A method for improving 193nm laser irradiation hardness of calcium fluoride crystals, which is characterized in that the method avoids the formation of F centers, H centers and M centers in the calcium fluoride crystals by controlling the content of impurity cations in raw calcium fluoride materials with low s orbitals and d orbitals overlapped with F center wave functions, reduces the concentration of fluorine vacancies in the calcium fluoride crystals and reduces the concentration of oxygen in the calcium fluoride crystals to improve the irradiation hardness of the calcium fluoride crystals.

2. The method according to claim 1, characterized in that it comprises: purifying the calcium fluoride raw material to enable the content of impurity cations of a low s orbit and a d orbit which are overlapped with an F heart wave function in the calcium fluoride raw material to be below 1ppm, adopting the purified calcium fluoride raw material and an oxygen scavenger as initial raw materials, growing the calcium fluoride crystal by a Bridgman method or a temperature gradient method, and annealing in a fluorine-containing atmosphere to obtain the calcium fluoride crystal with improved laser irradiation hardness.

3. The method of claim 1 or 2, wherein the impurity cations having low s-orbitals and d-orbitals overlapping with the F-cardiowave function comprise Y, La, Ce, Gd, Tb, Lu.

4. A method according to claim 2 or 3, characterized in that the calcium fluoride raw material is purified by chemical precipitation and/or zone melting to make the content of impurity cations having low s-orbitals and d-orbitals overlapping with the F-core wave function in the calcium fluoride raw material below 1 ppm.

5. The method according to claim 4, wherein the calcium fluoride raw material is purified by a chemical precipitation method so that the content of impurity cations having low s orbitals and d orbitals overlapping with F heart wave functions in the calcium fluoride raw material is between 1 and 10ppm, and then further purified by a zone melting method so that the content of impurity cations having low s orbitals and d orbitals overlapping with F heart wave functions in the calcium fluoride raw material is less than 1 ppm.

6. The method of any of claims 2 to 5, characterized in that the oxygen scavenger is lead fluoride and/or polytetrafluoroethylene.

7. The method according to any one of claims 2 to 6, wherein the oxygen scavenger is present in an amount of 0.3 to 4wt.% of the calcium fluoride raw material.

8. The method according to any one of claims 2 to 7, wherein the fluorine-containing atmosphere is CF ₄ Gas, or HF gas, or CF ₄ Mixed gas of gas and inert gas, or mixed gas of HF gas and inert gas, or CF ₄ A mixed gas of a gas, an HF gas and an inert gas.

9. The method of claim 8, wherein the mixed gas comprises CF ₄ The volume fraction of the gas is 3-25%.

10. The method according to claim 8 or 9, wherein the inert gas is Ar and/or N ₂ 。

Images