CN114941170B - Method for improving 193nm laser irradiation hardness of calcium fluoride crystal - Google Patents
Method for improving 193nm laser irradiation hardness of calcium fluoride crystal Download PDFInfo
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- 239000013078 crystal Substances 0.000 title claims abstract description 174
- WUKWITHWXAAZEY-UHFFFAOYSA-L calcium difluoride Chemical compound [F-].[F-].[Ca+2] WUKWITHWXAAZEY-UHFFFAOYSA-L 0.000 title claims abstract description 126
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- BFKJFAAPBSQJPD-UHFFFAOYSA-N tetrafluoroethene Chemical group FC(F)=C(F)F BFKJFAAPBSQJPD-UHFFFAOYSA-N 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B11/00—Single-crystal growth by normal freezing or freezing under temperature gradient, e.g. Bridgman-Stockbarger method
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
- C30B29/10—Inorganic compounds or compositions
- C30B29/12—Halides
-
- C—CHEMISTRY; METALLURGY
- C30—CRYSTAL GROWTH
- C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
- C30B33/00—After-treatment of single crystals or homogeneous polycrystalline material with defined structure
- C30B33/02—Heat treatment
Abstract
The invention discloses a method for improving 193nm laser irradiation hardness of calcium fluoride crystals. The method increases the irradiation hardness of calcium fluoride crystals by controlling the impurity cation content of the calcium fluoride feedstock having lower s-and d-orbitals overlapping the F-heart wave function to avoid the formation of F-, H-, and M-centers in the calcium fluoride crystals, reducing the concentration of fluorine vacancies in the calcium fluoride crystals, and reducing the combination of one or more of the oxygen concentrations in the calcium fluoride crystals.
Description
Technical Field
The invention relates to a method for improving 193nm laser irradiation hardness of calcium fluoride crystals, and belongs to the technical field of calcium fluoride crystals.
Background
CaF 2 The transmission range of the single crystal covers the deep ultraviolet to mid-infrared band and has ultra-low dispersion properties (Abbe coefficient: V d =95.23), ultra-high refractive index uniformity, and negative refractive index temperature coefficient (dn/dt= -10.6x10) -6 K -1 ) Is an irreplaceable optical material in the exposure system of the photoetching machine. With the development of semiconductor lithography, lithography light sources are gradually evolving from mercury lamp light sources to deep ultraviolet laser light sources with shorter radiation wavelengths and larger single photon energies, of which ArF-193nm is typically represented. When the optical material is in a stronger ultraviolet/deep ultraviolet laser radiation state for a long time, the material characteristics are changed, a photo-induced shrinkage effect is generated, and the optical material is mainly represented by refractive index change and transmittance reduction, so that the stability of the whole system is affected. Thus, caF 2 The ultraviolet laser induced damage of the crystal is the key to influence the service performance of the crystal.
Both the intrinsic defects of the crystal and the surface and subsurface defects caused by processing affect the radiation damage resistance of the material. Subsurface defects resulting from surface processing can lead to photo-thermal effect damage and light field modulation enhancement effect damage in applications. CaF in addition to surface and subsurface defects 2 Various intrinsic defects are inevitably present in the crystal, and can be classified into point defects, line defects, surface defects and bulk defects from small to large in scale. Point defects are generally composed of color centers and impurity ions. The line defects in the crystal are mainly referred to as dislocations. CaF (CaF) 2 The common surface defects in the crystal are generally small angle grain boundaries, twin grain boundaries, faults and the like. The bulk defects are defects such as bubbles, cavities, inclusion and the like on a larger scale. The irradiation damage resistance of the calcium fluoride crystal is the comprehensive manifestation of macroscopic service performance, and CaF is adopted in the action process of laser and the crystal 2 Absorption, scattering, strain field around the defect, etc. caused by the intrinsic crystal defect may reduce the stability of the optical performanceAnd (5) qualitative property. Therefore, the discovery of key factors affecting damage and further optimization of the crystal preparation process are important ways for improving the damage threshold of calcium fluoride laser and improving the comprehensive service performance of the calcium fluoride laser.
Disclosure of Invention
The inventors have found that there are critical cationic impurities in calcium fluoride crystals that affect 193nm radiation damage resistance, these impurity ions typically have low order s-orbitals and d-orbitals that overlap with the F-heart wave function. Meanwhile, F vacancy and residual O in the calcium fluoride crystal can reduce the radiation damage resistance to a certain extent. Aiming at the problems, the invention provides a method for improving 193nm laser irradiation hardness of calcium fluoride crystals, which specifically controls the impurity cation content with low s orbit and d orbit in the calcium fluoride crystals by means of reducing the cation content of key impurities, reducing the residual O content, reducing the F vacancy concentration in the crystals and the like, and controls the O content and the F vacancy concentration to be as low as possible, so that the irradiation damage resistance of the obtained calcium fluoride crystals is obviously improved.
In view of this, the present invention provides a method for increasing 193nm laser irradiation hardness of calcium fluoride crystals. The method increases the irradiation hardness of calcium fluoride crystals by controlling the impurity cation content of the calcium fluoride feedstock having lower s-and d-orbitals overlapping the F-heart wave function to avoid the formation of F-, H-, and M-centers in the calcium fluoride crystals, reducing the concentration of fluorine vacancies in the calcium fluoride crystals, and reducing the combination of one or more of the oxygen concentrations in the calcium fluoride crystals.
Preferably, the method comprises: purifying the calcium fluoride raw material to ensure that the impurity cation content of the low-order s orbit and d orbit which are overlapped with the F heart wave function in the calcium fluoride raw material is less than 1ppm, adopting the purified calcium fluoride raw material and an deoxidizer as initial raw materials, growing calcium fluoride crystals by a crucible descent method or a temperature gradient method, and annealing in a fluorine-containing atmosphere to obtain the calcium fluoride crystals with improved laser irradiation hardness.
Preferably, the impurity cations having lower s-orbitals and d-orbitals overlapping the F-heart wave function include Y, la, ce, gd, tb, lu.
Preferably, the calcium fluoride raw material is purified by adopting a chemical precipitation method and/or a zone melting method, so that the impurity cation content of the low-order s track and the d track which are overlapped with the F heart wave function in the calcium fluoride raw material is less than 1ppm.
Preferably, the calcium fluoride raw material is purified by adopting a chemical precipitation method to ensure that the impurity cation content of the calcium fluoride raw material with the low-order s orbit and the d orbit which are overlapped with the F heart wave function is between 1ppm and 10ppm, and then the calcium fluoride raw material is further purified by adopting a zone melting method to ensure that the impurity cation content of the calcium fluoride raw material with the low-order s orbit and the d orbit which are overlapped with the F heart wave function is less than 1ppm.
Preferably, the oxygen scavenger is lead fluoride and/or polytetrafluoroethylene.
Preferably, the oxygen scavenger comprises 0.3 to 4wt.% of the calcium fluoride raw material.
Preferably, the fluorine-containing atmosphere is CF 4 Gas, or HF gas, or CF 4 A mixed gas of a gas and an inert gas, or a mixed gas of an HF gas and an inert gas, or CF 4 A mixed gas of a gas and an HF gas and an inert gas.
Preferably, CF in the mixed gas 4 The volume fraction of the gas is 3-25%.
Preferably, the inert gas is Ar and/or N 2 。
The method of the invention utilizes means including reducing the cation content of key impurities, reducing the residual O content, and reducing the F vacancy concentration in the crystal. When the total content of the key impurity cations in the calcium fluoride crystal is below 1ppm and the O content and F vacancy concentration are as low as possible, the radiation damage resistance of the calcium fluoride crystal is obviously improved.
Drawings
FIG. 1 is a photograph showing crystals of a sample of calcium fluoride crystals prepared in example 1 before and after 193nm irradiation (damage);
FIG. 2 is an absorption spectrum of a sample of calcium fluoride crystal prepared in example 1 before and after 193nm irradiation;
FIG. 3 is an EPR spectrum of the irradiation of the calcium fluoride crystal samples prepared in example 1 and example 3;
FIG. 4 is a graph showing the bicrystal rocking curves of calcium fluoride crystal samples prepared in examples 1-5;
FIG. 5 is a graph showing the damage threshold of the calcium fluoride crystal samples prepared in examples 1 to 5.
Detailed Description
The invention is further illustrated by the following embodiments, which are to be understood as merely illustrative of the invention and not limiting thereof. The following illustrates the method of the present invention for increasing 193nm laser irradiation hardness of calcium fluoride crystals. May also be referred to as a method of increasing the damage to calcium fluoride crystals by 193nm laser irradiation.
By comprehensively analyzing the calcium fluoride crystal damage sample through absorption spectrum, electron paramagnetic resonance (electron paramagnanetic resonance, EPR), raman spectrum (Raman), damage morphology, typical line defects and the like and combining simulation calculation, key influencing factors influencing damage under ArF-193nm excimer laser irradiation are obtained. On the basis, the method regulates and controls the key influencing factors, and realizes the improvement of the irradiation hardness of the calcium fluoride crystal at 193 nm.
The absorption spectrum analysis is carried out on calcium fluoride crystals with different irradiation hardness, and the sample with obvious body damage is found to show a plurality of obvious color center absorption peaks, the color centers respectively correspond to F center (fluorine vacancy captures electrons) and V k Heart or H heart (V) k Dimer F with 100-directional interstitial fluorine and lattice fluorine as core 2 - The method comprises the steps of carrying out a first treatment on the surface of the Dimer F with H core of 111-direction interstitial fluorine and lattice fluorine 2 - ) M centers (aggregates of F centers) and color center absorption peaks associated with O impurities. These color center absorption bands are mainly caused by the overlapping of the lower s-orbitals and d-orbitals of the key impurity ions with the F-heart wave function. The EPR results indicate that: the single electron center in the sample where the bulk damage occurs is affected by the magnetic nuclei with spin quantum number i=5/2 in addition to F nuclei. By introducing different types and contents of cationic impurities, on the premise of ensuring high optical quality, the irradiation experiment at 193nm is used for determining that the key cationic impurities influencing 193nm irradiation resistance are low-order s orbit and d orbit impurity cations. Impurity cations having lower s-and d-orbitals,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 cores in the calcium fluoride crystal.
It was found that the total content of impurity cations Y, la, ce, gd, tb, lu having lower s-and d-orbitals in the purified calcium fluoride starting material is preferably less than <1ppm. The lattice distortion caused by the impurity content is in a controllable range, so that defects such as crystal cracking and the like are avoided, and the crystal growth quality can be ensured. Furthermore, no absorption in the specific application band range can be ensured. In practical applications, the requirements for other cationic impurities can be relaxed to effectively reduce the cost.
Purification methods include, but are not limited to, chemical precipitation and/or zone melting. The purpose of both purification methods is to reduce the critical cation content below 1ppm to prevent color centers from being induced during irradiation. The calcium fluoride feedstock may be purified to less than <1ppm total impurity cations using only chemical precipitation or zone melting. However, the single purification method has limitations, such as chemical purification, in which the purification effect is greatly reduced after the ion exchange resin is adsorbed twice, and the zone melting method has difficulty in effectively removing impurities from impurity ions with segregation coefficients close to 1. It is therefore preferred to use a combination of purification methods to control the content of the critical impurity cations.
In some technical schemes, raw materials are purified by a chemical precipitation method, the raw materials are further purified by a zone melting method, and the content of impurity cations with low-order s orbitals and d orbitals is monitored. Preferably, purifying the high-purity calcium fluoride raw material by adopting a chemical precipitation method, and controlling the content of cationic impurities such as Y, la, ce, gd, tb, lu to be 1-10ppm; and then purifying the calcium fluoride raw material by adopting a zone melting method, and controlling the content of cationic impurities such as Y, la, ce, gd, tb, lu to be less than 1ppm. Two purification methods of chemical precipitation and zone melting are combined to control the content of key impurity ions in the crystal. Experiments show that the cations with low-order s orbit and d orbit which can overlap with F heart wave function are mainly trivalent heavy metal cation impurities. Thus, further purification using ion exchange resins is contemplated. The cation exchange resin has stronger adsorptivity to high-valence cations with large ionic radius, and is suitable for extracting key impurity ions in calcium fluoride raw materials.
As an example, the steps of purifying the calcium fluoride raw material by chemical precipitation are:
the high-purity calcium fluoride raw material required by crystal growth is synthesized by adopting a direct precipitation method. The reaction raw materials are analytically pure calcium nitrate and potassium fluoride. To remove the critical impurity cations in the feed, the calcium nitrate and potassium fluoride solutions are first purified. 150g of calcium nitrate was formulated as a 0.15g/mL calcium nitrate solution, 1000mL sodium pyridine-2, 6-dicarboxylate was added, the pH was adjusted to between 1.6 and 2 by adding nitric acid, and then the mixture was pumped at a rate of 2-3mL/min into a column containing 500mL Berle Bio-Rad AG1-X4 anion exchange resin, and after passing through the column, a purified starting material was obtained. The above purification steps were repeated three times to obtain a purified starting material. In this process, pyridine-2, 6 dicarboxylic acid forms an anionic complex with impurity cations, and the complex is adsorbed on an anion exchange resin when the mixed solution flows through a chromatographic column, thereby achieving the purpose of purification. The potassium fluoride is subjected to impurity removal by adopting a similar step. 150g of potassium fluoride was prepared as a potassium fluoride solution of 0.15g/mL, 1000mL of sodium pyridine-2, 6-dicarboxylate was added, the pH was adjusted to between 1.6 and 2 by adding nitric acid, and then the mixture was pumped into a column containing 500mL of Berle Bio-Rad AG1-X4 anion exchange resin at a rate of 2 to 3mL/min, and a once purified starting material was obtained after passing through the column. The above purification steps were repeated three times to obtain a purified starting material. Directly introducing purified potassium fluoride into purified calcium nitrate solution for reaction, aging for 3 hours after the reaction is complete, obtaining calcium fluoride gel through high-speed centrifugation, and removing K adsorbed by the calcium fluoride gel through ultrasonic dispersion + And (3) repeating the centrifugation and washing processes for 3 times, and freeze-drying and grinding to obtain the high-purity calcium fluoride raw material.
Preferably, the calcium fluoride powder synthesized by the direct precipitation method is deoxidized and fluorinated in a high vacuum furnace to obtain the powder raw material with purity superior to 99.9999% (6N level). The reaction temperature is 500-90 DEGThe temperature is 0 ℃ and the heat preservation time is 6 to 20 hours, and the fluorinating agent is CF 4 . According to the difference of boiling points of fluoride of different impurity elements, impurity ions and oxygen-containing impurities in the raw materials can be further removed by adopting a high-temperature melting mode through a physical sublimation method, so that the crystalline raw materials with higher purity are obtained.
As an example, the steps of purifying a calcium fluoride raw material by the zone melting method are: by analyzing the distribution of impurity elements in the crystal, the distribution rule of partial 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, the method is based on the impurity such as Ce, la, gd and the like in CaF 2 The segregation characteristic in the crystal lattice, the growth interface (micro-convex) and the crystallization rate (atomic layer stacking) are precisely controlled, so that the purification of the crystal is realized by utilizing the self-impurity-removing capability of the crystal lattice in the crystal growth process; on the other hand, the method adopts the scheme of twice growth and impurity removal and combines multiple chemical purification (calcination, dissolution and precipitation), and finally breaks through the existing limit of the impurity content of Ce, la, gd and the like, so that the purity of the crystal raw material reaches 7N level.
In other technical schemes, firstly, the calcium fluoride raw material is purified, and the cation content of key impurities is controlled to be 1-10ppm; and then adopting a chemical precipitation method to continuously purify the calcium fluoride raw material, and controlling the content of impurity cations to be less than 1ppm.
By analyzing the formation process of typical color centers such as F center, H center and M center, the formation of the color centers is related to F vacancy in the crystal, so that the control of the concentration of F vacancy in calcium fluoride crystal is another means for improving irradiation hardness. And F-containing atmosphere is introduced in the growth process of the calcium fluoride crystal, so that the concentration of F vacancies in the crystal is reduced. In addition, absorption spectrum analysis on a sample damaged by a generator shows that an absorption peak at 480nm is related to oxygen impurities, and in consideration of the fact that F vacancies in crystals are increased due to oxygen doping, pollution of oxygen is difficult to avoid in the calcium fluoride raw material and the crystal growth process, so that reducing residual oxygen in the crystals as much as possible is an important means for improving irradiation hardness.
The purified calcium fluoride raw material and the deoxidizer are used as initial raw materials, the growth of calcium fluoride crystals is carried out under vacuum atmosphere by a crucible descent method or a temperature gradient method, and the calcium fluoride crystals with high laser irradiation hardness are obtained by annealing in fluorine-containing atmosphere.
Proper amount of deoxidizer is added in the growth process of calcium fluoride crystal, so that the O content in the crystal can be reduced. The oxygen scavenger may be lead fluoride or polytetrafluoroethylene. The mass percentage of the deoxidizer in the calcium fluoride raw material is 0.3-3 wt%. As an example, the mass percent of oxygen scavenger to the calcium fluoride raw material is 2.5wt.%.
The fluorine-containing annealing atmosphere is CF 4 Gas, or HF gas, or CF 4 A mixed gas of a gas and an inert gas, or a mixed gas of an HF gas and an inert gas, or CF 4 A mixed gas of a gas and an HF gas and an inert gas. In some technical schemes, CF of mixed gas 4 The concentration range of (2) is 3% -25%.
The steps of growing calcium fluoride crystals and annealing in a fluorine-containing atmosphere are as follows:
and (3) batching: 2.5wt.% PbF was incorporated in a raw material from which critical cationic impurities were removed using a drug product of grade 5N purity 2 As an oxygen scavenger.
Mixing: and (5) putting the weighed raw materials into a mixing bucket, and stirring the raw materials on a mixer for 24 hours to uniformly mix the raw materials.
Crystal growth and atmosphere annealing: after the charging is finished, vacuumizing to be more than 5 multiplied by 10 -3 Pa, heating the material, firstly heating to 200 ℃, and preserving heat for 10 hours to remove water and air in the raw materials; continuously heating to 800 ℃ at 20-50 ℃/h, and preserving heat for 15h; and then heating to 1400-1450 ℃ continuously, preserving heat and melting for 10h, and slowly lowering the crucible to grow crystals after melting is finished. The dropping speed is 0.1-0.5 mm/h, and the crystal is cooled to room temperature at the speed of 20-50 ℃/h after the crystal growth is completed. After standing for 5 hours, filling mixed gas of carbon tetrafluoride and argon with the volume fraction of 15 percent for annealing, heating to 900-1100 ℃ at the heating rate of 20-50 ℃/h, then keeping the temperature for 20-60 hours, and then cooling to room temperature at the cooling rate of less than 20 ℃/h.
The steps of growing calcium fluoride crystal and annealing in fluorine-containing atmosphere are as follows:
and (3) batching: 2.5wt.% PbF is incorporated in a raw material from which cationic impurities, which are key impurities, have been removed using a pharmaceutical product having a purity of 5N grade 2 As an oxygen scavenger.
Mixing: and (5) putting the weighed raw materials into a mixing bucket, and stirring the raw materials on a mixer for 24 hours to uniformly mix the raw materials.
Crystal growth and atmosphere annealing: and loading the mixed raw materials into a crucible, and placing the crucible into a temperature gradient zone of a temperature gradient method crystal growth device provided with a high vacuum closed furnace chamber, wherein the specific position selection is different according to the temperature distribution design of the crystal growth device. Preferably, the bottom of the lower charging bin is level with the edge of the bottom of the heating body. The diffusion pump or the turbomolecular pump is adopted to pump the furnace chamber into a high vacuum state (which is better than 10 -3 Pa magnitude). In the growth process, the cavity is kept in high vacuum or is filled with high-purity (better than 99.999%) argon and CF4 is a protective atmosphere. Heating the crucible to a temperature 10-100 ℃ higher than the melting point of the raw material at a heating rate of 20-80 ℃/h for melting (based on the temperature measured by a thermocouple placed at the bottom of the crucible), keeping the temperature between 150-300 ℃ for 5-30 h to dry the moisture in the raw material, and keeping the temperature between 600-900 ℃ for 12h to play the role of an deoxidizer. After the material melting temperature is kept constant for 5-30 h, the temperature is reduced by 100-200 ℃ at a temperature reduction rate of 0.2-5 ℃/h, preferably by 150 ℃ at a temperature reduction rate of 0.5-2 ℃/h, and the crystallization process of the crystal is completed. Finally, the temperature is reduced to normal temperature at a cooling rate of 10-30 ℃/h. After standing for 5 hours, charging mixed gas of carbon tetrafluoride and argon with the volume fraction of 15%, heating to 900-1100 ℃ at the heating rate of 20-50 ℃/h, then keeping the temperature for 20-60 hours, and then cooling to room temperature at the cooling rate of less than 20 ℃/h.
In fact, due to the noossen diffusion mechanism, there is still a certain amount of oxygen diffusion at very low pressure, and therefore, it is difficult to ensure that oxygen is removed from the crystal by simply evacuating, and thus, it is necessary to remove oxygen from the crystal by chemical reaction. In order to ensure the control of the crystal growth interface in the crystal growth process, the vacuumizing growth and the atmosphere annealing mode are adopted to remove oxygen. Tetrafluoroethylene is introduced, and the main deoxidization mechanism is as follows:
C 2 F 4 +2CaO+O 2 →2CO 2 ↑+2CaF 2
in the prior art, the mixed gas of carbon tetrafluoride gas and argon is introduced to fluorinate the raw material before growing the crystal, but the vacuum pumping mode is still adopted for growing in the crystal growing process. The method can mainly remove small molecular compounds adsorbed on the surface of the crystal, and the improved transmittance is mainly the transmittance reduction phenomenon caused by infrared band small molecular vibration absorption. However, oxygen vacancy pairs affecting the irradiation hardness of calcium fluoride crystals still appear during the vacuum growth, so that the invention fills a fluorine-containing atmosphere such as a mixed gas of carbon tetrafluoride and argon gas in an annealing step to improve the oxygen vacancy pairs appearing during the crystal growth. At the same time, there is another important object of the fluorine-containing atmosphere here, namely to reduce fluorine vacancies in calcium fluoride crystals under an atmosphere with an excess of fluorine.
The prior art adds PbF from the point of crystal growth 2 Or polytetrafluoroethylene is used as an oxygen scavenger because a certain partial pressure of oxygen may be generated during the crystal growth even under vacuum, and in addition, some impurity elements in calcium fluoride exist in the form of oxides, so that it is a relatively common means to add an oxygen scavenger during the growth of fluoride crystals from the viewpoint of production. Based on the analysis of laser irradiation damage experimental results, three key factors influencing the irradiation damage resistance of the calcium fluoride crystal are provided: the 5d orbital related critical impurity ion content, fluorine vacancies, and lattice oxygen. The invention provides a basis for controlling oxygen to be an analysis of a damage mechanism, wherein lattice oxygen refers to oxygen entering a lattice position in a crystal, and can form an oxygen vacancy pair with a vacancy at the lattice, and the vacancy captures electrons to form a core, so that the crystal is damaged. The best method for inhibiting lattice oxygen is to mix oxygen scavenger with raw material, because the calcium fluoride raw material is mostly high-purity powder, because the specific surface area of the powder is larger, and fluorine is the same as hydrogenThe hydrogen bonding effect can be formed, the adsorption effect of the oxygen scavenger tray on small molecular compounds in the air is larger, so that the small molecular compounds like hydroxyl and water molecules adsorbed on the surface of the raw material powder are difficult to exhaust only by adding the oxygen scavenger powder into the oxygen scavenger tray, or oxygen possibly entering a crystal lattice is eliminated by a mixing mode. The invention further improves PbF 2 Oxygen scavenger proportion and increase mixing time to eliminate the existence of lattice oxygen as much as possible. Pb impurities exist only in PbF form in the crystal growth system 2 Or PbO, and the two substances can be volatilized and discharged by setting a temperature control program in the actual crystal growth process. It was also confirmed in practical tests that Pb impurities were substantially discharged, and that there was substantially no effect on the hardness of 193nm laser irradiation. Moreover, the fluorine-containing atmosphere is not used for fluorinating the raw material, but is used in the annealing step of the crystal, and the main purpose is to reduce or eliminate fluorine vacancies.
In conclusion, the three means of reducing the cation content of key impurities, reducing the residual O content and reducing the F vacancy concentration in the crystal are used together, so that the formation of color centers can be avoided from the source to improve the irradiation resistance of the crystal. In some technical schemes, the total content of impurity ions such as Y, la, ce, gd, tb, lu is controlled to be lower than 1ppm through raw material treatment and growth process control, 0.3-4wt% of deoxidizer is added, and when 3-25% of fluorine-containing atmosphere is introduced, the irradiation damage resistance of calcium fluoride crystals at 193nm is obviously improved, and no damage condition exists after seven-millions of pulses are irradiated.
Orientation and processing of the grown crystals to obtain(111) plane calcium fluoride crystals. The crystal is subjected to ultra-precise polishing, and the roughness is less than 1nm. The MLI-FBG excimer laser is adopted to adjust the energy density, the frequency and the pulse width, and the irradiation damage test at 193nm is carried out on different crystals.
The present invention will be further illustrated by the following examples. It is also to be understood that the following examples are given solely for the purpose of illustration and are not to be construed as limitations upon the scope of the invention, since numerous insubstantial modifications and variations will now occur to those skilled in the art in light of the foregoing disclosure. The specific process parameters and the like described below are also merely examples of suitable ranges, i.e., one skilled in the art can make a suitable selection from the description herein and are not intended to be limited to the specific values described below.
In the examples described below, calcium fluoride crystals were prepared with different impurity levels, different O levels and different F vacancy concentrations, respectively. And growing the crystal by adopting a crucible descent method. The impurity content is mainly regulated by different raw material purities. The different O content is regulated 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) are commercially available. Sodium pyridine-2, 6-dicarboxylate, berle Bio-Rad AG1-X4 anion exchange resins were also commercially available. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu of the calcium fluoride raw material is 30.124ppm.
CaF growth by Bridgman method 2 And (3) single crystals. The selected crucible material is graphite crucible, the bottom of the crucible is placed in the direction of the normal line of the directional end face of the X-ray diffractometer to be [111 ]]CaF of (F) 2 And (3) a single crystal rod, and performing crystal growth in a high vacuum atmosphere. The method comprises the following steps:
and (3) batching: using pharmaceutical raw material of grade 5N purity, 0.5wt.% PbF was incorporated 2 As an oxygen scavenger;
mixing: putting the weighed raw materials into a mixing bucket, and stirring for 24 hours on a mixer to uniformly mix the raw materials;
crystal growth: after the charging is finished, vacuumizing to be more than 5 multiplied by 10 -3 Pa, heating the material, firstly heating to 200 ℃, and preserving heat for 10 hours to remove water and air in the raw materials; continuously heating to 800 ℃ at 20-50 ℃/h, and preserving heat for 15h; and then heating and raising the temperature to 1400-1450 ℃ continuously, preserving the heat and melting the materials for 10h, and slowly lowering the crucible to grow crystals after the melting is finished. The dropping speed is 0.1-0.5 mm/h, and the crystal is cooled to room temperature at the speed of 20-50 ℃/h after the crystal growth is completed.
Example 2
The calcium fluoride raw material is purified by a chemical precipitation method. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu of the purified calcium fluoride raw material is 8.623ppm.
The method is characterized in that a direct precipitation method is adopted to synthesize high-purity calcium fluoride raw materials required by crystal growth, reaction raw materials are analytically pure calcium nitrate and potassium fluoride, and in order to remove key impurity cations in the raw materials, calcium nitrate and potassium fluoride solution are purified at first. 150g of calcium nitrate was prepared as a 0.15g/mL calcium nitrate solution, 1000mL sodium pyridine-2, 6-dicarboxylate was added, the pH was adjusted to between 1.6 and 2 by adding nitric acid, and then the mixture was pumped into a column containing 500mL Berle Bio-Rad AG1-X4 anion exchange resin at a rate of 2 to 3mL/min, a once purified starting material was obtained after passing through the column, and the above purification steps were repeated three times to obtain a purified starting material. In this process, pyridine-2, 6 dicarboxylic acid forms an anionic complex with impurity cations, and the complex is adsorbed on an anion exchange resin when the mixed solution flows through a chromatographic column, thereby achieving the purpose of purification. The potassium fluoride adopts similar steps to remove impurities: 150g of potassium fluoride was prepared as a potassium fluoride solution of 0.15g/mL, 1000mL of sodium pyridine-2, 6-dicarboxylate was added, the pH was adjusted to between 1.6 and 2 by adding nitric acid, and then the mixture was pumped into a column containing 500mL of Berle Bio-Rad AG1-X4 anion exchange resin at a rate of 2 to 3mL/min, a once purified starting material was obtained after passing through the column, and the above purification steps were repeated three times to obtain a purified starting material.
Directly introducing purified potassium fluoride into purified calcium nitrate solution for reaction, aging for 3 hours after the reaction is complete, obtaining calcium fluoride gel through high-speed centrifugation, and removing K adsorbed by the calcium fluoride gel through ultrasonic dispersion + And (3) repeating the centrifugation and washing processes for 3 times, and freeze-drying and grinding to obtain the high-purity calcium fluoride raw material. The prepared CaF can also be treated in a high vacuum furnace 2 Deoxidizing and fluoridation to obtain powder material with purity higher than 99.9999% (6N level), reaction temperature of 700 deg.c, holding time of 13 hr and fluoridation agent selectionIs CF (CF) 4 。
The crystal growth process was the same as in example 1. The difference is that the chemical precipitation method is adopted to remove the drug raw material after the key cation 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 the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the chemical precipitation method is 8.623ppm.
The calcium fluoride raw material purified by the chemical precipitation method is further purified by a zone melting method. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the zone melting method is 0.801ppm. The specific operation of zone-melting purification is to adopt a crucible descent method to carry out crystal growth, after the growth is finished, taking an upper isodiametric (two thirds of the equal diametric position of the crystal is taken upwards) crystal as a raw material for secondary purification, and after the secondary growth, taking an upper grade (two thirds of the crystal is taken upwards in the equal diametric direction of the crystal) crystal as a raw material for final crystal growth.
The crystal growth process was the same as in example 1. The difference is that chemical precipitation and zone melting are adopted to remove the key cationic impurities.
Example 4
The calcium fluoride raw material was purified by chemical precipitation in the same manner as in example 2. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the chemical precipitation method is 8.623ppm.
The calcium fluoride raw material purified by the chemical precipitation method was further purified by the zone melting method in the same manner as in example 3. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the zone melting method is 0.801ppm.
The crystal growth process was substantially the same as in example 1, except that the oxygen scavenger content was different. CaF growth by Bridgman method 2 And (3) single crystals. The selected crucible material is graphite crucible, the bottom of the crucible is placed in the direction of the normal line of the directional end face of the X-ray diffractometer to be [111 ]]CaF of (F) 2 And (3) a single crystal rod, and performing crystal growth in a high vacuum atmosphere. In particular to：
And (3) batching: 2.5wt.% PbF is mixed into the purified drug material by chemical precipitation and zone melting 2 As an oxygen scavenger;
mixing: putting the weighed raw materials into a mixing bucket, and stirring for 24 hours on a mixer to uniformly mix the raw materials;
crystal growth: after the charging is finished, vacuumizing to be more than 5 multiplied by 10 -3 Pa, heating the material, firstly heating to 200 ℃, and preserving heat for 10 hours to remove water and air in the raw materials; continuously heating to 800 ℃ at 20-50 ℃/h, and preserving heat for 15h; and then heating and raising the temperature to 1400-1450 ℃ continuously, preserving the heat and melting the materials for 10h, and slowly lowering the crucible to grow crystals after the melting is finished. The dropping speed is 0.1-0.5 mm/h, and the crystal is cooled to room temperature at the speed of 20-50 ℃/h after the crystal growth is completed.
Example 5
The calcium fluoride raw material was purified by chemical precipitation in the same manner as in example 2. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the chemical precipitation method is 8.623ppm.
The calcium fluoride raw material purified by the chemical precipitation method was further purified by the zone melting method in the same manner as in example 3. The total content of the cations of key impurities such as Y, la, ce, gd, tb, lu in the calcium fluoride raw material purified by the zone melting method is 0.801ppm.
The crystal growth process was substantially the same as in example 1, except that the oxygen scavenger content was different and annealing was performed under a fluorine-containing atmosphere. The growth method of the single crystal is a crucible lowering method. The selected crucible material is graphite crucible, the bottom of the crucible is placed in the direction of the normal line of the directional end face of the X-ray diffractometer to be [111 ]]CaF of (F) 2 Single crystal bar, crystal in CF 4 And growing in a mixed atmosphere with Ar. CF in mixed atmosphere 4 Is 40% by volume. The method comprises the following steps:
and (3) batching: 2.5wt.% PbF is mixed into the purified drug material by chemical precipitation and zone melting 2 As an oxygen scavenger;
mixing: putting the weighed raw materials into a mixing bucket, and stirring for 24 hours on a mixer to uniformly mix the raw materials;
crystal growth and atmosphere annealing: after the charging is finished, vacuumizing to be more than 5 multiplied by 10 -3 Pa, heating the material, firstly heating to 200 ℃, and preserving heat for 10 hours to remove water and air in the raw materials; continuously heating to 800 ℃ at 20-50 ℃/h, and preserving heat for 15h; and then heating to 1400-1450 ℃ continuously, preserving heat and melting for 10h, and slowly lowering the crucible to grow crystals after melting is finished. The dropping speed is 0.1-0.5 mm/h, and the crystal is cooled to room temperature at the speed of 20-50 ℃/h after the crystal growth is completed. After standing for 5 hours, filling mixed gas of carbon tetrafluoride and argon with the volume fraction of 15 percent for annealing, heating to 900-1100 ℃ at the heating rate of 20-50 ℃/h, then keeping the temperature for 20-60 hours, and then cooling to room temperature at the cooling rate of less than 20 ℃/h.
The calcium fluoride crystals prepared in examples 1-5 were labeled as sample # 1, sample # 2, sample # 3, sample # 4, sample # 5, respectively.
TABLE 1 oxygen content test results
The test results in table 1 show that oxygen incorporation into the crystal can be reduced to some extent by increasing the amount of oxygen scavenger used.
FIG. 1 is a photograph showing the crystals of calcium fluoride prepared in example 1 before and after 193nm irradiation. The crystal used for irradiation is(111) plane calcium fluoride crystals. The crystal is colorless and transparent before irradiation, and obvious black damage appears in the damaged area of the crystal surface after irradiation.
FIG. 2 shows the absorption spectra of the sample of example 1 before and after 193nm irradiation. The crystal has no obvious absorption peak before irradiation, and the absorption spectrum of the crystal after irradiation mainly shows four absorption peaks, which are respectively 326nm (V k Heart or H-heart); 378nm (F core); 480nm (oxygen dependent); 600nm (related to F core aggregates), which are mainly caused by the absorption level formed by hybridization of the d-orbitals of impurity ions with the 1 s-orbitals and 2 p-orbitals of the F core.
FIG. 3 shows an embodiment 1 and an embodiment3 EPR profile of the irradiated sample. The EPR spectrum of the example 1 crystal after irradiation shows six peaks of near-isointensity, mainly caused by the hyperfine interaction of the single impurity cation with d-orbital activity with F-core, indicating that the color center appears in the example 1 crystal to be mainly dominated by the impurity cation. Whereas the EPR spectrum of example 3 shows an intensity proportional to (1+x) after the crystal irradiation 6 Seven lines of binomial expansion coefficients of (c), which are mainly caused by the hyperfine interactions between F atoms and F centers in calcium fluoride crystals, illustrate that the color centers in example 3 are mainly caused by vacancies existing in the crystals.
FIG. 4 is a bicrystal rocking curve of calcium fluoride crystals prepared in examples 1-5. As is clear from the graph, when the impurity ion content in the crystal is large, the approaching performance of the crystal is affected, wherein the crystallization performance of example 1 is worst, the half-width is 0.213 degrees, and when the impurity ion content in the crystal is reduced, the crystal can maintain a good crystallization performance, and the half-width is about 0.010 degrees.
FIG. 5 is a graph showing the damage threshold of calcium fluoride crystals prepared in examples 1 to 5. It has been found that the impurity cation content with d-orbital activity in the crystal is reduced by chemical precipitation and zone melting; and the irradiation hardness of the crystal is improved to a certain extent by improving the content of the deoxidizer and filling the fluorine-containing atmosphere to reduce the number of hollow bits in the crystal. Wherein, examples 1-4 show damage in irradiation experiments, and example 5 shows that the crystal has stronger irradiation hardness after being treated by the three combined processes, wherein the damage does not exist after the irradiation of seventy-ten million pulses.
Claims (8)
1. A method for improving 193nm laser irradiation hardness of calcium fluoride crystal, characterized in that the method improves irradiation hardness of calcium fluoride crystal by controlling impurity cation content of low-order s-orbitals and d-orbitals overlapping with F-heart wave function in calcium fluoride raw material to avoid formation of F-center, H-center and M-center in calcium fluoride crystal, reducing fluorine vacancy concentration in calcium fluoride crystal and reducing oxygen concentration in calcium fluoride crystal in combination; the method comprises the following steps: purifying the calcium fluoride raw material to ensure that the impurity cation content of the low-order s orbit and d orbit which are overlapped with the F heart wave function in the calcium fluoride raw material is less than 1ppm, adopting the purified calcium fluoride raw material and an deoxidizer as initial raw materials, growing calcium fluoride crystals by a crucible descent method or a temperature gradient method, and annealing in a fluorine-containing atmosphere to obtain the calcium fluoride crystals with improved laser irradiation hardness; the impurity cations having lower s-orbitals and d-orbitals overlapping the F-heart function include Y, la, ce, gd, tb, lu.
2. The method according to claim 1, wherein the calcium fluoride raw material is purified by chemical precipitation and/or zone melting to have an impurity cation content of 1ppm or less in the calcium fluoride raw material having lower s-orbitals and d-orbitals overlapping with the F-heart wave function.
3. The method according to claim 2, wherein the calcium fluoride raw material is purified by chemical precipitation method to make the impurity cation content of the low-order s orbit and d orbit which are overlapped with the F-heart wave function in the calcium fluoride raw material be 1-10ppm, and then the calcium fluoride raw material is further purified by zone melting method to make the impurity cation content of the low-order s orbit and d orbit which are overlapped with the F-heart wave function in the calcium fluoride raw material be <1ppm.
4. The method of claim 1, wherein the oxygen scavenger is lead fluoride and/or polytetrafluoroethylene.
5. The method of claim 1, wherein the oxygen scavenger comprises 0.3-4 wt.% of the calcium fluoride source material.
6. The method of claim 1, wherein the fluorine-containing atmosphere is CF 4 Gas, or HF gas, or CF 4 A mixed gas of a gas and an inert gas, or a mixed gas of an HF gas and an inert gas, or CF 4 A mixed gas of a gas and an HF gas and an inert gas.
7. The method according to claim 6, wherein the CF in the mixed gas 4 The volume fraction of the gas is 3-25%.
8. The method according to claim 6, wherein the inert gas is Ar and/or N 2 。
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