193_nm激光下不同含量...2晶体辐照损伤研究（英文）_王华进.pdf

第 38 卷 第 2 期 无 机 材 料 学 报 Vol.38 No.2 2023 年 2 月 Journal of Inorganic Materials Feb.,2023 Received date:2022-05-27;Revised date:2022-06-04;Published online:2022-08-26 Foundation item:Strategic Priority Program of the Chinese Academy of Sciences(XDA25020313);National Natural Science Foundation of China(61925508);Science and Technology Commission of Shanghai Municipality(20501110300,20511107400);CAS Project for Young Scientists in Basic Research(YSBR-024)Biography:WANG Huajin(1996),male,Master candidate.E-mail: 王华进(1996),男,硕士研究生.E-mail: Corresponding author:KOU Huamin,associate professor.E-mail:;SU Liangbi,professor.E-mail: 寇华敏,高级工程师.E-mail:;苏良碧,研究员.E-mail: Article ID:1000-324X(2023)02-0219-06 DOI:10.15541/jim20220300 Irradiation Damage of CaF2 with Different Yttrium Concentrations under 193 nm Laser WANG Huajin1,2,KOU Huamin2,WANG Yongzhe2,JIANG Dapeng2,ZHANG Bo2,QIAN Xiaobo2,WANG Jingya2,ZHU Linling4,ZENG Aijun4,YANG Qiuhong1,SU Liangbi2,3(1.School of Materials Science and Engineering,Shanghai University,Shanghai 200444,China;2.State Key Laboratory of High Performance Ceramics and Superfine Microstructure,Shanghai Institute of Ceramics,Chinese Academy of Sciences,Shanghai 201899,China;3.Center of Materials Science and Optoelectronics Engineering,University of Chinese Academy of Sciences,Beijing 100049,China;4.Shanghai Institute of Optics and Fine Mechanics,Shanghai 201800,China)Abstract:Radiation resistance of CaF2 crystal is one of the critical properties in the application of deep ultraviolet lithography,but the damage process under 193 nm laser irradiation is still unclear.This paper reports the damage behavior of CaF2 crystals under 193 nm laser irradiation and the key defect factors affecting the damage.Through the 193 nm laser irradiation experiment,it is found that the crystal damage is mainly manifested as the radiation-induced color centers inside the crystal and the radiation-induced damage pits on the surface.Irradiation-induced color centers were analyzed by UV-visible spectrophotometer,and linear fitting was performed between absorption coefficients of different color centers and Y impurity contents.The results show that Y ion has a low-order orbit that overlaps with the F center structure wave function,and hybridizes to form a stable structure.There is a linear relationship between Y ions contents and intrinsic color centers of CaF2 crystals,confirming that Y element is the key impurity ion affecting the formation of color centers.Energy dispersive X-ray spectrometer(EDS)results show that the content of calcium in the damage pits increases and the content of fluorine decreases,which confirms that the diffusion of H centers and the aggregation of F centers lead to irradiation damage.Electron backscatter diffraction(EBSD)results show that surface irradiation damage occurs preferentially at dislocations.Therefore,reducing the impurity content and dislocation density is an important way to improve the anti-irradiation damage performance of calcium fluoride crystals under 193 nm laser.Key words:CaF2 crystal;color center;laser irradiation damage;impurity content;dislocation density CaF2 has high transparency in the range from 130 nm to 9 m and is widely used in deep ultraviolet(DUV)to far infrared(IR)optical components.It is even more irreplaceable in applications in the extreme UV edge.The most demanding application for CaF2 is DUV optical microlithography,where CaF2 is a crucial lens material.Although the transmittance of CaF2 crystal meets the 193 nm lithography requirements,even when the laser energy density is below the damage threshold,they cannot avoid the optical damage caused by cumulative ultraviolet radiation.This behavior is known as the incubation effect1-4.To improve the resistance of CaF2 to DUV laser irradiation,investigating the interaction between the DUV laser and CaF2 and clarifying the mechanism of laser damage is very important.After irradiation with a 193 nm laser,high-quality CaF2 still show coloration effects to some extent.The investigation of these coloration effects,the so-called laser damage,has been studied intensively in recent years5-9.The coloration of fluorite(natural CaF2)was studied widely.Smakula,et al10-12 obtained the coloring effect of calcium fluoride by X-ray,-ray,neutron irra-diation,and electron irradiation respectively.According to Mies theory13-15,the coloration effects were due to 220 无 机 材 料 学 报 第 38 卷 the formation of colloidal particles in the mineral.Besides,Przibram16 showed that many coloration effects could be associated with impurity ions,especially rare-earth ions.Bill and coworkers17-18 extensively studied the coloration of natural fluorite and doped CaF2 with many impurity ions.Alig,et al19-20 believed that the orbital hybridization of impurity ions such as Y,La,Ce,Gd,Tb,and Lu with intrinsic defects resulted in color center absorption.Among the impurity ions,the Y ion was the main impurity element,Y is easily incorporated into CaF2 lattice due to very similar ionic radius and chemical properties to Ca.Mizuguchi and coworkers21 have shown that the damage resistance of CaF2 crystals doped with only 5 g/g of Y decreased strongly comparing to pure CaF2 crystals(Y content 1 g/g).All these studies show that impurity ions promote the formation of calcium colloids.However,the growth behavior of calcium colloids in crystals is still unclear.In this study,CaF2 crystals with different Y impurity content were fabricated to study the radiation-induced color centers,and the relationship between Y impurity contents and different color centers was investigated.The formation and growth behavior of calcium colloids were also analyzed combining the EDS,EBSD and morphologies analysis in the damage area.The damage process and mechanism under 193 nm laser irradiation were proposed,which was of great significance to improve the calcium fluoride crystal damage performance.1 Experimental 1.1 Sample preparation and materials properties The CaF2 crystals used in the experiment were grown by vertical Bridgman method.The raw materials were high-purity CaF2 with different Y impurity contents.Appropriate PbF2 were added as an oxygen scavenger.The furnace used was vacuum-tight and the maintained pressure was below 5103 Pa during the entire progress of crystal growth.Crystals were oriented using X-ray diffraction methods and cut perpendicular to the(111)axis.Each sample was cut into a circular piece with 25 mm in diameter and 4 mm in thickness.Samples were both ends polished for optical transmission measure-ments and other experiments.The actual concentration of Y ions in these samples was measured by inductively coupled plasma mass spectroscopy(ICP-MS).UV-VIS-NIR spectrophotometer was used to measure the optical absorption spectra.Scanning electron microscope(SEM,FEI Magellan 400)was used to characterize the structure of subsurface damaged regions.Energy dispersive X-ray spectroscope (EDS,Oxford X-MAX80)and electron backscatter diffraction(EBSD,Oxford NordlysNano),integrated in the SEM were used to characterize elements and structure on top and below the damaged mirror surface to clarify the chemical modification of the bulk material.All measurements mentioned above were carried out at room temperature.Etch pits were obtained by etching the cleaved surface of sample in HCl aqueous solution at 60 for about 10 min.The etched samples were photographed under an optical microscope with a ma-gnification of 63 times to obtain the etched morphology.The morphology and absorption spectrum of the samples before irradiation are shown in Fig.1.The yttrium impurity ion content of sample 1#is 0.47 g/g,and the values for sample 2#and sample 3#are 1.60,5.74 g/g respectively.The content of other impurity cations is less than 0.01 g/g.The samples are numbered as 1#3#according to the Y content.1.2 Laser damage experiment The samples were exposed to UV light with a solid-state pulse power switch from an argon fluoride excimer laser(ArF,193 nm,MLI-FBG).Damage experiments used a square beam profile(3 mm3 mm,106(mJcm2)/pulse).The frequency was fixed to 800 Hz.According to ISO 21254 laser damage test method,the laser damage tests were carried out in the mode S-on-1,and the number of pulses expresses the radiation resistance of crystals.2 Results and discussion 2.1 Irradiation-induced color centers analysis Laser radiation damage is characterized by a loss of transmittance of the material due to the formation of absorption and scattering centers.The radiation damage resistance of the crystal is expressed as the number of pulses at which transmittance loss is observed.The typical morphologies of the laser-induced damage are shown in Fig.2(a),obvious black shading was found in the crystal.Fig.1 Absorption spectra and morphologies of samples before irradiation 第 2 期 WANG Huajin,et al:Irradiation Damage Study of CaF2 with Different Yttrium Concentrations under 193 nm Laser 221 Fig.2 Radiation damage result(a)Damage morphology;(b)Relationship between Y ions and radiation resistance;(c)Damaged region absorption spectra mpps:million pulsesper second The results of the laser damage experiment are shown in Fig.2(b).With the increase of Y ions concentration,the radiation damage resistance of the crystal decreased significantly.The absorption spectra of the samples after irradiation are shown in Fig.2(c).There are four obvious absorption bands located at 326,378,480,and 600 nm,respectively.This spectrum is similar to the absorption spectrum of the color center induced by X-ray and high-energy electron irradiation22-26.The absorption around 326 nm was aroused by the formation of H center(a fluorine atom at an interstitial site covalently bound to a lattice anion),while 378 nm band belongs to the F center absorption(an electron trapped at an anion vacancy).The absorption peak at about 600 nm is considered to be caused by the M center(M centers are two adjacent F centers).The absorption peak at 480 nm is mainly due to the oxygen incorporated into the lattice26.Referring to the color center structure and performance,the loss of crystal transmittance can be summarized as the following process.Generation of electron hole pairs:Irradiation0eh+(1)Generation of Frenkel defects:IrradiationF0FVi+(2)Anion vacancies capture electrons to form F center:IrradiationFFVeVe+(3)Reversible conversion process(F center and M center):ThermalFF22 VeeV+(4)The formation process of color centers are shown in Fig.3(a).Under the 193 nm laser irradiation,electrons were excited to the conduction band through a two-photon absorption process as show in Eq.(1),producing self-trapped excitons(STE).STE is a kind of metastable defect that can release energy and transform into a pair of F(Eq.(2)and Eq.(3)and H centers27.Since the yttrium ions has low-lying s and d valence orbitals which overlap the F-center wave functions.Orbital hybridization as shown in Fig.3(b)occurs21,leads to the emergence of color center absorption peaks.In addition,since the F center has a more robust localized structure and symmetry than the H center,the diffusion energy barrier of the F center is much higher than that of the H center in the CaF2 crystal6.These characteristics lead to a gradual increase of the F center in the irradiated area during the Fig.3 Schematic diagrams of the formation process of color centers(a)F center generated by self-excited defects formed by two-photon absorption;(b)defect-impurity orbital energy levels of the YFC 222 无 机 材 料 学 报 第 38 卷 irradiation process.The F centers gradually gather into M center(Eq.(4)and other F center clusters.The creation of M centers indicates the beginning of the calcium colloid generation.During the long-term irradiation,the F center clusters aggregate to form stable calcium colloids,which is responsible for the black shading shown in Fig.2(a).Calcium colloids results in absorption and scattering,significantly decreasing the crystals transparency.In the process of irradiation damage,impurity ions reduce the F-H pair formation energy and stabilize the F center and H center.This behavior has been confirmed by various experimental data and calculation results3,5,11,13.In order to understand accurately the effect of Y ions on the damage during irradiation,the absorption coefficient was used in this work to assess the damage degree.The linear fitting results of different color center absorption bands are shown in Fig.4.The slopes of the fitted curves at 326 nm(H center)and 378 nm(F center)are extremely similar,indicating that these two color centers increase synchronously with the increase of Y ion content during the irradiation process.H center appeared once F center are generated,which is a typical Frenkel defect(Eq.(2).During the irradiation process,STE directly produces F-H pairs to form H and F centers,and M centers are formed by the aggregation of F centers,so the fitted curve at 600 nm(M center)also has a linear relationship with the Y ion content,but its slope is much smaller than that at 326 and 378 nm.The linear fitting results indicate that Y element is the key impurity in the formation of crystal intrinsic defects(F,H and M centers).No good linear fitting results were obtained at 480 nm,indicating that the color center at 480 nm was not dominated by the content of Y ions.2.2 Damage pits analysis The damage pits shown in Fig.5 was found by observing the damaged area with the electron microscope.Fig.5(a)is the morphology of the damage pits in the irradiation area.Fig.5(b)is the damage pits clustered together after further magnification.The morphology is similar to the melting damage,and there are small granular substances in the damaged area.Fig.5(c)shows a single damage pit,which is triangular in shape,and there are three black lines(marked with white dashed circle)approximately 120 in the middle of the pit.This morphology is consistent with the dislocation etch pits obtained by HCl corrosion in Fig.5(d),but the side length of the damaged pits is only 10 m,which is 1/4 of the etch pits.This phenomenon indicates that the melting damage may occur centered on dislocations.To clarify the distribution of damage craters,the damaged areas of the crystals using electron backscatter diffraction(EBSD)were analyzed.The result is shown in Fig.6.Secondary electron imaging is shown in Fig.6(a),Fig.4 Linear fitting of color center absorption coefficient and Y ion concentration Fig.5 SEM images of damaged area(a)Complete morphology of the damage pits;(b)Aggregate damage pits;(c)Single damage pit;(d)Etch pits of dislocations(HCl etching)Fig.6 Damage morphologies of sample 2#under different modes of SEM(a)Secondary electron image;(b)Stress distribution diagram,blue,green and red regions indicate the variation of stress from low to high,respectively and the triangle shape of the damage pits is consistent with the crystal dislocation etch pit in Fig.5(d).Moreover,the Kernel average misorientation(KAM)distribution was calculated to de