Toshiro Mabuchi

High-index fluoride materials for 193-nm immersion lithography

BaLiF3 single crystal has been studied as the lens material for the candidate of the next generation high index immersion lithography system. Although the refractive index of BaLiF3 is 1.64 at 193nm which is not sufficient for the requirement, other optical properties such as 193nm transparency and laser durability might fulfill the requirement, and intrinsic birefringence is relatively lower than other candidate materials. It is estimated that the cause of scattering in the BaLiF3 crystal is aggregation of excess LiF component. The special annealing process to eliminate excess LiF component was applied to improve the transparency. The internal transparency was improved to more than 97%/cm by optimizing growth conditions and annealing conditions.

Birefringence simulations of annealed ingot of calcium fluoride single crystal by considering creep behavior of ingot during annealing process

We developed an analysis system for simulating birefringence of an annealed ingot of CaF2 single crystal caused by the residual stress after annealing process. In the residual stress calculation, we can select either the elastic thermal stress analysis using a stress-free temperature or more exact stress analysis considering creep deformation. When we use the residual stress calculated from the creep deformation analysis of a CaF2 ingot, we can obtain reasonable results in comparison with the experimental results.

Properties of ultra-large CaF2 crystals for the high NA optics

CaF2(Calcium fluoride) lens materials are required for the most important component of the ArF laser lithography stepper for the reason of its excellent transparency in DUV region and excellent laser durability as compared with quartz. Last year we reported to succeed in growing both <111> and <100> crystals, which had 210m diameter and 150mm length by the Czochralski (CZ) method with high productivity1). The obtained CaF2 crystal had low stress birefringence by way of the optimized annealing process. Although, for the higher NA system, larger CaF2 single crystals are required for the lens materials having the high quality and high productivity. To meet the above mentioned demands, we made efforts to produce and succeeded the first in the world ultra-large (φ300mm over) and high quality CaF2 single crystal by the CZ method with high productivity. The crystal had very low stress birefringence, good transparency in DUV region and good homogeneity. It was also easy to control the orientation of the crystal by the selection of seed crystal.

Fluoride single crystals for the next generation lithography

BaLiF3 single crystal has been studied as the candidate for the last lens material of the next generation high index immersion lithography system. Although the refractive index of BaLiF3 is 1.64 at 193nm which is not sufficient for the requirement, other optical properties such as 193nm transparency and laser durability fulfill the requirement. It is estimated that the cause of both high SBR part and inhomogeneity of refractive index of BaLiF3 seems to present along the faces of slip planes which are observed by crossed Nicol observation. As a result of comparative study of various direction perpendiculars to the growth axis, good crystallinity with less slip planes has been obtained by shifting the growth axis from <100> which is adequate for the last lens production. MgF2 single crystal studied as the polarizer material for high power ArF laser oscillator, and crystal with excellent laser durability and large diameter (>100mm) has been developed by CZ technique. In addition crystals oriented along both c-axis and a-axis were successfully grown.

Birefringence Simulation of Annealed Ingot of Calcium Fluoride Single Crystal

We developed amethod forsimulating birefringence ofan annealed ingot ofcalcium fluoride single crystal caused bythe residual stress after annealing process. Themethod comprises theheatconduction analysis thatprovides thetemperature distribution during theingot annealing, theelastic thermal stress analysis usingtheassumption ofthestress-free temperature that provides theresidual stress after annealing, andthebirefringence analysis ofanannealed ingot induced bytheresidual stress. Thefinite element method wasapplied totheheatconduction analysis andtheelastic thermal stress analysis. Inthese analyses, thetemperature dependence of material properties andthecrystal anisotropy weretaken into account. Inthebirefringence analysis, thephotoelastic effect gives thechange ofrefractive indices, fromwhichtheoptical pathdifference intheannealed ingot iscalculated byJones calculus. Anapproximate method forcalculating theoptical pathdifference using theaverage stress alongthewave normal isalso proposed andtherelation between theJones calculus andtheapproximate method isdiscussed. Itisfound that theresult oftheapproximate method agrees verywell withthat oftheJones calculus. Thedistribution oftheoptical pathdifference intheannealed ingot obtained fromthe present calculation agrees reasonably wellwiththatofthe experiment. Itscalculated value also agrees reasonably well withthat oftheexperiment, whenastress-free temperature is adequately selected.

Birefringence simulations for annealed ingots of the -and - growth calcium fluoride single crystals with consideration of creep deformation

We developed an analysis system for simulating birefringence of an annealed ingot of CaF2 single crystal caused by the residual stress after annealing process. The analysis system comprises the heat conduction analysis that provides the temperature distribution during the ingot annealing, the stress analysis to calculate the residual stress after ingot annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. The finite element method was applied to the heat conduction analysis and the stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the residual stress calculation, we considered the time-dependent nonlinear deformation behavior of a material called creep. In the birefringence analysis, the distributions of optical path difference were calculated by using average stress method with consideration of the crystal anisotropy. We can perform the birefringence analysis of an ingot of CaF2 single crystal with any growth direction, using this analysis system, and we performed the analyses of the crystals with the and growth directions. From these analyses, we obtained reasonable results of optical path difference in comparison with the experimental results.

Birefringence simulations of annealed ingot of calcium fluoride single crystal: consideration of creep behavior of ingot during annealing process

We developed an analysis system for simulating birefringence of an annealed ingot of CaF2 single crystal caused by the residual stress after annealing process. The analysis system comprises the heat conduction analysis that provides the temperature distribution during the ingot annealing, the stress analysis to calculate the residual stress after ingot annealing, and the birefringence analysis of an annealed ingot induced by the residual stress. In the residual stress calculation, we can select either the elastic thermal stress analysis using the assumption of a stress-free temperature or more exact stress analysis considering the time-dependent nonlinear behavior of a material called creep. When we use the residual stress calculated from the creep deformation analysis of a CaF2 ingot, we can obtain reasonable results both for the optical path difference values and for its distributions in comparison with the experimental results.

Factors affecting the transmission and stability in complex fluorides in VUV spectral region

Transmittance and radiation induced absorbance in VUV-UV-visible spectral region were measured in several binary and complex fluoride single crystals at room temperature. Influence of the intentional doping and material stochiometry is demonstrated. X-ray induced coloration and degradation of transmittance characteristics are observed and discussed in terms of creation of various electron (F-like) and hole (VK- and H-like) centers and in terms of near band-edge transitions arising due to imperfect periodicity of the lattice in a general sense. It is shown that VUV characteristics cannot be derived or predicted from those observed in UV-visible spectral region.

Birefringence Simulation of Annealed Ingot of Magnesium Fluoride Single Crystal

We developed a method for simulating birefringence of an annealed ingot of MgF2 single crystal for lithography optics. This method provides the optical path difference caused by crystal symmetry and residual stress existing in the crystal. The method consists of the residual stress analysis and the birefringence analysis. The residual stres analysis comprises the heat conduction analysis and the elastic thermal stress analysis. The heat conduction analysis provides the temperature distribution during ingot annealing and the elastic thermal stress analysis provides the residual stress after annealing by using the result of the heat conduction analysis. Because there exists no experimental data on the inelastic behavior of MgF2 single crystal, the residual stress was estimated with the elastic thermal stress analysis by assuming a stress-free temperature, by which the inelastic behavior could be treated approximately. The finite element method was applied to both the heat conduction analysis and the elastic thermal stress analysis. In these analyses, the temperature dependence of material properties and the crystal anisotropy were taken into account. In the birefringence analysis, the distributions of optical path difference were calculated by an approximate method using the result of the residual stress analysis. This approximate method uses the average stress along the wave normal and is equivalent to the exact method in case of low stress dealt with the present study. In this analysis, it is possible to consider both the intrinsic birefringence and the stress birefringence in any crystal orientation.

Structure and optical property of large size CaF2 single crystals grown by the CZ method

CaF2 single crystals are required for the most suitable lens materials for the ArF laser lithography stepper because of its excellent transparency and laser durability in the DUV region. We have succeeded in growing the large size and high quality CaF2 single crystals with both <111> and <100> orientation, and the diameter of 300mm by means of the Czochralski (CZ) method. The refractive index homogeneity is one of the most important optical properties required for the lithography lens material. Particularly, the residual homogeneity which is a high-order refractive index distribution after subtraction of Zernike 36 coefficient is very important. The vein-like striations were observed in the residual homogeneity pattern of the CaF2 single crystals. The structure of CaF2 single crystal correlating with the residual homogeneity is characterized by using the reflection X-ray topography. It is observed that the structure of CaF2 single crystal composed with sub-grains parted by the small-angle tilt boundaries. The sub-grains are grown along the growth direction. It is understood that the large angle tilt type sub-boundaries among the domains composed of small sub-grains correspond to the vein-like striations. In the growth of CaF2 single crystal, the sub-grain structure of seed is introduced into grown crystal. While the bending of crystal lattice plane from the neck toward the shoulder lead to the large angle tilt type sub-boundaries. Use of high crystalline seed and control of crystal lattice plane in the shoulder are effective in the growth of high crystalline CaF2 single crystal getting good residual homogeneity.