Magnesium fluoride (MgF2) single crystal is expected as the alternative of Quartz for polarizing materials in high
power lithography system. MgF2 is anisotropic crystal and its physical properties are different along each crystal axes.
Therefore it is difficult to make large diameter single crystal by using Bridgman method which is mainly used for growth
of fluoride crystals.
We have been studying on making large diameter and high quality single crystal by using Czochralski (CZ) method
[1,2]. Previously we reported the stable growth of it with 150mm diameter. This time we succeeded to grow the crystal
with over 200mm diameter.
Additionally, by improving the purification process and growth process, we succeeded to reduce 75 percent of the
amount of color center induced by irradiation of ArF laser.
In the semiconductor lithography technology, the polarized illumination system is applied to make the resolution more
microscopic, therefore the polarizer material with excellent durability against the high power ArF laser has been
required. Magnesium fluoride (MgF2) is one of a suitable material because of its laser durability and high transparency
in VUV region. Previously we reported MgF2 single crystal with diameter of 100mm by using the Czochralski (CZ)
method. By optimizing the crystal growth conditions, MgF2 single crystals with over 150mm in diameter have been
stably grown. Also these crystals show good optical properties and crystallinity.
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.
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.
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.
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.
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.
We tried to investigate various kinds of metal fluoride materials which have higher gravity than CaF2 and cubic crystal system, and we found out barium lithium fluoride (BaLiF3) and potassium yttrium fluoride (KY3F10) as candidates for the last lens material. We have developed unique Czochralski (CZ) machines and techniques for the growth of large calcium fluoride single crystals. And we applied these technologies to the growth of fluoride high index materials. We have succeeded to grow the large BaLiF3 single crystal with 120mm in diameter and a KY3F10 single crystal, and measured their basic properties such as refractive index, VUV transmittance, birefringence, and so on. As a result of our basic research, we found out that BaLiF3 single crystal is transparent at VUV region, and the refractive index at 193nm is 1.64, and KY3F10 single crystal has the index of 1.59 at the wavelength of 193nm which is slightly higher than fused silica. We expect that these fluoride high index materials are useful for the last lens material of the next generation immersion lithography.
CaF2 (Calcium fluoride) lens materials are required for the composition of F2 laser lithography stepper. Recently, it is reported that both <111> crystal and <100> crystal are necessary for the lens blanks, which can resolve the intrinsic birefringence of CaF2. Although CaF2 single crystal has been produced by Bridgman method, some problems are pointed out on the optical properties and production yield especially on <100> crystal with large diameter. So it is worried that the amount of the supply of CaF2 will be short when F2 stepper will start on a large scale. To resolve the above mentioned problems, we tried to grow a large CaF2 single crystal by Czochralski (CZ) technique, because we expected that CZ technique could provide higher productivity and higher quality which mean lower residual stress than conventional method. CZ technique can also easily control the growing crystal axis by the selection of seed crystal. Consequently we succeeded in growth both <111> and <100> direction single crystal which has 210mm diameter and 150mm length. The obtained crystal is a single crystal over a whole boule and it shows high transparency in vacuum ultraviolet region. Moreover it shows very low birefringence value and good homogeneity after annealing process. We hope our challenge should prompt the development of F2 lithography.
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.