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.
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.