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Study of wavelength-dependent birefringence in CaF₂ single crystals

  • Journal of the Korean Crystal Growth and Crystal Technology
  • Abbr : J. Korean Cryst. Growth Cryst. Technol.
  • 2024, 34(6), pp.195-201
  • DOI : 10.6111/JKCGCT.2024.34.6.195
  • Publisher : The Korea Association Of Crystal Growth, Inc.
  • Research Area : Engineering > Materials Science and Engineering
  • Received : September 12, 2024
  • Accepted : October 4, 2024
  • Published : December 31, 2024

Won-Bae Sohn 1 Jeon Su Jong 1 KangJuneHyuk 1 Doo Gun Kim 1 Jin Hyeok Kim 1 Seon Hoon Kim 1

1한국광기술원

Accredited

ABSTRACT

CaF2 (calcium fluoride) single crystal is in increasing demand as an objective lens for semiconductor lithography equipment with deep ultraviolet (DUV) light source. High crystallinity, excellent and homogeneous optical properties, and low birefringence value are required to be used as a material for UV lens. In this study, <111> CaF2 single crystals were grown via czochralski growing method. Growth direction, single crystallinity, high purity and high optical transmittance of CaF2 crystal were confirmed. In addition, the crystal shows average refractive index of 1.437124, 1.433864, and 1.43248 according to 486.1 nm, 587.6 nm, and 656.3 nm wavelengths, respectively. Calculated uniformity of refractive index was 17.4 ppm, 10.5 ppm, and 10.5 ppm, which shows excellent optical homogeneity. Moreover, residual stress was analyzed through comparing birefringence at 193 nm and 570 nm wavelength. CaF2 single crystal shows average birefringence of 1.021 nm/cm under 193 nm light source, while 0.788 nm/cm at 570 nm source. Considerable changes of birefringence pattern according to light source wavelength were not observed. Conclusively, we confirmed that our as-grown CaF2 single crystal has remarkable optical and structural properties such as uniform refractive index and low birefringence.

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