NIST scientists discover an unexpected optical property important to microchip manufactures. (News Briefs).

Scientists at NIST have determined that calcium fluoride, one of the principal materials used for optics in the ultraviolet spectral region, exhibits an unexpected, intrinsic birefringence in the deep ultraviolet. The presence of the effect has been confirmed both experimentally and theoretically, and its magnitude is sufficiently large that calcium fluoride may cause unacceptably large aberrations in optical components intended for use by chip makers developing advanced 157 nm lithography processes. At a minimum, the industry will need to correct for the birefringence.

Ordinarily, a crystal with a cubic structure such as calcium fluoride does not exhibit birefringence in stress-free material because of its high symmetry. However, when the wavelength of light is only a few hundred times the crystal interatomic spacing, the symmetry-breaking effect of the finite value of the photon wavevector starts to become significant and gives rise to the effect. These findings were reported at two SEMATECH meetings earlier this year. NIST scientists are attempting to nullify the birefringence by using mixed crystals of calcium fluoride and barium fluoride, a substance that exhibits opposite birefringence behavior. For additional information see: http://physics.nist.gov/duvbirefring. [J. H. Burnett, Z. H. Levine, and E. L. Shirley, Intrinsic birefringence in calcium fluoride and barium fluoride, Phys. Rev. B 64, 241102 (2001).]

CONTACTS: John Burnett, [phone]; john. [e-mail] or Zachary Levine, [phone]; [e-mail] or Eric Shirley, (301) 9752349; [e-mail].