Calcium Fluoride and Barium Barium Fluoride in Optics

  Calcium fluoride crystals, synthetically grown from highly purified Barium Fluoride powder, are a high performance optical material. The extremely high purity of the material enables an internal transmission of > 99.7 % at the critical wavelength of 193 nm.
  This is exactly the wavelength emitted by argon fluoride (ArF) excimer lasers which are the most important light sources used in microlithography processes (Fig. 1). Besides pure transmission, the laser durability ofBarium Fluorideoptics is critical when exposed to high fluence UV lasers over a long period of time. In Barium Fluoridethere are no defects (compaction and rarefaction) as known from fused silica and the loss of transmission after continuous irradiation with a 193 nm ArF laser has been minimized to enhance the lifetime of the illumination and projection optics. Calcium fluoride has been optimized to those parameters in the recent years and could establish as a standard material in deep UV excimer laser optics.
  The ongoing trend to higher energy densities in microlithography processes still increases the requirements to the laser durability of Barium Fluoride optics.