Abstract

Increasing regulatory and environmental concerns trigger a growing demand for replacing ThF₄ with a non-radioactive, low-index infrared (IR) coating material with (at least the same) good evaporation characteristics and durabilty of the deposited films. Naturally, many other fluorides have been investigated^1,2,3 for their suitability as a ThF₄ substitute, and YF₃ has been purported commercially as a good candidate.⁴ Unfortunately, its thermal expansion coefficient does not match well other common IR materials, so that the substrate temperature during deposition is normally kept below 200 °C. Such low substrate temperature causes relatively porous films which adsorb water vapor both during and after deposition.⁵,⁶ The rather loose packing often causes these films to fail severe abrasion tests commonly required for IR coatings.

© 1992 Optical Society of America