Ion Assisted Deposition (IAD) of YF3 Films on Ge and ZnSe Substrates

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 investigated1,2,3 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.