Determination of the stoichiometry and the Yb²⁺/Yb³⁺ ratio in YbF _x optical IAD films by RBS and in situ XPS analysis

Abstract

Thin films of YbF₃ are interesting as a possible component in antireflection coatings matching the CO₂ laser radiation. Ion assisted deposition (IAD) techniques for producing high density films were applied to this material and proved to be most successful in optimising the figures of merit for this purpose (low absorption values, high damage thresholds). Our XPS and RBS measurements, however, reveal a deficiency of fluorine in these IAD films that becomes more pronounced with increasing ion bombardment. The deficiency correlates with an increasing Yb²⁺/Yb³⁺ ratio determined by XPS. It can be concluded that the fluorine deficiency is well compensated by a valence state transition of the Yb ion and the formation of a corresponding amount of YbF₂ in the film. This process appears to be essential for avoiding metallic precipitation and achieving good optical film properties. By applying an in situ XPS technique, results have been obtained that are representative for the bulk of the films as has been confirmed by RBS.