Deposition of optical coatings using vapor phase precursor materials

The inclusion of particulates within multiilayer thin films structures has been highlighted as one of the key factors influencing the perfomance of optical coatings at high laser fluences. The problem is especially severe for a number of refractory materials, where the particulates arise from a number of sources in the coating process, but their density is significantly enhanced by the strong electrostatic fields present within most conventional equipment. While advanced techniques such as molecular beam evaporation have demonstrated significant gain in the realization of coatings with high laser damage thresholds, the Knudsen sources generally employed in such equipment do not allow the evaporation of refractory materials, especially oxides. This paper addresses this shortcoming by examining the feasibility of using vapor phase precursors in various activated processes. Currently available techniques are reviewed and some of the obstacles that have to be overcome highlighted. While such techniques can also be susceptible to particulate formation for different reasons to those met in electron beam evaporation, it has been found that under controlled conditions, particularly when critical supersaturation conditions are avoided, it is possible to deposit films which are ostensibly free from inclusions. Moreover, it has been found that it is possible to deposit films at lower temperatures, so avoiding problems associated with thermal stress in conventional processes. The properties of titania films deposited by such a process are described.