Ion-assisted processing of optical coatings

Abstract

The use of ion guns in evaporation chambers has opened a number of material processing possibilities. While ion-assisted deposition (IAD) is certainly the most widely known, other applications have also appeared such as ion processing of substrates and ion beam polishing (IBP). The first part of this presentation is devoted to the study of IAD as a means of growing dense layers which are less affected by moisture than those grown by conventional evaporation. We have applied this technique to a number of materials to study its effects on microstructure, chemical composition, and crystallinity. As a result, several major subsequent applications of IAD are starting to emerge. The first is illustrated by the ability of IAD to tailor the chemical composition of compounds in order to tune their properties. In particular, we describe the synthesis by reactive IAD of aluminum oxynitride (AION), a compound with composition-dependent refractive indices which we have successfully used to fabricate rugate filters. A second application we are at present developing is the control of the microstructure and the intrinsic stress. Finally a third application of IAD is the fabrication of nitrides which can be grown by thermal evaporation but only when nitrogen ions are available at the surface of the growing film. Other uses of ion beams are ion beam processing of subtrates and IBP. We present some results regarding the increase in roughness observed in ion bombarded substrates such as germanium, ZnSe, and ZnS. Then, we describe IBP as a means of smoothing surfaces. IBP has been developed to polish rough diamond films produced by microwave-assisted chemical vapor deposition. By achieving r.m.s. roughness values below 5 nm we have opened a range of optical applications for diamond-coated silicon.