ION-INDUCED PROCESSES IN OPTICAL COATINGS (BOMBARDMENT, THIN FILMS).

Abstract

Nearly all the deficiencies of conventional vacuum evaporated coatings trace to a single physical property of condensed films: low packing density. One way to increase packing density is to bombard the growing film with ions during deposition, called ion-assisted deposition (IAD). The beginning chapters of this dissertation analyze IAD as a perturbation of the conventional vacuum evaporation process. The experimental chapters begin with an examination of the effect on moisture penetration behavior of oxygen-ion bombarding completed optical filters. Moisture adsorption and desorption is retarded after bombardment in filters composed of titania and silica, but not in those of zirconia and silica. Bombardment evidently induces a crystalline-to-amorphous transition in titania, causing the surface to swell and occluding the pores. The transition in zirconia is the reverse, and no impediment to moisture appears. Argon-ion-assisted magnesium fluoride (MgF₂) can show ultraviolet (UV) absorption. The primary mechanism is probably the formation of F-centers (single fluorine-ion vacancies), although an unsaturated oxygen bond may also be responsible. Absorption can be removed by baking and often by irradiation with UV. After baking, fluorine is lost and replaced by oxygen. Absorption-free MgF₂ films can be deposited by minimizing the substrate temperature and bombardment flux. Ion-assisted films contain up to 2% argon and up to 170 parts-per-million of tungsten from the ion gun filaments. They show a slightly higher refractive index, are much less porous, and are much more resistant to damage by abrasion and exposure to fluorine gas. Ion-assisted aluminum oxide (alumina, Al₂O₃) films show a small increase in UV absorption after argon-ion bombardment; however, a mixture of argon and oxygen ions avoids the problem. Excess oxygen is often incorporated into alumina films, and depresses both the mass density and the refractive index. IAD increases refractive index and decreases porosity. Ion-assisted alumina films are somewhat more stable in humid environments. Ion-assisted deposition has been shown by this study to cause substantial improvements in many of the physical and some of the optical and chemical properties of evaporated magnesium fluoride and aluminum oxide films.