PREPARATION AND CHARACTERIZATION OF OXIDIZED SILVER THIN-FILMS

Silver reacts readily with atomic oxygen, which is present in oxygen p lasmas and in low earth orbit. To study the oxidation process, silver films were deposited by r.f. sputtering or by thermal evaporation, the n exposed to an oxygen plasma from an electron cyclotron resonance (EC R) source. In-situ spectroscopic ellipsometry (SE) was used to monitor deposition and oxidation, and determine final thicknesses and optical constants. SE indicated that oxidation began at the surface of the si lver and proceeded downward, with a rough interface which increased st eadily in thickness. Oxide films were nearly transparent over the visi ble spectrum, where the refractive index was above 2, and were strongl y absorbing below 400 nm. Completely oxidized films were twice as thic k as the original silver films. They appeared smooth to the eye, and w ere relatively stable in ambient air. Films that were not oxidized all the way through were much less stable in air, changing interference c olor and appearing rough within a few days. Oxide films deposited by r eactive sputtering of silver in an O-2 background had higher refractiv e index (>2.5) than the ECR oxidized silver films. They were also rela tively stable in air, unless deposited onto silver, in which case the samples changed color and appeared rough within a few days, similar to the partially oxidized silver films.