Plasma controlled surface texturing of silver foils

Abstract

Several industrial processes involve the use of silver in one form or another, two of which there are in the use of conductive adhesives within the Personnel Computer Boards manufacturing industry and as a catalyst for the epoxidation of ethylene. Both these processes could be improved significantly if the silver surface were altered morphologically. One possible way to improve either of the aforementioned technologies would be to plasma modify the silvers surface. The aim of the work described in this thesis is to produce a high surface area silver using a high frequency rf plasma, with the hope that with further research this technology may be used to modify silver powder for use in conductive adhesives or as a catalyst for the epoxidation of ethylene This thesis describes the use of high frequency continuous wave (CW) oxygen plasmas for the treatment of silver foils, in particular the effects on surface morphology of changes in rf power (15-50 W), gas pressure (0.2-0.8 mbar), exposure time (lmin-2hrs), orientation of the foil in the reactor with respect to the gas flow (perpendicular or parallel) and distance of the foil from the live electrode (0-17cm). Scanning Electron Microscopy was used to observe these morphological changes. Further to observing the surface topographies of the oxides under these various conditions, a brief, but no means complete, study of the structure of the various oxides produced at these various conditions was performed using X-ray powder diffraction. Reduction of these oxidised foils using a hydrogen plasma (CW) was investigated, with particular emphasis being placed on how rf power (15-50W), gas pressure (0.2-0.8 torr) and exposure times (lmin-2hrs) affected the extent of reduction of the oxidised foil (using X-ray Crystallograhy) and its surface morphology (using Scanning Electron Microscopy ).Pulsed oxygen plasma treatments of the silver foils and CW oxygen plasma treatment of earthed silver samples were performed to try and elucidate the mechanism behind any surface modification. From the above investigations it was found that oxygen plasma treatment of silver foils resulted in significant modification of the surfaces, and this depended on the energy density of the plasma. The optimum surface texture (in terms of apparent surface area) was observed at rf powers of 15W, gas pressures of 0.8 torr, exposure times of 30 min, and at distances of 8-9 cm from the live electrode. The oxidised foil could be fully reduced back to silver metal using a hydrogen plasma, with little change in the optimum surface morphology. The pulsing and earthing studies indicated that the formation of the new surface textures was either due to surface diffusion of a charged silver species or was caused by ion bombardment of the silver surface resulting in a facetted surface texture. The oxides observed were Ag0 and Ag(_2)0, but to what extent each was present appeared to depend on the energy density of the plasma.