Techniques for the sputtering of optimum indium-tin oxide films on to room-temperature substrates

Abstract

New ion-assisted, successive-layer plasma anodisation processes are described for the preparation of indium-tin oxide films. They use unbalanced planar magnetron sputtering to provide controlled argon-ion bombardment of the growing film, and also use room-temperature substrates. In both processes the magnetron was used both to deposit the metal and to provide the activated reactive atmosphere, either by moving the substrate between different units or by controlled pulsing of the admission of the oxygen to change the function of the magnetron. These processes have been used to determine the optimum stoichiometry through control of the amount of oxygen incorporated into the metal oxide, and the optimum doping of the indium oxide with tin to produce films with the highest electrical conductivity and visible transparency. The experiments confirm results which were obtained with continuous deposition processes, which demonstrated that the addition of any tin will result in a loss of performance as compared to that achieved with control of the stoichiometry. The continuous processes used both oxide and metal targets with the supply of oxygen being carefully controlled, when necessary, from process parameters. We generally used plasma-emission monitoring (PEM) of the radiation emitted by the sputtered indium, when excited by the plasma, to indicate the partial pressure of oxygen and the state of the target. The continuous sputtering of a compacted oxide target was the least critical of the processes: only a small amount of flow-controlled oxygen had to be added to the sputtering atmosphere to give adequate electrically conducting and visibly transmitting films. Better films could be made more quickly reactively from metal targets, but were much more critical in the control of the conditions which they required. The use of successive ion-assisted plasma anodisation techniques made this a much less critical process, while in all cases the limiting resistivity was found to be around 4×10 6Ω·m. This was obtained via control of the stoichiometry through the oxygen content of an indium film. The role of tin was to make the achievement of optimum films more easily attainable.