An ellipsometric study of the RF sputter oxidation of lead—indium alloys

A simple ellipsometer and an inexpensive 13-MHz sputteretch assembly have been used with a conventional evaporator to make in, situ measurements of the growth of thin oxide layers (≤ 10 nm) on pure lead and pure indium films, and also on a series of alloy films containing up to 46 at % In. Our lead results agree with results of earlier workers. For pure indium, we find that equilibrium oxide thicknesses are typically smaller than for lead, and that equilibrium time constants are longer; moreover, increased RF power leads to increased, rather than decreased, oxide thicknesses. Alloy films show an intermediate behavior in general, but in the range 26-36 at % In the oxide thickness rises to a maximum before falling to its equilibrium value. We agree with results obtained by other methods of oxidation and analysis which show the oxide to have a disproportionately large In2O3content, and, in particular, that there is 100 percent In2O3for all alloys containing more than 35 at % In. We show that the difference between PbO and In2O3as regards the dependence of oxide thickness on RF power can be accounted for by slightly modifying the oxide growth model of Greiner, to make the diffusion length of oxygen ions very slightly dependent on RF voltage. To account for the intermediate behavior, we introduce a simple coupled Greiner model involving separate columns of two oxides, in which oxygen ions can interdiffuse between columns as they find their way to the alloy surface. As the parameters of the model are changed, the growth behavior changes from a simple monotonic growth of thickness with time, through a behavior in which there is an intermediate maximum, and then back to a monotonic growth behavior again.