Theory of plasma chemical transport etching of gold in a chlorine plasma

Abstract

The behavior of the reactive ion etching of gold in a chlorine plasma reported earlier is shown in this paper to be interpretable in terms of a theory of chemical vapor transport, modified to account for the effect of species generated in the plasma and transported across the plasma sheaths to the reacting surfaces. The interpretation of the experiments in terms of such a theory is required because the volatile compound of gold generated at the etched surface must be transported to another surface where it is consumed by a reaction which is the reverse of etching, and gold is deposited. It is shown that the mass transport of the volatile product is not only due to the differences in temperature between the surfaces where the reactions occur, but is also driven by differences in the surface reaction effective free energy. These free energy differences arise from differences in the transport of the plasma-produced species across the plasma sheaths to the reacting surfaces. The dependences of the etch rate on the film area (the “loading”), on the gas flow and on the relative temperatures of the surfaces between which transport occurs also arise from these considerations and explain the salient features of the reactive ion etching of gold in a chlorine plasma.