The oxidation of aluminum films in low-pressure oxygen atmospheres

Abstract

The oxidation kinetics of aluminum films exposed to oxygen atmospheres at room temperature in the pressure range of from 10 −3 to 10 Torr are followed by simultaneous measurement of the film's oxygen uptake and work function as a function of time under the conditions of constant pressure. For successive increases in pressure, the experimental data for both the oxygen uptake and work function can be fitted to pressure-dependent, direct logarithmic time laws. On decreasing the oxygen pressure more or less abruptly to the vacuum level of the system, the total amount of oxygen taken up is observed to remain constant while the work function decays in accordance with a direct logarithmic time law. A theoretical treatment shows that the observed experimental behavior of both the oxygen uptake and work function of the oxidized aluminum film can be accounted for by a two-stage reaction. At the temperature and pressures employed in the experiment the first stage of the oxidation process is an activated oxygen chemisorption onto the oxidized surface of the film. The second stage is an activated, place-exchange process between the chemisorbed oxygen and the underlying aluminum atoms to form the oxide. An analysis of the experimental data in terms of the theory yields values for the aluminum-aluminum oxide-oxygen gas system parameters which are consistent with previously published results for this type of oxide film formation on aluminum.