Effect of Heating Single Crystal Copper in H2 and N2 on Thin‐Film Oxidation Kinetics

Abstract

The oxidation rate of spectroscopically pure polycrystalline and single crystal copper at 200°C in 1 atm O2 was measured by coulometric reduction of the oxide. Growth of films up to 4000Aå thick follows two‐stage logarithmic kinetics. Pretreatment of filed polycrystalline Cu surfaces in H2 at 450°C for 30 min decreases by 50% the thickness of oxide forming subsequently in O2 at 200°C. Similar pretreatment in N2, on the other hand, has little or no effect. Hydrogen pretreatment of electropolished single crystal Cu decreases oxidation of (100), has no effect on oxidation of (111), and increases oxidation of the (110) face. Pretreatment in N2 at 450°C has no appreciable effect on subsequent oxidation of (100), contrary to H2 pretreatment, and it increases oxidation of both (111) and (110) faces.The oxidation rates of untreated Cu faces decrease in the order in agreement with data of Gwathmey and his collaborators. Hydrogen pretreatment leads to oxidation rates, whatever the original orientation, approaching that of the (111) face; N2 pretreatment leads to rates approaching that of the (100) face. The results appear to be best explained by a rearranged surface or submicroscopic faceting resulting from hydrogen adsorption, the preferred orientation of which approximates the (111) face. Pretreatment in N2, on the other hand, probably because of adsorbed traces of oxygen in gas or metal, favors rearrangement having the approximate (100) orientation. Observed two‐stage logarithmic kinetics for oxidation of rearranged surfaces, and the results by others of low energy electron diffraction, suggest that the first‐formed adsorbed oxygen‐metal film is extremely stable, maintaining its characteristic structure and orientation throughout the oxidation process. It thereby establishes oxidation rates in the thin film region independent of original crystal face. It is concluded that gaseous or vacuum pretreatment of Cu surfaces can exert a large effect on subsequent oxidation behavior.