Surface film resistivity of Cu and Cu-alloy crosswires

The surface film resistivity on metals can be determined by means of the contact resistance between crossed rods as a function of applied force. This method has not been in much use since its development by R. Helm, but is especially valuable because it permits determining, in addition to film resistivity, any two among electrical bulk resistivity, Young\´s modulus and hardness. Since incidental surface films on metals are overwhelmingly insulating so that current conduction through them occurs by means of electron tunneling, the thicknesses of such films can be determined via the correlation between film thickness and film resistivity known through R. Helm\´s work. After a discussion of the underlying theory, measurements of the oxidation of copper and a few copper alloys in atmospheric air, at room temperature and at 58 and 85/spl deg/C are reported. In this case, as for "clean" metals in general, the surface film includes an adsorbed moisture layer which is thinned into a double molecular layer at the contact spots. Its contribution to the film resistivity is in accordance with the additional thickness of about 0.5 nm. If it is present alone, the film resistivity is at a minimum 5/spl times/10/sup -13/ W m/sup 2/. Differences in the oxidation rate and the mechanism by which oxidation occurs are revealed by the measurements.