Study on the sputter-cleaning processes of Ni by means of Kelvin probe

A new device has been established to study the surface properties of materials under low energy ion irradiation by means of a Kelvin probe. The serious impact of space charging on the probe was observed, and verified unambiguously via a simulation test. The appropriate measures applied to the system function quite well, and suppress the detrimental influence effectively. The preliminary experiment of helium ion irradiation on nickel resulted in a decrease in work function (WF) at the low fluence, followed by an increase with increasing the fluence till the final saturation was achieved. The behavior can be explained using a two-layer surface model, i.e., the desorption of the topmost loosely bound adsorbed layer due to the irradiation decreases the surface dipole moment towards the bulk, in turn the WF decreases; then the native oxide layer on the bulk is sputtered away gradually, which induces the increase in the WF; finally, the saturation is reached as a result of the balance between the sputtering and the adsorption of the species from the residual gases. The desorption/adsorption experiment further verified the validity of the model.