Energy dependence of the ionization probability of sputtered Cu and Ni

Despite its great sensitivity, the usefulness of secondary ion mass spectrometry (SIMS) for many applications has been limited by an inadequate understanding of the probability of sputtering an atom in an ionized state. To determine this ionization probability for clean Cu and Ni surfaces, I have measured the energy distribution of sputtered neutrals and ions by quadrupole mass filtering and retarding potential analysis using potential modulation differentiation. Analysis of sputtered neutrals was accomplished by electron impact ionization. Because the neutrals outnumber the ions by at least two orders of magnitude, the ratio of sputtered ions to neutrals is an accurate measure of the ionization probability. For energies below 20 eV the dependence of the ionization probability on energy goes as P(E) α Eⁿ, where n = 0.65 for clean Cu. The absorption of oxygen on the Cu surface increases the total ion yield while causing a reduction in the value of the exponent n. Similar results are found for nickel, where n = 0.54 for the clean surface.