Secondary-ion emission from clean and oxidized aluminum as a function of incident ion mass and energy

Measurements of the intensities of low-energy secondary ions emitted from clean and oxidized polycrystalline aluminum surfaces under (15–275)-KeV ${\mathrm{He}}^{+}$, ${\mathrm{Ne}}^{+}$, ${\mathrm{Ar}}^{+}$, ${\mathrm{Kr}}^{+}$, and ${\mathrm{Xe}}^{+}$ bombardment are reported. An additional measurement of the secondary-ion mass spectrum obtained from a clean Al surface under 40-MeV ${\mathrm{Br}}^{5+}$ bombardment is also presented. By varying both the beam energy and mass over such a large range, we have been able to identify three distinct categories of secondary ions. The first category includes the metallic-ion species emitted from the oxide surface. The intensities of these ions are observed to be proportional to the sputtering yield of aluminum, indicating that they are produced by means which are consistent with several of the mechanisms already proposed to explain oxygen-enhanced secondary-ion emission from metals. The second category includes both singly and multiply charged Al ions from the clean surface as well as multiply charged ions from the oxide surface. Under high-energy bombardment, these ions all appear to be produced by the kinetic mechanism which has been proposed to explain multiply charged ion emission from Al under lower-energy bombardment. The third category of secondary ions identified includes only ${\mathrm{O}}^{+}$ emitted from the oxide surface. This species, although usually a small component of the spectrum, is produced by means wholly unrelated to elastic energy deposition. Its production cannot be explained by any of the proposed models of metallic secondary-ion emission. The possibility that ${\mathrm{O}}^{+}$ is produced by a mechanism similar to that of electron-stimulated desorption is discussed.