Ions Sputtered from Copper

Abstract

Positive ions sputtered from copper under a variety of different bombardment and surface conditions have been mass analyzed and found to include species characteristic of the incident ions (``reflected'' ions), of copper and some of its compounds, and of alkali metal impurities. A model is described which accounts for most of the properties of the ``reflected'' ions, not in terms of reflection but in terms of the amount of gas trapped and released by the surface. The ionizing agent is unknown but is thought to be the bombarding ions (or their secondary electrons) rather than the surface. The Cu⁺ ions, on the other hand, seem to be surface ionized sputtered particles originating from patches of high local work function. Yield data seem to correlate qualitatively with probable changes in work function caused by adsorption of impurities, sputtering, or vacuum annealing. The CuO⁺ peak is believed to be caused by dissociation and ionization of cuprous oxide molecules under the action of the ion bombardment. The rates of formation and disappearance of this peak were studied under various circumstances. An unusually high sputtering rate was found to occur at an argon bombarding energy of 75 ev. Na and K impurities sputtered from and ionized by the copper surface showed some interesting transient effects which can be interpreted in terms of a low sputtering probability compared to the base metal. The relevance of these experiments to sputtering theories, ion pumping, oxidation, cleanup, and surface ionization theory is discussed.