Graduate Thesis Or Dissertation
 

Atomic ejection in the single crystal sputtering of nickel with 40-600 eV argon ions

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  • A threestage, differentially pumped ion accelerator is used to produce low energy argon ions, which in turn, are used to bombard the surfaces of plane nickel single crystals. Nickel atoms ejected from the crystals are counted using the radioac tive tracing method. From the analysis of the angular distri butions of ejected atoms, the study of low energy single crystal sputtering is made possible. The experimental procedure consisted of pumping down the accelerator to a pressure of 10⁻⁹ torr, outgassing the target by electron bombardment, generating a welldefined and singly ionized argon ion beam, and bombarding the target with ions for a time which is shorter than that required for gases to form a monolayer on the target surface at the existing residual partial pressure of absorbable gases. Three targets, consisting of Ni ⁶³ being electroplated on the surfaces of two (110) and one (111) nickel crystals respec tively, were used in this work. Each target was bombarded with 40-600 eV argon'ions at normal incidence except one of the (110) crystals which was mounted so that its surface normal made an angle of 20° with respect to the incident ion beam. Nickel atoms ejected from the target were collected on a thin molyb denum foil. Immediately after each bombardment, the collector foil was cut into narrow strips, and the residual radioactivity of each strip was analyzed by placing it under a thin endwindow GM counter. The angular distributions of nickel atoms ejected from the crystals were found to peak at directions corresponding to the closepacked crystal directions. The atomic intensity of each peak increased as the ion energy increased while its halfwidth remained about 8.5°. These results indicate that at low energy ion bombardment atoms are ejected intensely around the close packed direction from a single crystal and focused collisions may indeed narrow the distribution of the ejected atoms around this direction. The sputtering yield curves for nickel atoms ejected along the [110] direction from a (110) and a (111) nickel crystals showed that the atomic ejection process was affected by the structure and atomic binding of the crystal surface and by the ion energy. At the same ion energy, the [1107 sputtering yield obtained from a (110) crystal was higher than that from a (111) crystal. For ejecting nickel atoms with low energy argon ions along a closepacked crystallographic direction, the most probable threshold energy for a (110) nickel was found to be 10 eV. For a (111) nickel crystal, the threshold appeared to have a value of about 15 eV. These results indicate that the surface binding energy may be a dominant factor in the atomic ejection process because the binding energy in a (111) nickel crystal is higher than that in a (110) nickel crystal. The experimental thresholds are higher than the threshold values predicted by Harrison Magnuson's. theory. This may indicate that the atomic ejection process is a complicated event. Further investigations, both experimental and theoretical, are desired in order to explain the ejection phenomenon near the threshold and to claim the final value of threshold energy.
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