Graduate Thesis Or Dissertation
 

Atomic ejection in the beam sputtering of nickel, by argon ions, in the threshold to 600 ev energy range

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  • Using a low-energy, three stage, differentially pumped ion accelerator, similar to that used by Hagstrom for Auger electron ejection studies, permits, with the application of radiotracer methods, a new approach to the study of low energy sputtering. The experimental approach consisted of pumping down the experimental tube to 10⁻⁹ torr, outgassing the target by electron bombardment, introducing argon into the system, and bombarding a Ni⁶³ target for a time which is short compared to the time required for a monolayer of adsorbed gases to form at the existing residual partial pressure of adsorbable gases. The target consisted of Ni⁶³ plated on a cold-rolled nickel substrate. Heating the target above its recrystallization temperature produced an 80 percent oriented polycrystalline surface which approximated the (100) plane of a nickel single crystal. Mounting the target so that its surface normal made an angle of 30⁰ with respect to the incident beam direction and bombarding with argon ions produced the characteristic pattern for atoms sputtered from the close-packed directions or "chains" which terminate on the surface of a (100) plane. The target was mounted inside a cylindrical collector, slightly behind center. The ion beam impinged on the target through a narrow slit in the collector. Sputtered atoms were collected on a removable molybdenum foil which lined the outer collector cylinder support. After each bombardment, the molybdenum foil was removed, cut into narrow strips, and then the strips were analyzed by placing them under a G-M counter. The tracer technique permitted measuring the relative sputtering yields from individual crystallographic directions with respect to the angle of incidence of the ion beam. The method thus permitted a direct check on the Harrison-Magnuson theory of single-crystal sputtering thresholds. Contrary to their predictions, there was very little dependence of the threshold on angle of ion incidence with respect to a given chain direction. The observed sputtering yield curves flattened out or "saturated" at a much lower energy than those appearing in the literature. The saturation yield proved highly dependent on the angle of ion incidence. It appears, on the basis of the single direction yield curves, that the measurements appearing in the literature represent a superposition of independent yield curves--from <110> and from <100> directions, respectively. Reflection maxima of sputtered nickel atoms, which appeared behind the target on the collector surface, appear to be due to specular reflection from the primary maxima. A plot of the reflection ratios as a function of incident ion energy suggests that the specular reflection arises from resonance scattering from the interaction potential that the nickel atoms encounter at the molybdenum surface.
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