Schematic of the low-energy ion beamline scattering apparatus showing various sections: (a) ICP plasma source and extraction optics, (b) high-voltage floating region including both steering magnets and decelerator, (c) scattering chamber, and (d) scattered product detector.
Design schematic of the ICP plasma source and ion beam extraction electrodes. Both the reactor and electrode set have cylindrical symmetry. The extraction aperture where the virtual plasma sheath forms is electrically floating with the plasma itself. Also, two versions of the plasma reactor (pyrex and alumina) were used.
Design schematic of the ion beam decelerator, high-voltage beamline electrical break, and target region.
Simplified schematic of the power supply bias scheme for the beamline. All acceleration, focusing, and steering lens supplies float atop the main beamline bias (20–1500 eV) which controls the final impact energy. A separate plasma bias supply is ramped up in direct correspondence with the beamline float bias.
(a) Simplified schematic of the scattered product detector showing differential pumping stages, ionizer, energy and mass filter, and Daly-type ion counting detector. (b) Example ion trajectory calculation for 50 eV at 15 eV pass energy showing various lenses in the scattered product detector (, , , , , , , , and ). All lenses float off the sector retard voltage used to scan the kinetic energy pass band.
Beam currents measured via Faraday cup (0.5 mm inlet diameter) at the target position for a plasma running at 3.5 mT and 500 W. Beamline transport voltage was and the mass slit was set to 3 mm width. and species result from attack of the pyrex plasma reactor used for this experiment. Also, was added to the plasma mix to prevent -like polymer deposition on the reactor walls at high blending ratios.
Current density at the target for: (a) and (b) and ion beams for different final beam energies. In each case, the beamline transport voltage was maintained at , decelerator was tuned to optimize beam current through the 2 mm diam beam flag aperture. A neat Ar plasma and plasma at and 3–5 mTorr were used.
Ion beam energy distributions for measured at the target position using a miniature hemispherical sector energy analyzer with channeltron detector. For each case, the difference between the plasma float potential and the average beam energy (11–12 V) is dictated by the Ne plasma self-potential. Narrow FWHMs over a wide impact energy range can be seen.
Energy scans of and exit channels from scattering off an Ag target at 110 eV and 90°. Neutrals were detected with the ionizer running at 2 mA emission current, 70 eV electron energy, and the capacitor deflector in the first pumping stage set to . In the hyperthermal case, the ion creation potential in the ionizer is above ground, giving the fast neutrals leaving the surface a kinetic energy kick of 15 eV when electron impact ionization occurs. Thus, the single scattered which is neutralized to by the surface should occur at above the SS position shown for the ion peak.
(a) Experimental exit energies for single-scattered and off several targets showing the constant kinematic energy transfer factor. (b) Summary of experimental kinematic factors for collision energies between 50 and 1000 eV, measured from the slope of the vs data. The theoretical prediction from the BCA model for 90° scattering is given by the line. For Al and Si targets, only the data below was used for the -factor determination.
(a) Exit energies of and resulting from single binary collisions of projectiles with polycrystalline Al for 90° lab scattering angle in specular reflection. Elastic scattering behavior for a single collision is indicated with . Error bars on the energy data are partially shown to avoid clutter. (b) Binary collision inelasticities determined from the data in (a). Energy requirements for various electronic excitations of and are shown, along with the theoretically predicted overlap distance of the orbitals of the collision partners (grey area). The collision has been calculated using the Thomas–Fermi–Molière potential.
Charged exit channels leaving an Si(100) surface for: (a) and (b) projectiles. and ion species were not seen in the scattered ion spectrum for any impact energy tested. The total scattered ion intensity has been normalized by the incident beam current and was obtained by integrating the detector counts of each species (quad at fixed mass) obtained during an energy sweep of the hemispherical sector. plasmas running at 1–5 mTorr and 500–700 W were used.
Design and operational specifications for the low-energy ion beamline scattering system.
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