Low-energy ion beamline scattering apparatus for surface science investigations

Abstract

We report on the design, construction, and performance of a high current ͑monolayers/ s͒, mass-filtered ion beamline system for surface scattering studies using inert and reactive species at collision energies below 1500 eV. The system combines a high-density inductively coupled plasma ion source, high-voltage floating beam transport line with magnet mass-filter and neutral stripping, decelerator, and broad based detection capabilities ͑ions and neutrals in both mass and energy͒ for products leaving the target surface. The entire system was designed from the ground up to be a robust platform to study ion-surface interactions from a more global perspective, i.e., high fluxes ͑Ͼ100 A/cm 2 ͒ of a single ion species at low, tunable energy ͑50-1400± 5 eV full width half maximum͒ can be delivered to a grounded target under ultrahigh vacuum conditions. The high current at low energy problem is solved using an accel-decel transport scheme where ions are created at the desired collision energy in the plasma source, extracted and accelerated to high transport energy ͑20 keV to fight space charge repulsion͒, and then decelerated back down to their original creation potential right before impacting the grounded target. Scattered species and those originating from the surface are directly analyzed in energy and mass using a triply pumped, hybrid detector composed of an electron impact ionizer, hemispherical electrostatic sector, and rf/dc quadrupole in series. With such a system, the collision kinematics, charge exchange, and chemistry occurring on the target surface can be separated by fully analyzing the scattered product flux. Key design aspects of the plasma source, beamline, and detection system are emphasized here to highlight how to work around physical limitations associated with high beam flux at low energy, pumping requirements, beam focusing, and scattered product analysis. Operational details of the beamline are discussed from the perspective of available beam current, mass resolution, projectile energy spread, and energy tunability. As well, performance of the overall system is demonstrated through three proof-of-concept examples: ͑1͒ elastic binary collisions at low energy, ͑2͒ core-level charge exchange reactions involving 20 Ne + with Mg/ Al/ Si/ P targets, and ͑3͒ reactive scattering of CF 2 + /CF 3 + off Si. These studies clearly demonstrate why low, tunable incident energy, as well as mass and energy filtering of products leaving the target surface is advantageous and often essential for studies of inelastic energy losses, hard-collision charge exchange, and chemical reactions that occur during ion-surface scattering.

Evidence of Simultaneous Double-Electron Promotion inF+Collisions with Surfaces

“…The measured average exit energy of F is 35-40 eV lower than that predicted by the binary collision approximation (BCA) for a 90 elastic deflection [9]. The simultaneous appearance of these two phenomena above a critical collision energy is a clear indicator of electronic transitions taking place in the hard collision step as the AO's of the colliding partners intermix at small R min .…”

“…Furthermore, the threshold R min calculated for 4f MO promotion in the F-Si quasimolecule agree quite well with the experimental onset of F 2 production (R min 0:5-0:6 A). Experiments were performed in a custombuilt ion beamline scattering apparatus described in detail elsewhere [9]. Ions were produced in a 13.56 MHz rf-driven inductively coupled plasma discharge with a feed of 1:1 CF 4 :O 2 (5 sccm), operating at 2 mTorr and 600 W. Ion energy was adjusted by floating the plasma above ground with dc.…”

“…However, careful inspection of the energy offset in region 1 reveals that straggling losses (a few eV in our case [8]) cannot be solely responsible for the deviation. Identical experiments with Ne projectiles on bare and fluorinated Si (F=Si) show that the surface Si x F y layer (which commonly forms in F etching environments [12]) significantly increases straggling: i.e., Ne off bare Si is perfectly elastic (within 3-5 eV [9]), while Ne and F off F=Si show much larger deviations. This observation can be rationalized on the grounds that chemisorbed fluorine will draw a considerable amount of electron density toward the surface which could likely increase the total electron drag on the projectile.…”

Intramolecular water-splitting reaction in single collisions of water ions with surfaces

“…15,16 The ions were produced in an inductively-coupled plasma reactor, with tunable plasma potential. Ions were extracted, collimated, and launched into the ion beam line, held typically at –15 kV, then underwent magnetic mass filtering.…”

Dynamic Deuterium Enrichment in Cometary Water via Eley–rideal Reactions

“…Laboratory experiments were carried out in an ultra-high vacuum (UHV) ion scattering system, described elsewhere (Gordon & Giapis 2005). Isotopically pure beams of H 2 O + , HDO + , and D 2 O + , with tunable energy and fluxes between 2 and 5 μA, were directed to oxidized Si, highly ordered pyrolytic graphite (HOPG), and polycrystalline Pt surfaces, held at room temperature.…”