Potential Sputtering of Lithium Fluoride by Slow Multicharged Ions

Abstract

Thin polycrystallineLiF films have been bombarded by slow ((1 keV) multicharged Ar~+ ions (q ( 9), in order to study the resulting total sputter yields by means of a quartz crystal microbalance.More than 99% of sputtered particles are neutral and show yields, at given impact energy, in proportion to the potential energy of projectile ions. The respective "potential sputtering" process already takes place far below 100 eV impact energy. It can be related to defect production in LiF following electron capture by the multicharged ions, and removes about one LiF molecule per 100 eV of projectile potential energy. PACS numbers: 79.20.Rf, 79.20.Nc, 79.90.+b Recently, Neidhart et al.[1] observed total sputter yields of about 0.5 LiF molecule per primary ion, for impact of rather slow ()5 eV) He+, Ne+, and Ar+ ions on polycrystalline lithium fluoride. Since for metal surfaces no sputtering takes place at such low impact energies, this sputtering of LiF has been related to "electronic effects" initiated by electron transfer from the LiF surface into projectile ions. F+ secondary ion yields resulting from impact of slow singly and doubly charged noble gas ions on LiF differ by more than 1 order of magnitude [2], which has been explained by the comparably more efficient Auger neutralization (AN) and/ or resonance neutralization (RN) from the LiF valence band into doubly charged ions. Generally, for impact of slow multicharged ions (MCI) on alkali halide surfaces a rapid increase of the secondary ion yields with projectile charge q has been demonstrated [3]. When etching a KC1 surface previously bombarded with slow Arq+ or Krq+ at given ion fluxes, higher charged ions turn out to cause larger etching patterns [4]. Based on such evidence, Bitenskii, Murakhmetov, and Parilis [5] have proposed a so-called "Coulomb explosion" sputter mechanism for insulators under slow MCI impact. Conceivably, this process should be initiated by the strong negative charge depletion in the uppermost target layers due to the rapid electron capture into incoming MCI, leaving behind positive target ion cores which then may push each other out of the solid. Sputtering of metals is exclusively caused by kinetic energy transfer from projectiles onto the target particles, producing nonthermal velocity distributions of sputtered particles with steeply decreasing yields below typically 500 eV impact energy [6]. Ion induced sputtering from alkali halides results in much slower (thermal) particle velocities, similarly as for electron and photon stimulated desorption (ESD and PSD) [7,8] from alkali halides. Only a small fraction shows comparably high velocities as for sputtering of metals. Most particles from ion induced sputtering of alkali halides are neutral [9], as for ESD and PSD. For the latter processes, recent studies [7,8] explain the observed, rather large neutral desorption yields by efficient creation of defects in the near surface region, which suggests that similar mechanisms may also account here for the interesting ion induced sputtering o...

Threshold for Potential Sputtering of LiF

“…For comparably highly charged ions on GaAs, a model involving structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitations [11] was invoked to explain the observed high sputtering yields [4]. For projectile ions in somewhat lower charge states (q # 27), a large amount of experimental data for various target surfaces (among them alkali halides and SiO 2 ) [1,2,12] are at variance with the Coulomb explosion mechanism [13]. They are, however, consistent with the so-called "defect-mediated desorption" model originally developed for electron-and photon-stimulated desorption [14] for alkali halides.…”

“…Further stepwise increase of the sputtering yield with higher projectile potential energies provides evidence for additional defect-mediated sputtering mechanisms operative in alkali halides. PACS numbers: 79.20.Rf In recent studies on the impact of slow multiply charged ions on insulator surfaces, a dramatic increase of the yields for sputtering [1][2][3][4] and secondary ion emission [5][6][7][8] with projectile charge state has been observed for certain target species as, e.g., LiF and SiO 2 . In contrast to the well established process of kinetically induced sputtering, ablation of target atoms and ions due to the potential energy of the projectile, henceforth called potential sputtering (PS), is largely unexplored.…”

“…Kinetic sputtering was suppressed or at least kept at a minimum by decelerating the projectiles to impact energies as low as 20 eV. Our experimental technique has been described in detail elsewhere [1,2,12,13,17]. In short, total sputter yields (including both neutral and charged secondary particles) for ion-surface collisions are determined with a sensitive quartz-crystal microbalance technique.…”

Kinetically Assisted Potential Sputtering of Insulators by Highly Charged Ions

“…The "Coulomb explosion" model [9,10] has long been favored, but with the exception of proton sputtering from hydrocarbon covered surfaces [8,11] has failed to provide even a semiquantitative interpretation of experimental data [12]. For GaAs a model to explain the observed high sputtering yields [5] was recently suggested, which involves structural instabilities arising from the destabilization of atomic bonds due to a high density of electronic excitation [13] produced during the neutralization and penetration of very highly charged ions with typically 500 keV where the kinetic energy exceeds the available potential energy.For slow medium charge-state projectile ions (q # 27) on alkali halides and SiO 2 the so-called "defect-mediated desorption" model has been most successful [12] in describing the experimental data [1,3,14]. This model requires a target material with strong electron-phonon coupling, where electronic excitations can be localized by forming lattice defects via self-trapping [e.g., "self-trapped excitons" (STE) and "self-trapped holes" (STH)] [15].…”

Slow multicharged ions hitting a solid surface: From hollow atoms to novel applications

“…4). This effect has been termed potential sputtering [10,11], as compared to the more conventional kinetic sputtering by momentum transfer between impinging ion and recoiling target atoms in a collision cascade.…”