Direct Evidence for Projectile Charge-State Dependent Crater Formation Due to Fast Ions

Abstract

We report on craters formed by individual 3 MeV=u Au q ini þ ions of selected incident charge states q ini penetrating thin layers of poly(methyl methacrylate). Holes and raised regions are formed around the region of the impact, with sizes that depend strongly and differently on q ini . Variation of q ini , of the film thickness and of the angle of incidence allows us to extract information about the depth of origin contributing to different crater features. DOI: 10.1103/PhysRevLett.101.167601 PACS numbers: 79.20.Rf, 61.80.Jh, 61.82.Pv, 81.16.Rf Energetic atomic [1-3], molecular or cluster ions [4] impacting solids may leave tiny holes at the surface often surrounded by a raised region of displaced material. The observed surface morphology [1][2][3][4] is similar to what is found in ablation craters produced by an intense laser pulse [5,6], in macroscopic craters produced by meteorite impacts on planets [7], or by balls dropped into granular media [8], although their spatial scales may differ by about 17 orders of magnitude. Depending on the energy regime of the ions and the type of material being bombarded, the shape of the impact features and the underlying mechanisms of formation may differ [9,10]. As the energy deposited by swift ions of equal kinetic energy, but different charge-states may vary substantially close to the surface, a detailed knowledge of charge-state dependent effects is of great importance for ion-beam based techniques of materials structuring (such as ion-track etching), particularly considering the new demands for smaller pattern sizes and the use of thinner layers [11]. Our results give direct evidence for a strong dependence of the surface modifications introduced by single fast ions on their charge state. For track-etching procedures, this means it is possible to control the etching sensitivity at the surface and charge equilibration below the surface should yield different etched shapes, dependent on the incident charge state. Moreover, by employing a series of well-defined nonequilibrium charge states, we could derive depth information on the near-surface effects induced by single fast ion impacts.In this Letter, we focus on cratering induced by high velocity ions. At specific kinetic energies of a few MeV per nucleon, such ions transfer more than 99% of their energy to the target-electron system. A considerable fraction of the energy deposited in the track of a swift heavy ion is concentrated in a core region ( % 1 nm), where ionizations are directly produced by the ions. Fast secondary electrons spread the rest of the energy over larger distances. The exact size of such regions depend on the material and on the velocity of the ions, while the total amount of energy loss per path length, S e ¼ dE e =dx, is well understood [12]. The subsequent electron dynamics, however, is a nonlinear phenomenon and it may result in large sputter yields and cratering, due to the coupling of electronic and atomic degrees of freedom [13][14][15]. For reviews on ion-induced electronic cratering pr...

Graphitic nanostripes in silicon carbide surfaces created by swift heavy ion irradiation

“…The ion irradiation-induced damage we observe in SiC clearly differs from the modifications observed in other dielectrics (for example, Al 2 O 3 , SrTiO 3 , TiO 2 , SiO 2 , CaF 2 , polymethylmethacrylate) that have been irradiated under similar conditions 17,[32][33][34][35] . In those materials, the surface typically shows elongated protrusions (or chains of protruding hillocks) aligned along the ion beam direction.…”

Atomic force microscopy and Raman scattering studies of femtosecond laser-induced nanohillocks on CR-39

“…The nanocrater formation at a polymer surface with an ultrashort laser is similar to the observed craters induced by high energy ions [32]. This fast energy deposition can be explained on the basis of short-time models for atom dynamics [32], according to which mainly the energy density close to the surface is responsible for particle ejection and cratering of the polymer as a consequence of ion bombardment.…”

Confinement Effects of Ion Tracks in Ultrathin Polymer Films

“…1(f)]. These are typical surface features seen on polymers [9,14] and other organic materials [15] due to massive mass transport and particle ejection emerging from the core of MeV-GeV ion tracks. As the morphology and size of the surface features do not change with further increase in target thickness, this is what we call the "bulk" condition for this particular radiation effect.…”

“…The extreme levels of energy deposited via electronic excitation (reaching up to several keV/nm) along narrow (∼1-10 nm) tracks are the basis of all unique radiation effects produced by high-energy ions in matter, including bond breaking [5], creation of defects [2], sputtering [6,7], and cratering [8,9]. Because of that, ion track formation and damage production by individual ions has been a central and persistent topic of research in radiation physics for decades [2,3].…”

“…The samples were irradiated at the UNILAC accelerator at GSI, Darmstadt, by 923 MeV Pb eqþ and 2187 MeV Au eqþ beams (eq þ indicates that the charge state of the ions had a distribution around equilibrium). Data obtained previously from samples irradiated with 597 MeV Au 45þ were also used for comparison [9]. Ion impact densities were ∼10 ions=μm 2 and the angles of incidence θ were set at 0°and 79°with respect to the surface normal.…”