V.I. Shulga

Round Robin computer simulation of ejection probability in sputtering

Abstract We have studied the ejection of a copper atom through a planar copper surface as a function of recoil velocity and depth of origin. Results were obtained from six molecular dynamics codes, four binary collision lattice simulation codes, and eight Monte Carlo codes. Most results were found with a Born-Mayer interaction potential between the atoms with Gibson 2 parameters and a planar surface barrier, but variations on this standard were allowed for, as well as differences in the adopted cutoff radius for the interaction potential, electronic stopping, and target temperature. Large differences were found between the predictions of the various codes, but the cause of these differences could be determined in most cases. A fairly clear picture emerges from all three types of codes for the depth range and the angular range for ejection at energies relevant to sputter ejection, although a quantitative discussion would have to include an analysis of replacement collision events which has been left out here.

Depth of origin of sputtered atoms for elemental targets

The mean depth of origin of sputtered atoms is an important characteristic of the sputtering process. There exist several theoretical and experimental determinations of the escape depth with different results. To clear up the situation, in the present work a systematic computer simulation study of the mean depth of origin of sputtered atoms is performed. The Monte Carlo program TRIM.SP and the lattice code OKSANA are applied to calculate the distribution of depth of origin and the dependences of the mean depth of origin on the atomic density, N, projectile energy, E, the angle of incidence, α, projectile and target atomic species, Z1 and Z2, as well as the simulation model.

Computer simulation of single-crystal and polycrystal sputtering I

Abstract Sputtering of Cu single-crystal and polycrystal targets by 27 keV Ar ions has been simulated using the new binary collision cascade computer program OKSANA. The sputtering yield, the sputtering and reflection efficiencies, and the absolute and relative contributions to sputtering from various components have been calculated in a broad range of incidence angles. The obtained angular dependences of the sputtering yield have proved to agree with experimental data. Some features of sputtering due to semichannel focusing of incident particles have been found. The contributions to sputtering from several types of linear collision chains and from the primary knock-on atoms are considered in most detail. It has been shown, in particular, that the pure focused, pure defocused, and mixed focused-defocused collision chains contribute noticeably to sputtering. The contribution from the primary knock-on atoms is angle-dependent and reaches its maximum in the range of glancing angles for both single-crystal an...

Ion-induced alkali-silicon interfaces: Atomistic simulations of collisional effects

The bombardment of Si with Cs is investigated by computer simulation with the program TRIDYN. The dynamic target changes due to the bombardment as composition profiles and the corresponding changes in sputtering yield and reflection coefficient are studied as functions of incident conditions. The incident energy is varied from 0.1 to 100 keV, and for 4 and 40 keV the dependence on the incident angle is considered. The dynamic results are compared with data from static calculations (low fluence). The influence of interaction potentials and of the choice of three surface binding energy models on the results are discussed.

Analysis of the primary process in isotope sputtering

Abstract Knockon sputtering from Mo isotopic mixtures has been studied by binary-collision and (in part) molecular-dynamics simulation. Differential and integrated partial sputter yields have been determined for homogeneous, mostly polycrystalline targets. Simulations have been carried out on 100Mo92Mo and 100Mo50Mo mixtures and accurate scaling relations with mass ratio have been established. The reliability of the simulation code was tested by computation of absolute elemental sputter yields, and the influence of key input parameters on these yields was determined. Relative sputter yields of isotopic mixtures do not depend sensitively on most of those parameters, with the notable exception of the target-target interatomic potential, the surface barrier, and the treatment of nonbinary collisions. Most of our simulations refer to Ar bombardment, and ion energies covered range from less than 100 eV to 100 keV. At high ion energies sputtering is found to be preferential in the light species in agreement with predictions from transport theory, and the magnitude of this effect appears consistent with the few available experimental results. A weak dependence of the yield ratio on emission angle is found which is identified as a surface scattering effect. At low ion energies very pronounced preferential sputtering is found which occasionally even may go in the opposite direction, i.e., preferential emission of the heavier species. This effect is governed by threshold processes and characterized by a predominant contribution from primary recoil atoms to the sputtered-particle flux. It is sensitive to the angle of incidence. Although the relative importance of threshold processes decreases rapidly with increasing energy their signatures remain visible at fairly high energies because isotope effects in higher generations of recoil atoms are weak. In the 1–5 keV range which is important for numerous applications and where most experimental and previous simulational work was carried out, the two types of processes compete. This complicates the analysis and explains why it has been difficult to reconcile discrepancies in the past.

Angular distribution of atoms sputtered from amorphous and polycrystalline targets

Abstract The angular distributions of atoms sputtered from amorphous and polycrystalline targets under 1 and 10 keV Ar ion bombardment (normal incidence) have been calculated using the binary-collision simulation. A large set of targets, from 3 Li to 92 U, was considered. The distributions were approximated by a function cosnθ. It has been shown that the exponent n is by far not a universal target-independent constant. In case of amorphous targets, which have been considered in detail, the exponent can be approximated by n=1+ANp/Uq, N being the target atomic density, U the surface binding energy, A, p and q fitting parameters. For polycrystalline targets, a pronounced decrease of n is noted for the targets predisposed to the generation of focused collision sequences. In both cases, the exponent is strongly dependent on the interatomic potential used. Results are compared with data from the literature with special emphasis on sputtering from iron, platinum, gold and germanium.

Simulation of energy-dependent isotope sputtering

Abstract The flux of atoms sputtered from an isotopic mixture is known to be enriched in the light component(s). Measured enrichment factors have been reported that significantly exceed those predicted by high-energy sputter theory. Moreover, a dependence on the emission angle is observed which was not predicted either but had been found in numerical simulations. To resolve this puzzle, we have simulated the sputtering of isotopic mixtures for ion energies ranging from 100 eV to 100 keV and found the isotopic enrichment to be very sensitive to the energy of the bombarding ion beam. Our computed enrichment factors are consistent with values measured at low beam energies. They also reach an asymptotic value in agreement with the theoretical high-energy prediction. Similarly, a pronounced angular dependence is found at low beam energies which becomes much smaller in the high-energy limit.

Stopping of copper clusters in copper

Abstract The molecular-dynamics simulation technique is used to study the penetration of 0.1 and 1 keV/at. copper clusters through a thin polycrystalline copper foil. Just as in the case M 1 ⪢ M 2 ( M 1 = cluster atom mass; M 2 = target atom mass) studied earlier, the stopping power per cluster atom proves to be much lower compared with the case of bombardment with single atoms. The results obtained are interpreted by analyzing the stopping powers of the front and rear cluster atoms and by examining some elementary collision processes under cluster and atom bombardment.

Interaction of slow (100 eV/atom) copper clusters with thin gold films: reflection, transmission, and sputtering at normal and oblique incidence

Abstract The interaction of a Cu 13 cluster with a thin Au crystal has been studied by molecular dynamics simulation as a function of the angle of incidence. Initial orientations of the cluster and the target were randomized. Trajectories are shown for two representative events. The average number, energy spectrum, and angular distribution of reflected and transmitted cluster atoms are computed as well as the corresponding quantities for target atoms sputtered from the upstream or downstream surface. Surprisingly strong clearing-the-way effects account for a significant decrease of reflection coefficients and correspondingly enhanced transmission in comparison with atomic bombardment. Also sputter yields are affected significantly. Collisions between cluster atoms account for a broadening of the energy spectrum of transmitted and reflected cluster particles, in particular so for oblique incidence. Multihit processes account for a pronounced high-energy tail in the spectrum of sputtered atoms. Due to computational limitations, the simulations are incomplete from the point of view of sputtering. Nevertheless, evidence is presented that suggests that the sputter yield per cluster atom may be smaller than the one for atomic bombardment in certain situations.

The density effects in sputtering of amorphous materials

Abstract The effects of the target atomic density on sputtering of amorphous targets under 1 keV Ar ion bombardment have been investigated using binary-collision simulation. Attention was given to the sputtering yield, and the angular and energy distributions of sputtered atoms. A large set of targets, from 3 Li to 92 U was considered and three interatomic potentials were applied. It has been shown that both the sputtering yield and the angular and energy distributions of sputtered atoms are undoubtedly dependent on the target atomic density. Results are compared with the data from the literature.