Y. Yamamura

Theoretical studies on an empirical formula for sputtering yield at normal incidence

Abstract Theoretical investigations on the Matsunami empirical formula for the sputtering yield are presented, and a new empirical formula is proposed. The first Matsunami empirical formula includes implicitly the effect of the inelastic stopping in one of the adjustable parameters, while in the new empirical formula the inelastic part and the elastic part are explicitly separated. It is found that the new empirical formula can predict well the energy-dependence of the sputtering yield over a wide range of the ion-target combinations. Another interesting conclusion is that one of the adjustable parameters of the new formula, which corresponds to Sigmund α shows the Z2-oscillation.

Sputtering by cluster ions

Abstract Using the Monte Carlo simulation code DYACAT, the sputtering by cluster ions (more than 100 atoms) has been investigated. In the DYACAT program which is based on the binary collision approximation, trajectories of ions and recoil atoms are followed dynamically. In order to simulate the nonlinear cascade development, three types of collision events are taken into account, i.e., between two moving particles, between a moving atom and a target atom and between a moving particle and an interstitial. The time-dependent material change is also followed. The energetic cluster ions, 100 eV/atom (Ar) n ( n being 10 to 200), have been bombarded on a carbon target. It is found that the energy spectra of slowing down ions which are spread until 200 eV is much different from those of a monatomic ion. Although the incident energy is in the near-threshold region, nonhnearity in sputtering is clear for large n .

Computer studies of the energy spectra and reflection coefficients of light ions

Abstract A new computer code, i.e., the ACAT code, has been developed to simulate atomic collisions in amorphous targets using binary collision approximation, where random targets are simulated employing the so-called cell model which is successfully used in the liquid theory. The ACAT code is used to study the backscattering and the implantation of 0.01-10 keV hydrogens on Au and on Cu. In comparing the calculated results of the ACAT code with those of the MARLOWE and TRIM codes, as a whole, an agreement between these three codes is satisfactory, and the ACAT and TRIM codes give almost the same results for energy spectra of backscattered particles and other related quantities. Using the ACAT computer code, it was found that in light ion backscattering the potential parameter as well as the inelastic energy loss parameter has an appreciable influence on the particle and energy reflection coefficients.

An empirical formula for angular dependence of sputtering yields

Abstract An analytic empirical formula for the angular dependence of light-ion sputtering and heavy-ion sputtering has been proposed. The present empirical formula has two adjustable parameters which are determined by the least-squares method. One of the two parameters corresponds to Sigmundf, and the other is the angle of incidence at the maximum yield. The present empirical formula is found to be very useful for preliminary descriptions of the angular dependence of sputtering yield. This work was carried out under the collaborating Research Program at the Institute of Plasma Physics, Nagoya University. Lindhards theory of the nuclear stopping power is developed to take into account the binding force between the lattice atoms of solids. It is shown that the nuclear stopping power is proportional to the velocity (not the energy) of the moving atom at the low velocity limit. Debye temperature dependence of the nuclear stopping power is discussed.

Energy and angular distributions of sputtered atoms at normal incidence

Abstract The Monte Carlo simulation code ACAT has been applied to investigate the angular distribution and the energy distribution of atoms sputtered from Cu and Nb targets by normally incident Ar+ ions. It is found that there are two important effects which affect the angular distributions and the energy distributions of sputtered atoms, i.e., the anisotropic effect and the bulk recoil effect. The former effects means that the recoil flux keeps the memory of the incident ion-beam direction because of the incomplete cascade, while the latter one means the contributions of recoils generated at the deeper layer to the angular and the energy distributions of sputtered atoms. The anisotropic effect is important in the low energy region, and it makes the angular distribution under-cosine and the high energy tail of the energy distribution fall off faster than the Thompson distribution. The bulk recoil effect makes angular distribution be over-cosine and the peak position of the energy distribution be shifted t...

Over-cosine angular distributions of sputtered atoms at normal incidence

Abstract The angular distribution of sputtered atoms for normal incidence ions has been investigated theoretically and by computer simulation. For low energy ions the angular distribution is under-cosine, while for relatively high energy ions we obtain an over-cosine angular distribution for the sputtered atoms. It is found that the outward-peakness of the angular distribution for relatively high energy ions is due to the geometrical asymmetry near the surface. Using the Monte Carlo simulation code ACAT, which is based on the binary collision approximation, the angular distributions of sputtered atoms are calculated for various incident energies of Ar ions incident normally on an Fe target. It is found that one needs to take into account the surface roughness in order to obtain good agreement with experiment. The surface roughness is believed to reduce the degree of the over-cosine distribution because a rough surface has a larger effective surface area as compared with an unirradiated surface.

A new empirical formula for the sputtering yield

Abstract A new empirical formula for the sputtering yield at the normal incidence has been proposed. It is found that the new empirical formula can predict well the energy-dependence of the sputtering yield for various ion-target combinations. Another interesting conclusion is that one of the adjustable parameters of the new formula shows the Z2-oscillation.

Shell correction of screening length in interatomic potential

Abstract In the computer studies on the interaction of an charged particle with solids, authors treat the nuclear collision by the Thomas–Fermi screened Coulomb potential. For better agreement with experiment, authors sometimes modify the screening length. We investigate the theoretical background for the correction factor of the screening length in the interatomic potential which is deduced from two steps. The first step is to determine the correction factor of an isolated atom so as to adjust the average radius of the Thomas–Fermi electron distribution to that of the Hartree–Fock electron distribution. The second step is to determine the screening length of two-atom encounter, using the combination formula corresponding to the Firsov formula. The obtained correction factors of the screening length are in good agreement with those determined by the ICISS computer analysis.

Computer studies of 180° Ne neutral atoms backscattered from a Pt(111) surface

Abstract The 180° neutral impact-collision ion scattering spectroscopy (NICISS) intensity has been evaluated using the ACOCT program code based on the binary collision approximation. The computer simulations are performed for the case of 2 keV Ne+ ions incident along the [ 1 1 2] direction of a Pt(111) surface. It is found that there are the influences of not only first or second layer atoms at the surface but also several layer atoms below the surface on the 180° Ne NICISS intensity. Also, the ACOCT results show that the Moliere approximation of the Thomas-Fermi potential with a reduced Firsov screening length multiplied by a factor of 0.85 and the vertical component of surface Debye temperature 143 K are suitable for the 180° Ne NICISS intensity at the Pt(111) surface.

Large-angle surface scattering of low-energy ions in the two-atom scattering model

Abstract The ion surface scattering near 180° scattering angle is investigated, based on the two-atom scattering model. The importance of the shadowing effect and the focusing effect on the intensity of large-angle backscattering is discussed in detail.