G. Betz

Energy and angular distributions of sputtered particles

Abstract A review is presented on energy and angular distributions of atomic, molecular and cluster particles sputtered from solid surfaces under ion bombardment. The review is preferentially focused on experimental results and the corresponding experimental methods; theories about sputtering phenomena are used more as guide-lines for discussion and comparison. The four main areas considered are sputtering of (1) metals, (2) alkali halides, (3) frozen gases, and sputtering by (4) MeV ion impact on insulators. Analysis of the experimental data on energy and angular distributions has revealed a large variety of sputtering modes such as elastic and inelastic collisional sputtering, ion-induced thermal evaporation, ejection of excited atoms, sputtering due to exciton decay, formation of clusters or ejection of grains, sputtering by shock waves and formation of craters associated with an explosive gas flow. The comparison of experimental and theoretical data shows that the theory of linear collision cascades is a good basis for an understanding of the elastic sputtering processes. Also, recent computer simulations of sputtering give valuable insight into collisional processes. The situation is more complex when electronic excitation is the driving interaction of the sputtering process. In this case, depending on the type of target material and the energy loss of the primary ions, a large number of theoretical attempts have been made to describe individual phenomena attributed to electronic sputtering.

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.

Molecular dynamics studies of cluster emission in sputtering

Abstract The emission of neutral clusters can constitute a sizeable fraction of the total sputtered flux under energetic (keV) ion bombardment of metal surfaces. We have performed Molecular Dynamics (MD) simulations of the sputtering process for Ar on Cu(111) and ion energies up to 5 keV. Using a many-body interaction potential we have analysed in detail the processes leading to cluster emission, in order to gain a microscopic understanding of these phenomena. Results obtained for cluster emission will be presented and compared with single atom emission. Their yields and energy distributions (up to pentomers) will also be compared with recent experimental results using (non)resonant laser multiphoton post-ionisation in combination with the Time of Flight technique. From the results of the MD calculations we propose a new model for the emission of large clusters, which is still a puzzling phenomenon in sputtering.

Electronic excitation in sputtered atoms and the oxygen effect

Abstract Bombardment of surfaces by ions gives rise to a variety of inelastic collision events leading to the ejection of excited atoms and ions. Such excited sputtered particles have been studied since more than 80 years through their optical emission, when they decay in front of the target to the electronic ground state, having lifetimes of 10−9 to 10−7 s, typically. Information on the energy distribution of such excited states can be obtained by two different techniques: light vs distance measurements (LvD) and by studying line profile broadening in light emission due to the Doppler effect. Only recently it has become possible to study in addition metastable excited atoms using laser induced fluorescence spectroscopy (LIF). Relative sputtering yields and energy distributions have been measured for such metastable states and two types can be distinguished. States with a very low excitation energy (0–0.3 eV), being sublevels of the electronic ground state, were found to have yields and energy distributions comparable to the electronic ground state, while metastable states at higher excitation energies (above 1 eV) seem to behave similar to short lived excited states, typically observed in secondary photon emission (BLE) with excitation energies in the range of 2–6 eV. This behaviour is also clearly visible with respect to oxygen surface coverage or increased near surface oxygen concentration where, similar to secondary ion emission, drastic changes in the yield by orders of magnitude have been found for excited atoms as well as for ions. In addition, under the same conditions a strong decrease in the sputtering yield of neutral ground state atoms has been observed for a number of metals. LIF results for highly excited metastable states are compared with recent results obtained by studying line profile broadening in light emission for Ca, Al and Cr targets. Different mechanisms that have been proposed to account for the observations will be discussed.

Experiment and simulation of cluster emission from 5 keV Ar → Cu

The abundance distribution of neutral Cun clusters sputtered by 5 keV Ar impact from a polycrystalline Cu surface is measured using single-photon laser post-ionization. Molecular dynamics computer simulation is used to gain insight into the cluster sputtering process. Three diAerent codes and two potentials are used to check the sensitivity of the results on numerics and physical input. DiAerences in the results obtained by the various codes and the diAerent potentials used are discussed. While the total sputter yield is consistent with experiment, the fraction of atoms bound in clusters, and in particular the dimer fraction, are overestimated by at least a factor of 4. This is also true for a many-body potential which has been fitted to describe both bulk Cu and dimers. In detail, the simulation shows that larger clusters are emitted at later times from the target. Clusters originate mainly from regions of the surface, which are around the melting temperature of bulk Cu. Large clusters are emitted preferably from ion impacts with a high individual sputter yield. Finally, we simulate sputtering from a model Cu material with an artificially decreased cohesive energy. Here, drastic high-yield events (up to Ya 78) can be observed, which produce clusters abundantly. ” 1998 Elsevier Science B.V. All rights reserved.

Cluster bombardment of solids: A molecular dynamics study

Abstract We have used the molecular dynamics technique employing many body potentials to study the interaction of Al clusters of different sizes (60, 210, 504 and 1080 atoms) with a Cu (111) surface at room temperature in the energy range between 0.1–30 eV per cluster atom. This energy region covers the full range from deposition without any sputtering of target or reflection of cluster atoms up to bombarding energies, where reflection, sputtering and crater formation become dominant. One main motivation was to investigate the influence of cluster size on thin film deposition with cluster beams. From the results we conclude, that the mean kinetic energy (“temperature”) of the target atoms near the impact point only depends on the bombarding energy per cluster atom. However, the time this region remains at an elevated temperature increases strongly with cluster size. A deposition “temperature” can be chosen by varying the bombarding cluster energy. Thus, the “temperature” can be tuned to a suitable value, at which the duster melts at the surface, recrystallises and at most weak mixing with the first few surface layers takes place, but the “temperature” is not high enough for evaporation (sputtering), pronounced mixing or penetration. Sufficient time to complete the melting and re-crystallisation can be provided by using sufficiently large clusters.

Molecular dynamics study of sputtering of Cu (111) under Ar ion bombardment

Abstract We have used the molecular dynamics (MD) technique using many-body interaction potentials to analyse in detail the processes leading to sputter emission, in order to gain a microscopic understanding of low energy bombardment phenomena. Calculations were performed for a Cu (111) single crystal surface bombarded with Ar atoms in the energy range from 10–1000 eV. The results presented for low bombarding energies are mainly concerned with the near sputtering threshold behaviour, yields and depth of origin of sputtered atoms. Furthermore, it is found, that in addition to sputtered atoms, a large number of ad-atoms at the surface are generated during the evolution of the collision cascade. At higher energies the question of cluster emission and especially their energy distribution and angular distribution are addressed. It was found that the energy distributions for the dimers and monomer atoms exhibit a similar dependence on emission energy as has been observed recently also experimentally. For atoms ...

Energy distributions of sputtered metal Al-clusters

Abstract The energy distributions of neutral clusters escaping from metal targets under ion bombardment have been measured by nonresonant laser post-ionization and a time-of-flight (TOF) technique. Measurements of various Al- and Sn-atoms, dimers and trimers from pure Al and AlSn alloy targets show that the ionization process is crucial for the interpretation of the measured spectra. In particular, fragmentation of large clusters can easily mask and disturb the measured energy distribution of clusters with fewer atoms and of monomers, if the ionization probability is low, as it is the case for multiphoton ionization. The results obtained for the energy distribution of Al-atoms, dimers and trimers show that simple theories for the cluster formation cannot predict the correct energy distributions, but molecular dynamics simulations agree well with the experiments.

Collisional and electronic processes under ion, electron and photon bombardment of alkali and alkaline-earth halides

Abstract The interaction of energetic ions or electrons with alkali halides and alkaline-earth halides results in elastic and inelastic energy transfer to the near surface region. The emission of secondary sputtered and desorbed particles has been studied to identify these processes. In particular, we have compared the emission of neutral ground state and excited alkali and alkaline-earth atoms from NaCl, LiF and CaF2. The intensity of the sputtered and desorbed particles and their velocity distributions have been measured as a function of the target temperature between room temperature and 400 ° C. For NaCl under ion bombardment neutral ground state Na atoms desorb predominantly (ion-beam-induced) thermally, even at room temperature, while Ca atoms from CaF2 desorb mainly thermally above 300 °C and purely collisional below 250 °C. For 400 eV electron bombardment desorption of thermal neutral ground state atoms is observed whenever a thermal contribution is present under ion bombardment. Excited Na and Ca atoms with energies in the eV range have been observed for ion bombardment. For electron bombardment excited Na has been identified with thermal velocities at all target temperatures. Contrary, excited Ca was not found even at target temperatures where substantial ground state Ca is desorbed. For NaCl as well as CaF2 and electron as well as ion bombardment inelastic processes play an important role for particle ejection above a certain target temperature.

Modelling of cluster emission from metal surfaces under ion impact

Using the molecular–dynamics technique, cluster emission for 5 keV Ar bombardment of a Cu (111) surface has been investigated using a many–body (tight binding) potential for the Cu–Cu interaction. The calculations allow us to analyse the basic processes underlying cluster emission. It is found that two distinct processes can be distinguished which lead to cluster emission under energetic ion bombardment. The first process causes the emission of small clusters, which are emitted by a collective motion during the development of the collision cascade within the first picosecond after impact. Thus, emission times of such clusters agree with the emission times of atoms in sputtering. Such a process can be envisioned if, for example, a few layers below the surface, an energetic recoil causes the development of a subcascade. Energy transferred by this event to the surface is strongly directional and can lead to the simultaneous emission of a group of neighbouring surface atoms, which in some cases will remain bounded and form a cluster after emission. Typically, clusters emitted by this mechanism consist of atoms, which are neighbouring in the target and are almost exclusively surface atoms, similar to all sputtered atoms. Emission of large clusters (cluster sizes of 10 or more atoms), as observed experimentally, is a puzzling phenomenon. From our calculations we conclude that the emission of such large clusters does not occur during the collisional phase of sputtering, but happens much later (5–10 ps after ion impact). Emission can occur for spike events, where all the energy of the impinging ion is deposited locally in a small volume near to the surface, and the sputtering yield is 3–5 times the average yield. Such events are rare, but we have found a few cases in our calculations where stable clusters consisting of more than 20 atoms were emitted. Melting of the spike volume occurs, and the high temperatures and pressures produced can cause emission of large fragments during the thermal phase. The composition of such large clusters is quite different from that of small clusters. They consist of atoms from different layers and the constituents are also generally not next–neighbour atoms. This change in origin of the cluster atoms reflects the mixing and diffusion processes occurring in the melted zone before emission. The calculations indicate that hydrodynamical phenomena might play a role in the emission of large fragments. Additional calculations, where the energy was distributed ‘thermall’ in a three–dimensional volume under the surface for 500 fs, give very similar results, even in such cases where the kinetic phase of the collision–cascade development was absent.