Cluster Surface Collisions Research Articles

Inelastic processes under gas cluster ion bombardment of metals

Total currents of positive and negative particles arising under bombardment of metal surfaces with argon gas cluster ions were measured for the energy range 2–15 keV. Supposing that the negative current was defined by secondary electrons, and the positive current is the flow of the reflected argon ions, the possible mechanisms of electron emission and charge transfer are discussed. The discussion is supported by molecular dynamics simulations of cluster-surface collisions.

Cluster-induced desorption investigated by means of molecular dynamics simulations-Microsolvation in clusters of polar and non-polar constituents.

The desorption of surface-adsorbed molecules induced by cluster-surface collisions of neutral molecular clusters, in particular, SO2 clusters, was investigated by means of molecular dynamics simulations. The desorption efficiency was found to be in general much higher for clusters of polar molecules when compared to nonpolar cluster constituents, for both nonpolar and polar adsorbates. In all cases, desorption is shown to proceed via dissolvation of the analyte in the cluster. In systems with nonpolar cluster constituents, the process is mainly driven by the increase in the entropy of the dissolved analyte in a larger cluster fragment. The latter process is enhanced by polar cluster constituents since the respective clusters show lower fragmentation at comparable kinetic energy and thus provide in average larger cluster fragments for the analytes to be dissolved in. In systems with clusters of polar constituents and polar adsorbates, the process is most efficient due to the additional energetic stabilization of the desorbed molecule in the solvation shell formed in the cluster fragment.

Cluster ion beam process technology – 20 years of R&D history

More than 20 years have passed since the author first began to explore the feasibility of processing by gas cluster ion beams at the Ion Beam Engineering Experimental Laboratory of Kyoto University. Processes employing ions of gaseous material clusters comprised of a few hundred to many thousand atoms are now being developed into a new field of ion beam technology. Cluster–surface collisions produce important non-linear effects which are being applied to shallow junction formation, to etching and smoothing of semiconductors, metals and dielectrics, to assisted formation of thin films with nano-scale accuracy and to other surface modification applications.

Cluster Ion Beam Process for Nanofabrication

AbstractThis paper reviews gas cluster ion beam (GCIB) technology, including the generation of cluster beams, fundamental characteristics of cluster ion to solid surface interactions, emerging industrial applications, and identification of some of the significant events which occurred as the technology has evolved into what it is today. More than 20 years have passed since the author (I.Y) first began to explore feasibility of processing by gas cluster ion beams at the Ion Beam Engineering Experimental Laboratory of Kyoto University. Processes employing ions of gaseous material clusters comprised of a few hundred to many thousand atoms are now being developed into a new field of ion beam technology. Cluster-surface collisions produce important non-linear effects which are being applied to shallow junction formation, to etching and smoothing of semiconductors, metals, and dielectrics, to assisted formation of thin films with nano-scale accuracy, and to other surface modification applications.

Modelling transport phenomena and epitaxial behaviours of cluster–surface collisions viamolecular dynamics simulations

This paper studies the transport phenomena of disordered systems in the thermalrelaxation process by cluster impact on a solid surface. Constant-temperaturemolecular dynamics simulations were applied for nanoscale modelling. The surfacewetting and epitaxial behaviours of deposited clusters with various flow and internalkinetic energy were investigated. This revealed that the internal temperature ofclusters enhanced the lateral activity of atoms, while normal incident energy wasrelated to atomic implantation. By exchanging momentum and energy from thesystem to a heat sink with constant temperature, the collision system reachedthermal equilibrium after a long enough simulation time. The temperature decayprofiles of the system were presented as an exponential-type function, which isdependent on the efficiency of the cooling model and the heat conductivity of thematerial. Finally, the nanostructure growth mechanism by cluster deposition wasstudied. It showed that both epitaxial film and voids were constructed by thecluster–island and island–island collision mechanism, which was dependent on the incidentenergy of the cluster and the surface temperature. As a dynamic behaviour of ananostructure with a non-zero momentum of its mass centre, the rocking phenomenonwas presented as an important characteristic of thin films by cluster deposition.

Apparatus for reactive cluster-surface studies

We present a new instrument that has been specifically designed for systematic studies of reactive cluster-surface collisions under well-defined ultrahigh-vacuum conditions. As a key feature, it incorporates a temperature-controlled, high-pressure cluster source with an unsurpassed pulse-to-pulse stability for the generation of weakly bound molecular aggregates. Impact processes can be investigated in two experimental configurations complementing one another: In the first, the exploration of intracluster reactions of accelerated, size-selected, positively or negatively charged cluster ions (1–100eV∕molecule) is accomplished, employing an inert target surface combined with angular-integrated ion detection. This arrangement uses a compact, homebuilt dual time-of-flight mass spectrometer with excellent mass resolution m∕Δm>2000 and postacceleration capabilities. In the second configuration, the detailed analysis of chemical reactions between neutral clusters and a well-defined single crystal at lower kinetic energies (0.01–1eV∕molecule) is possible. Here, the surface interaction of a seeded supersonic cluster beam is probed with angular-, time-, and velocity-resolved detection of neutral reaction products using a high-performance quadrupole mass spectrometer.

Surface nanostructuring by implantation of cluster ions

A brief state-of-the-art review in the field of cluster ion implantation is presented. Ionised cluster beams are considered as a controllable and versatile tool for modification and processing of surfaces and near-surface layers on an atomistic scale as an alternative to ion implantation and ion-assisted deposition. The main effects occurring under cluster–surface collisions as well as advantages in the application of cluster ion beams are reviewed. The problem of surface erosion under impact of energetic cluster ions is emphasised in the paper. A model explaining crater and hillock formation on target surfaces with relation to the thermal-transfer effect and local target melting at the collision spot is discussed.

Molecular dynamics simulations of the micro-solvation of ions and molecules during cluster–surface collisions

We present molecular dynamics simulations of the disintegration of neat and Na + doped (H 2O) 1032 clusters induced by a surface collision at velocities of 1.75 and 2.91 km/s. The incorporated ion acts as a spectator to the collision and ends up in one of the largest cluster fragments. The micro-solvation shell of the Na + ion is exchanged during the collision even in the low velocity case where large portions of the cluster survive. Two different regimes can be discriminated in the micro-solvation dynamics, an impact-driven exchange initially and a diffusion channel evolving during the evaporative stabilization of impact heated cluster fragments.

Analysis of Solution-Deposited Alkali Ions by Cluster Surface Collisions

Sulfur dioxide clusters (SO2)x of mean size 〈x〉 = 1.7 × 103 collide at a velocity of 1.6 km s-1 with a metal surface that has been pretreated with dilute salt solutions containing Li+, Na+, K+, Cl-, and Br- ions in different concentrations. In the scattered gas plume, positive and negative cluster fragments of the formal composition (SO2)x(Li+, Na+, K+, SO2-) are detected. While the amount of charge observed in the various cationic channels correlates with the concentration of the respective cations in the solutions, no cluster fragments carrying chloride or bromide anions have been observed. This indicates, that the observed free charge carriers are not formed by a direct pickup of ions from the surface. Based on the assumption that the physical state for alkali adsorbates on metal surfaces is independent of the charge state of the adsorbing precursor, the findings are explained in terms of a known charge separation effect in cluster surface collisions involving neutrally deposited alkali adsorbates.

Design and capabilities of a cluster implantation and deposition apparatus: First results on hillock formation under energetic cluster ion bombardment

A description, advantages, and capabilities of a cluster implantation and deposition apparatus supplied by a pulsed cluster source from gaseous precursors are presented. A number of possible in situ and ex situ experimental methods to study cluster–surface collisions and modified substrate surfaces are discussed. Test experiments on cluster production show formation of Ar, N2, and O2 clusters with size up to 150 atoms for Ar and 60–70 molecules for the other gases. The possibility of cluster mass selection and acceleration up to 25 keV is reported. Nanosize hillock formation was found as a result of cluster–surface collisions with pyrolytic graphite and indium–tin–oxide. It is suggested that the hillocks’ parameters such as size and density per surface area can be controlled by varying the implantation parameters and substrate material and thus provide a promising technique for nanoscale surface modification.

Molecular-dynamics simulations of cluster–surface collisions: Emission of large fragments

Large-scale classical molecular-dynamics simulations of (H2O)n (n=1032,4094) collisions with graphite have been carried out. The clusters have an initial internal temperature of 180 K and collide with an incident velocity in the normal direction between 200 and 1000 m/s. The 1032-clusters are trapped on the surface and completely disintegrate by evaporation. The 4094-clusters are found to partly survive the surface impact provided that the surface is sufficiently hot. These clusters are trapped on the surface for up to 50 ps before leaving the surface under strong evaporation of small fragments. The time spent on the surface is too short for full equilibration to occur, which limits the fragmentation of the clusters. The size of the emitted fragment is roughly 30% of the incident cluster size. The cluster emission mechanism is found to be very sensitive to the rate of the surface-induced heating and thus to the surface temperature. The incident cluster velocity is less critical for the outcome of the collision process but influences the time spent on the surface. The trends seen in the simulations agree well with recent experimental data for collisions of large water clusters with graphite [Chem. Phys. Lett. 329, 200 (2000)].

MD simulation of cluster–surface impacts for metallic phases: soft landing, droplet spreading and implantation

An optimized version of the DL_POLY molecular dynamics simulation code [K. Kholmurodov, W. Smith, K. Yasuoka, T. Ebisuzaki, Comput. Phys. Commun. 125 (2000) 167–192] has been used to study the cluster–surface impact processes for metallic phases. The interaction of an energetic cluster of atoms with a solid surface has been investigated using the Finnis–Sinclair many-body potential. The characteristics of the cluster–surface collisions were studied in a wide range of the cluster impact energies ( E inc=0.035–3.5 eV/atom). Modification of the surface, exposed to the cluster-beams, was studied by monitoring the molecular dynamics configurations of the system in real time. The density and temperature distributions in the system under the energetic irradiations has been investigated in detail. The three major channels of the impact yield (viz., soft landing, droplet spreading and implantation) were distinguished and estimated. Based on the density and temperature distributions data the low energy cluster–surface impact has been analyzed and a novel interpretation of droplet spreading process is given.

Molecular dynamics simulations of cluster–surface collisions: Mechanisms for monomer emission

Molecular dynamics simulations of (H2O)4094-clusters impacting with a velocity of 470 ms−1 in the normal direction on a graphite surface kept at 1400 K were performed. The aim was to clarify the behavior of water molecules and other small fragments emitted during the collision event. The results agree well with previous experimental studies and with the results of a thermokinetic model for evaporation of small fragments during cluster scattering. About 80% of the evaporating water molecules come in close contact with the hot surface, and their translational degrees of freedom are partly accommodated to the temperature of the surface, especially in the direction normal to the surface plane, leading to a high translational temperature in this direction. The results stress the importance of energy transfer from the hot surface to the cluster and explain the high translational temperature determined from experimental angular distributions.

Fragmentation and ion-scattering in the low-energy collisions of small silver cluster ions (Agn+: n=1−4) with a highly oriented pyrolytic graphite surface

Fragmentation and ion-scattering processes in the low-energy (0–200 eV) collisions of Agn+ (n=1–4) with a highly oriented pyrolytic graphite (HOPG) surface have been investigated by employing a tandem time-of-flight mass spectrometer. It was found that the fragmentation of scattered cluster ions is due to unimolecular dissociation in the energy range studied. A marked difference between the fragmentation pattern of incident Ag3+ and that of Ag4+ has been found: The intact scattered cluster ion was observed for Ag3+ while only fragment ions for Ag4+. From the incident energy dependence of fragment ion intensities, it was deduced that internal energies of the scattered parent ions have upper and lower limits. These limitations are probably due to the adsorption and the implantation of the projectile clusters on the surface. Ion-scattering yield was found to increase with cluster size. Both the incident energy and cluster size dependencies of ion-scattering yields were reproduced by a model calculation in which the hole survival and sticking probabilities of the clusters, and the incident energy spread of the projectile ions are incorporated. The hole survival probability was evaluated from the time-dependent electron transfer rate for a elastically scattered sphere with an equivalent volume to that of the incident cluster ion. The calculation indicates that the average hole–surface distance at the moment of impact, which is determined by the dimensions of the incident cluster, is important for the hole survival in the cluster–surface collisions.

On the origin of surface smoothing by energetic cluster impact: Molecular dynamics simulation and mesoscopic modeling

Energetic cluster impacts have been applied for the production of high quality thin films as well as a tool for smoothing rough surfaces. Molecular dynamic (MD) simulations of cluster–surface collisions provide the following picture for the underlying microscopic mechanism: the impact of an energetic cluster onto a tilted part of the substrate induces a downhill particle current (consisting of both cluster and surface atoms) transferring the higher lying parts of the surface profile into the valleys. This effect is more pronounced for high kinetic energies. Based on these microscopic results, a mesoscopic model employing stochastic differential equations (SDE) has been formulated and allowing a quantitative explanation of experimental results from atomic force microscope measurements.

Interplay between emission of electrons and quasi-resonant ultrafast electron tunneling in cluster–surface collisions

Low energy collisions between clusters and solid surfaces are studied both theoretically and experimentally. The relative electron emission yield γ( N) has been measured as a function of the cluster size and collision energy. Results for different cluster types and surfaces are presented. A marked size dependence of γ( N) is obtained for collisions between Pt − N , Pb − N and Pb + N clusters with a graphite (HOPG) target. Moreover, the form of γ( N) changes dramatically with the collision energy. In contrast, if aluminum, for instance, is used as target, γ( N) shows a monotonically decreasing behavior for increasing cluster size. We present a theoretical description of the collision process using a microscopic model. The time-dependent charge exchange between the clusters and the targets was studied. In order to describe the neutralization dynamics we calculated the nonadiabatic survival probability P s( N) of the charged clusters. For surfaces with narrow densities of states P s( N) exhibits damped Stückelberg oscillations as a function of N. For metallic surfaces P s( N) shows a monotonically decreasing behavior for increasing cluster size N. We found that P s( N) shows the same behavior as γ( N). Our results suggest that the electron emission reflects the femtosecond neutralization dynamics during the collision and that the size dependence of γ( N) can give information on the cluster–surface interaction potential.

Cluster-surface collisions: Characteristics of Xe55- and C20–Si[111] surface bombardment

Molecular dynamics (MD) simulations are performed to study the cluster-surface collision processes. Two types of clusters, Xe55 and C20 are used as case studies of materials with very different properties. In studies of Xe55–Si[111] surface bombardment, two initial velocities, 5.0 and 10.0 km/s (normal to the surface) are chosen to investigate the dynamical consequences of the initial energy or velocity in the cluster-surface impact. A transition in the speed of kinetic energy propagation, from subsonic velocities to supersonic velocities, is observed. Energy transfer, from cluster translational motion to the substrate, occurs at an extremely fast rate that increases as the incident velocity increases. Local melting and amorphous layer formation in the surfaces are found via energetic analysis of individual silicon atoms. For C20, the initial velocity ranges from 10 to 100 km/s. The clusters are damaged immediately upon impact. Similar to Xe55, increase in the potential energy is larger than the increase in internal kinetic energy. However, the patterns of energy distribution are different for the two types of clusters. The energy transfer from the carbon clusters to Si(111) surface is found to be slower than that found in the Xe clusters. Fragmentation of the carbon cluster occurs when the initial velocity is greater than 30 km/s. At 10 km/s, the clusters show recrystallization at later times. The average penetration depth displays a nonlinear dependence on the initial velocity. Disturbance in the surface caused by C20 is discussed and compared to the damage caused by Xe55. Energetics, structures, and dynamics of these systems are fully analyzed and characterized.

Cluster–surface collisions by phase-space compressed guided-ion beam methods

A new instrument for production and analysis of samples created by mass-selected ion beam deposition is described. The deposition beamline makes use of a novel phase-space compressor that improves the focusability of the beam at low energies, allowing use with low current cluster ion sources. To illustrate the method, our initial results are presented for deposition of Cu + and Cu 2 + on molybdenum over the energy range from 6 to 220 eV. It is found that deposition of Cu + at low impact energies results in Cu depositing on the surface with sticking probability of ∼0.6. As the energy is increased above ∼100 eV, an increasing fraction of subsurface copper is observed. In the Cu/Mo system, diffusion of subplanted Cu to the surface is thermodynamically favored, and this probably accounts for the observation of no subplanted Cu at impact energies up to 100 eV. For Cu 2 +, the results are quite different. The apparent deposition efficiency is similar to that observed for Cu +, however, a significant fraction of sub-surface copper is observed at energies down to ∼40 eV (20 eV/atom). This greater tendency for production of subplanted copper is tentatively rationalized as resulting from a greater propensity for dimer impact to produce complex defects that stabilize subplanted copper. Possible production mechanisms for such defects are discussed.

Effect of surface reactivity on the nucleation of hydrocarbon thin films through molecular-cluster beam deposition

The effect of surface reactivity on thin film nucleation through cluster–surface collisions is examined using molecular dynamics simulations with a well-known reactive empirical-bond order potential for hydrocarbons. It has previously been shown through experiments that strongly adhering polymer thin films can be generated through the impact of organic clusters with mica, diamond, or glass surfaces. In this study, molecular clusters of ethylene and acetylene are impacted at hyperthermal velocities of about 1.3 keV per cluster against activated bare 1×1 diamond (111), reconstructed 2×1 diamond (111), and hydrogen-terminated diamond (111) surfaces. As a result of the collisions, the cluster molecules react with each other and the surface nucleating thin hydrocarbon films. The effect of surface and molecular reactivity on thin film nucleation and structure is discussed.

Internal excitation of ethanol molecules emitted during cluster-surface collisions

Results obtained from experimental studies of ethanol cluster collisions with graphite surfaces are presented. The initially cold clusters are heated during surface contact which induces fragmentation. The internal temperature of emitted ethanol molecules is found to equal the surface temperature up to 600 K. For surface temperatures of 600–1400 K, the internal temperature is close to 600 K independent of surface temperature. Angular and velocity distributions are well described by a model in which molecules evaporate from parent clusters gliding along the surface. The cluster decomposition dynamics are discussed and the results are compared with earlier studies of cluster-surface interactions.