Marek Sosnowski

Ion cluster beam deposition of thin films

Abstract The idea of using energized ionized clusters of atoms to control the quality of thin film deposition is due to Takagi and his associates at Kyoto University. They have worked to develop and demonstrate the benefits of bringing selected energy and momentum to the surface of a growing solid film without involving any other atomic species than that desired in the film. This technique is related to other ion assisted deposition methods, many of which involve bombardment of a growing film with rare gas ions of selectable energy and current density to improve film density and texture. The ionized cluster beam (ICB) concept involves the supersonic expansion of the atomic vapor of the material of interest from a crucible with a small exit aperture. Species from the expansion (atoms, dimers and larger atomic clusters which may form) are subsequently electron impact ionized and accelerated to a selected energy to bombard the surface of a growing film. ICB has been thought to involve clusters as large as 500 to 1000 atoms. Very high quality films, some of them epitaxial metals on silicon, have been formed with an ICB apparatus. This article reviews the present status of ICB. Based on recent measurements, the number of large clusters is so small that it seems very unlikely they have a dominant influence on the properties of deposited films. The high quality of ICB films seems instead to be due to ion assisted processes involving atomic ions or perhaps ions of quite small clusters.

An investigation of cluster formation in an ionized cluster beam deposition source

We have investigated the concentration of Agn clusters (25<n<1600) in a vapor of Ag expanding from the nozzle of a crucible heated to temperature as high as 1950 K (which corresponds to a silver vapor pressure of about 30 Torr). The goal of the experiment was to determine whether there were enough of these clusters present to have a significant effect on the ion‐enhanced deposition of Ag films, as postulated for ion cluster beam (ICB) deposition. Time‐of‐flight mass spectra were obtained by using an excimer laser at 248 nm (KrF) or 193 nm (ArF) to photoionize a fraction of the vapor. The ions were extracted normal to the direction of average velocity of the neutral species and detected with a Faraday plate after a 20 cm field‐free flight path. At the level of experimental uncertainty, no convincing evidence for the presence of large clusters was found for Ag vapor pressures in the 2–30 Torr range (typical for ICB). We conclude that under the conditions of our experiment, not more than one atom in 5000 is ...

Transient enhanced diffusion from decaborane molecular ion implantation

Transient enhanced diffusion (TED) from implantation of 5 keV B10H14 and 0.5 keV B ions has been quantified and compared for nominal boron doses of 1014 and 1015 cm−2. Boron diffusivity during annealing was extracted from secondary ion mass spectroscopy depth profiles of diffused marker layers in boron doping-superlattices and the actual implanted B dose was independently measured by nuclear reaction analysis. Comparable enhancements were observed from both ions. Transmission electron microscopy analysis revealed that both boron- and decaborane-implanted samples were amorphized at a nominal 1015 cm−2 B dose. A comparison with data from low energy Si implants revealed a similar dependence of diffusivity enhancement on implant dose. These findings are consistent with the understanding that TED is caused by the interstitial supersaturation resulting from a number of excess interstitials approximately equal to the number of implanted atoms which can become substitutional in the silicon lattice. Accordingly, n...

Sputterring and smoothing of metal surface with energetic gas cluster beams

Abstract Sputtering is a process of surface erosion by energetic ions. One of the remarkable effects of sputtering with cluster ions is surface smoothing. To understand its mechanism we have developed a computational model, which utilizes molecular dynamics (MD) to simulate rapid atomic collisions in the central impact zone, and a finite-difference method to account for processes occurring on a longer time scale over a wider target area. A case of gas cluster of a few hundred Ar atoms impacting a Cu target with energy up to 20 keV has been considered. The MD simulation has revealed that the atoms ejected from the surface have a significant lateral momentum component (parallel to the surface) which may have a major effect on surface morphology. Evolution of surface morphology under cluster ion irradiation was described by the modified Kuramoto-Sivashinsky equation. Comparison of the simulations with experimental data shows qualitative agreement.

Surface smoothing with energetic cluster beams

Impacts of energetic clusters, consisting of hundreds or thousands of atoms on a solid surface, have some remarkable effects, one of which is surface smoothing. To understand its mechanism, we have examined the results of molecular dynamics (MDs) simulations of cluster collisions with a solid which revealed that cluster impacts form craters on the surface and that ejected atoms have a significant lateral momentum component (parallel to the surface). It is postulated that these energetic atoms which are not in thermal equilibrium with the surface are responsible for rapid surface diffusion and consequently the surface smoothing. A new model of surface smoothing by energetic clusters has been developed. The model is based on nonlinear dynamics of a surface profile described by the noisy Kuramoto–Sivashinsky equation. A Monte Carlo procedure is used for simulating cluster impacts by distributing on a surface craters whose volume is defined by MDs simulations. The results of calculations show a qualitative ag...

Simulation of cluster impacts on silicon surface

Abstract A new hybrid model, combining Molecular Dynamics (MD) with continuum mechanics and thermodynamics, has been developed for studying collisions of energetic particles with a solid surface. MD describes interaction of atoms in the central impact zone characterized by energetic atomic collisions and non-equilibrium states of matter while the continuum model is applied to a much larger volume outside. Appropriate boundary conditions at the interface of the two regions prevent the appearance of unphysical shock wave reflections. The hydrid model is very efficient in computations as it reduces the number of the systems degrees of freedom by minimizing the size of the central MD zone. The model was applied to collisions of a few keV Ar clusters containing approximately 100 atoms with Si(100) surface. The results show that cluster impacts create craters and local melting and that a number of displaced surface atoms have large lateral velocities. The latter may explain the experimentally observed surface smoothing by cluster bombardment.

Cluster-solid interaction experiments

Abstract Experiments to investigate the interaction of clusters with solids include a number of different types of observations, but with very little systematic study in most of them. In rough order of increasing cluster energy, experiments with non-metallic clusters involve surface cleaning, smoothing of surface topography, sputter erosion, crystallographic damage and impurity doping. The results of these experiments are different than if individual atoms of equivalent velocity had been incident on the solid. Studies of the impingement of metallic clusters on solids have been motivated primarily by interests in the deposition of thin films. The suggested advantages of clusters in this role are only beginning to be established experimentally.

Epitaxial growth of Cu films on Si by ionized cluster beam deposition

The growth of Cu films deposited by ICB method on Si(111) and Si(100) has been studied. Depositions were made in ultra high vacuum with substrates at near room temperature. The film growth was analyzed by in situ RED and XPS measurements. Channeling, x‐ray diffraction, Auger spectroscopy, and scanning ion microscopy measurements were made outside the deposition chamber. The growth of Cu films on Si(111) was initially two dimensional followed above ten monolayers by the appearance of three‐dimensional structures. 100 nm thick films had quite uniform crystal structure with epitaxial relation to the substrate. No intermixing or silicide formation at the interface was seen. Ion acceleration improved the smoothness of the film surface but had no appreciable effect on the film structure or interface. By contrast, in deposition on Si(100) ion acceleration strongly influenced the film structure and intermixing at the interface. Smooth films were obtained at acceleration voltage of 3 kV and higher while more disor...

Decaborane, an alternative approach to ultra low energy ion implantation

Ion beams of decaborane (B/sub 10/H/sub 14/) are used to form ultra shallow p-type junctions in Si. Because the ion energy is partitioned between the atoms of the molecule, B atoms are implanted with only approximately one tenth of the energy of the beam. Thus severe problems created by the space charge of ultra low energy (ULE) ion beams are minimized. Moreover, standard ion implanters equipped with a decaborane ion source may be capable of ultra shallow (tens of nm) implantation of boron. Ionization and ion beam properties of decaborane were studied in the energy range of 2-10 kV. Under proper conditions in the ion source, most of the extracted ions consist of 10 B atoms (B/sub 10/H/sub x//sup +/) and they can be transported through the implanter without significant break-up or neutralization. Boron depth profiles measured by SIMS in Si wafers implanted with B/sub 10/H/sub x//sup +/ and B/sup +/ ions of equivalent energy are the same but it appears that the retained dose achieved with the molecular ions is higher than with the monomer ions for the same B fluence. The effect may be due to a different Si sputtering yield per impinging B atom with the two types of ions. Si wafers with test MOS devices were implanted with decaborane ions and ULE BF/sub 2//sup +/ ions of equivalent energy. Measured device characteristics are very similar. The results confirm the potential of decaborane ion beams as an alternative technology for manufacturing of ultra-shallow p-type junctions in Si.

Irradiation Effects of Ar-Cluster Ion Beams on Si Surfaces

The effects of energetic Ar cluster ion impacts on Si(111) surfaces have been studied for cluster energies up to l5keV. The mean cluster size was about 1000 atoms, and the smaller sizes could be systematically excluded. Si samples irradiated at different cluster ion energies were analyzed by RBS, ellipsometry, and differential reflectometry. Implantation of Ar in samples irradiated with cluster ions was found by RBS to be detectable, but very small in comparison with samples irradiated with monomer ions of the same energy. The thickness of the damage layer as measured by both ellipsometry and differential reflectometry was also much smaller in the cluster ion irradiated samples.