Sputtering simulations

These are but a few of the scenarios being opened up by the implementation of classically based simulations. Indeed, the effectiveness of the theory used also allows for the irradiation-induced diffusion, segregation, and ripple topography growth apparent on and within various single and multicomponent substrates to be modeled (Liedke, et al. 2013). Note Although quantum mechanics would provide the more correct theoretical description, sputtering simulations based around such an approach would be far more expensive computationally with little benefit added. 

To examine the soUd as it approaches equUibrium (atom energies of 0.025 eV) requires molecular dynamic simulations. Molecular dynamic (MD) simulations foUow the spatial and temporal evolution of atoms in a cascade as the atoms regain thermal equiUbrium in about 10 ps. By use of MD, one can foUow the physical and chemical effects that induence the final cascade state. Molecular dynamics have been used to study a variety of cascade phenomena. These include defect evolution, recombination dynamics, Hquid-like core effects, and final defect states. MD programs have also been used to model sputtering processes. 

Simulations relying on short-range interactions Ion-induced sputtering... 

Molecular dynamics simulations yield an essentially exact (within the confines of classical mechanics) method for observing the dynamics of atoms and molecules during complex chemical reactions. Because the assumption of equilibrium is not necessary, this technique can be used to study a wide range of dynamical events which are associated with surfaces. For example, the atomic motions which lead to the ejection of surface species during keV particle bombardment (sputtering) have been identified using molecular dynamics, and these results have been directly correlated with various experimental observations. Such simulations often provide the only direct link between macroscopic experimental observations and microscopic chemical dynamics. 

SIMULATIONS RELYING ON SHORT-RANGE INTERACTIONS ION-INDUCED SPUTTERING... 

In secondary-ion mass spectrometery (SIMS) and its sister technique fast atom bombardment mass spectrometry (FARMS), a surface is bombarded with energetic particles, and the kinetic energy of the particles converts substrate and chemisorbed atoms and molecules to gas-phase species. The ejected (or sputtered) material is subsequently interrogated using various analytical tools, such as lasers and mass spectrometers, to indirectly deduce information about the initial surface. The relationships between sputtered material and the surface, however, are not always clear, and erroneous conclusions are easily made. Computer simulations have demonstrated that a fundamental understanding of the sputtering process is required to interpret experimental data fully ... 

Once the initial and boundary conditions are specified, the classical equations of motion are integrated as in any other simulation. From the start of the trajectory, the atoms are free to move under the influence of the potential. One simply identifies reaction mechanisms and products during the dynamics. For the case of sputtering, the atomic motion is integrated until it is no longer possible for atoms and molecules to eject. The final state of ejected material above the surface is then evaluated. Properties of interest include the total yield per ion, energy and angular distributions, and the structure and... 

Stability of sputtered molecules. In order to further mimic experimental conditions, many trajectories are evaluated by choosing an ensemble of impact points for the energetic particle within the surface unit cell. The experiments with which the simulations are compared are performed so that the majority of the bombarded surface is undamaged This makes direct comparisons between the simulated and experimental results possible. 

Molecular dynamics simulations have yielded a great deal of information about the sputtering process. First, they have demonstrated that for primary ion energies of a few keV or less, the dynamics which lead to ejection occur on a very short timescale on the order of a few hundred femtoseconds. This timescale means that the ejection process is best described as a small number of direct collisions, and rules out models which rely on many collisions, atomic vibrations and other processes to reach any type of steady state . Within this same short-timescale picture, simulations have shown that ejected substrate atoms come from very near the surface, and not from subsurface regions. 

In a similar study, Lo et al. have compared the characteristics of atoms sputtered from copper surfaces in simulations which used both pair-additive potentials and EAM potentials Significant differences were found for many... 

The NEB method has been applied successfiilly to a wide range of problems, for example studies of diffusion processes at metal smfaces, multiple atom exchange processes observed in sputter deposition simulations, dissociative adsorption of a molecule on a smface, diffusion of rigid water molecules on an ice Di siuface, contact formation between metal tip and a smface, cross-slip of screw dislocations in a metal (a simulation requiring over 100,000 atoms in the system, and a total of over 2,000,000 atoms in the MEP calculation), g d diffusion processes at and near semiconductor smfaces (using a plane wave based Density Fimctional Theory method to calculate the atomic forces). In the last two applications the calculation was carried out on a cluster of workstations with the force on each image calculated on a separate node. 

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