Molecular dynamics sputtering

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. 

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. 

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. 

Figure 7.44. Molecular dynamics simulation of the ion bombardment of a Ag crystal surface with a 15 keV Ga+ beam, and a 15 keV Cgo atomic cluster beattL The Cgo beam results in a larger crater and more material removed from the surface. In contrast, the Ga+ beam results in a destructive effect at greater sample depths, without successful sputtering of the sample. Reproduced with permission from Winograd, N. Anal. Chem. 2005, 77, 142A. Copyright 2005 American Chemical Society.

Fig. 15. Predicted probability distribution for sputtered Cu atoms per collision of an incident Ar+ ion. Molecular dynamics calculations axe from Ref. 72.

Kubota, A., and Economou, D. J., A molecular-dynamics simulation of ultrathin oxide films on silicon Growth by thermal O atoms and sputtering by 100 eV Ar ions. IEEE Trans. Plasma Sci. 27,1416-1425 (1999). 

Kubota, N. A., Economou, D. J., and Plimpton, S. J., Molecular-dynamics simulations of low-energy (25-200 eV) argon ion interactions with silicon surfaces Sputter yields and product formation pathways. 7. Appl. Phys. 83, 4055-4063 (1998). 

Sputtering has been modeled using both Monte Carlo and molecular dynamic computer simulations. A review of the simulation literature is given in Eckstein (1991). 

Being able to handle all above systems numerically, the modelling of the Fe AlN sputtering process may now be attempted by a molecular-dynamics approach [333]. To do so, a supercell of 768 bcc-Fe atoms is constructed from... 

Delcorte, A., Garrison, B.J. (2007) Sputtering polymers with buckminsterfullerene projectiles a coarse-grain molecular dynamics study. J. Phys. Chem. C, 111, 15312-15324. 

For an example of the enhancement in sputter yields resulting from large cluster ion impact, see Table 3.2. This lists sputter yields of H2O molecules from a 500-nm film of amorphous ice deposited on a Silver substrate resulting from Au, AU2, Au3 , and Cgo ion impact at 40° with respect to the sample normal as defined via Molecular Dynamics simulations (Szakal et al. 2006). 

TABLE 3.2 Sputter Yields of HjO Molecules as Determined by Molecular Dynamics Simulations from a 500-nm Film of Amorphous Ice on Silver Resulting from the Listed Projectiles all Incident at 40° (Szakal et al. 2006). 

Postawa Z, Czerwinski B, Szewczyk M, Smiley E, Winograd N, Garrison B (2003) Enhancement of sputtering yields due to C-60 versus Ga bombardment of Ag lll as explored by molecular dynamics simulations. Anal Chem 75(17) 4402 1407... 

The most common application of dynamic SIMS is depth profiling elemental dopants and contaminants in materials at trace levels in areas as small as 10 pm in diameter. SIMS provides little or no chemical or molecular information because of the violent sputtering process. SIMS provides a measurement of the elemental impurity as a function of depth with detection limits in the ppm—ppt range. Quantification requires the use of standards and is complicated by changes in the chemistry of the sample in surface and interface regions (matrix efiects). Therefore, SIMS is almost never used to quantitadvely analyze materials for which standards have not been carefiilly prepared. The depth resoludon of SIMS is typically between 20 A and 300 A, and depends upon the analytical conditions and the sample type. SIMS is also used to measure bulk impurities (no depth resoludon) in a variety of materials with detection limits in the ppb-ppt range. 

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