Nonequilibrium Pattern Formation In Erosion Processes

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0108494BarabasiIon-beam sputtering, the removal of material from the surface of solids through the impact of energetic particles, is a widely used thin film processing technique. Due to its relevance to a number of experimental techniques, measuring tools, cleaning and patterning methods, the morphological features of surfaces eroded by ion bombardment have been much investigated of late. On the other hand, ion-beam sputtering is attracting increased interest as a potential candidate for nanoscale surface patterning. Erosion with oblique ion beams has been known to create regular self-organization ripples, potential templates for quantum wires. But the interest in this technique has boomed recently following the recent demonstration that normal sputtering can lead to regular nanoscale islands or quantum dots. The observed high density islands, whose size can be tuned with the ion energy, display a high degree of spatial ordering and form a regular hexagonal lattice. These results signal the emergence of a novel and powerful technique for surface patterning with a number of desirable features that include material independence, tunable feature size, as well as suitability for inexpensive mass fabrication. To turn these advances into a viable patterning technology, we need to develop a better understanding of the fundamental processes that affect the surface morphology. Therefore, the research goal of this grant is to obtain a coherent understanding of the primary physical factors contributing to the morphology of surfaces eroded by ion bombardment. The research will use a combination of numerical and analytical tools to model the morphological evolution during ion bombardment. Since Bradley and Harper have calculated the ripple wavelength in agreement with experiments, it is generally accepted that continuum theories offer a surprisingly useful tool in addressing the surface morphology in ion sputtering. The PI has shown that nonlinear phenomena play a crucial role in determining the long-time features of the surface morphology, being able to account for ripple stabilization and quantum dot formation. These tools will be expanded to obtain better agreement with experiments, aiming to reproduce not only the large scale features, but also the island shapes and spatial ordering. In parallel, discrete microscopic models will be developed to understand the effect of various microscopic processes (surface diffusion, atom detachment, reattachment, etc) on the surface morphology. The simultaneous use of the continuum and microscopic approaches is expected to play a crucial role in uncovering the generic features of both pattern formation and roughening. These tools will be used to investigate several problems of immediate experimental and technological interest. First, preliminary theoretical work indicates that the size dispersion, as well as its dependence of the sputtered quantum dots, depends on erosion time, and exists an optimal time at which the dispersion is minimal. This time dependence of the dispersion on the erosion parameters (such as ion energy, ion cascade parameters and temperature) will be investigated, aiming to determine the optimal condition for island formation. Second, recent experiments have demonstrated that regular islands appear if the surface is rotated. Investigations will be made into the conditions under which the size dispersion benefits from sample rotation. Third, the sputtering yield, a key quantity for various surface characterization methods, is greatly affected by the surface morphology. Methods will be developed to predict the dynamic coupling between the morphology and the yield. This research is expected to lead to a comprehensive comparison between the continuum theory, modeling results and experiments, and could offer critical theoretical guidance to turn ion sputtering into a mature patterning technology. %%%Ion-beam sputtering, the removal of material from the surface of solids through the impact of energetic particles, is a widely used thin film processing technique. Due to its relevance to a number of experimental techniques, measuring tools, cleaning and patterning methods, the morphological features of surfaces eroded by ion bombardment have been much investigated of late. On the other hand, ion-beam sputtering is attracting increased interest as a potential candidate for nanoscale surface patterning. Erosion with oblique ion beams has been known to create regular self-organization ripples, potential templates for quantum wires. But the interest in this technique has boomed recently following the recent demonstration that normal sputtering can lead to regular nanoscale islands or quantum dots. The observed high density islands, whose size can be tuned with the ion energy, display a high degree of spatial ordering and form a regular hexagonal lattice. These results signal the emergence of a novel and powerful technique for surface patterning with a number of desirable features that include material independence, tunable feature size, as well as suitability for inexpensive mass fabrication. To turn these advances into a viable patterning technology, we need to develop a better understanding of the fundamental processes that affect the surface morphology. Therefore, the research goal of this grant is to obtain a coherent understanding of the primary physical factors contributing to the morphology of surfaces eroded by ion bombardment. ***