Nonlinear Stabilization Mechanisms in Amplitude Saturation During Sputter Ripple Formation on Silicon
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Nonlinear Stabilization Mechanisms in Amplitude Saturation During Sputter Ripple Formation on Silicon Jonah Erlebacher1 and Ari-David Brown1,2 1 Department of Materials Science and Engineering, Johns Hopkins University Baltimore, MD 21218, U.S.A. 2 Department of Physics and Astronomy, Johns Hopkins University ABSTRACT Sputter rippling refers to the formation of regular surface patterns during glancing incidence energetic ion beam etching of surfaces, usually as a result of a competition between etching (from the ion beam) and capillary action (driving smoothening via surface diffusion). Many different kinds of morphologies are often observed, including ripples oriented parallel and perpendicular to the projected ion beam direction and “quantum dots” arranged in hexagonal or rectangular arrays. Theoretical analyses of ripple evolution have concentrated on the initial stages of the surface instability leading to pattern formation, and the details associated with the nonlinear mechanisms leading to amplitude saturation and pattern stabilization remain a subject of active interest. The Si(111) surface is a single component surface with isotropic surface diffusion kinetics; for these reasons, this system provides a useful probe of surface evolution without complicating effects of compositional inhomogeneities and anisotropic terrace diffusion. Our examination of the Si(111) surface indicates that step-step interactions may play an important role in the evolution of sputter ripples in this system. To argue for this conclusion, a comparison with sputter ripple evolution on Si(001) is made. INTRODUCTION Sputter rippling refers to the formation of regular surface patterns during off-normal energetic ion beam etching of surfaces. Figure 1 shows an AFM micrograph of a representative example of sputter ripples formed on Si(111); experimental details are in the caption. The sample illustrates some generic features of sputter ripple evolution, characteristics both positive and negative from the vantage point of nanostructure fabrication: •
Ripples form spontaneously. As discussed in detail below, these patterns result from a competition between roughening due to material removal by the ion beam in competition with surface relaxation due to surface or near surface mass transport.
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Ripples sometimes possess well-ordered, well-defined topography. In the particular ripples shown in Figure 1, one sees that ~500 nm wavelength ripples are highly aligned, although not perfectly. Perfection is broken in two ways. First, there are edge-dislocation like defects, approximately 1 every 100 microns2. Second, there is a background of larger scale features that appear to be ripples oriented perpendicular to the highly ordered ripples.
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Ripples form in very narrow ranges of experimental parameter space. Generally, the competition between roughening and smoothening that leads to ripple formation is a sensitive function of both sample temperature and characteristics of the ion beam. For Si(001) and Si(111) and ion energies
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