Kinetics of Surface Roughening and Smoothing During Ion Sputtering
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ABSTRACT We have measured the kinetics of roughness evolution during low energy ion sputtering of SiO 2 surfaces using in situ X-ray reflectivity. Sputtering with heavy ions (Xe) leads to rapid roughening of the surface that can not be explained by a simple random removal process. Subsequent bombardment with light ions (He, H) leads to an exponential decrease in the surface roughness. These kinetics are explained quantitatively by a linear model that contains a balance between smoothing by surface diffusion and viscous flow and roughening by sputter removal of material. A curvature dependent sputter yield leads to amplification of a limited range of spatial frequencies on the surface and the formation of a ripple topography.
INTRODUCTION The evolution of surface morphology during sputtering is determined by the interaction of multiple processes. Some of these processes, such as defect production by the incident beam, tend to make the surface rougher. Others, such as surface diffusion and viscous flow, tend to make the surface smoother. The dynamic evolution of the surface can provide important insight into the interaction of these various surface processes, while fundamental understanding of this interaction can improve our ability to control surface morphology on the atomic-level. Ion bombardment of surfaces is a common phenomenon in many film processing environments, including sputter etching and deposition, ion beam assisted growth, reactive ion etching, and plasma assisted chemical vapor deposition [1]. Macroscopic roughening of surfaces by sputtering has long been observed and attributed primarily to variations in sputter yield with ion angle of incidence, surface contamination, or grain structure [2]. Cones, ripples, and other large scale periodic structures are often observed after high fluence sputtering. These structures have been observed on metals, semiconductors and amorphous materials for heavy ion bombardment at off-normal incidence. Development of ripple structure limits depth resolution in sputter depth profiling by secondary ion mass spectrometry [3].
Ion irradiation has also been observed to make surfaces smoother, particularly for the case of high energy ion irradiation, where the sputter yield is small. In the case of metals this has been
attributed to surface diffusion [4], while for silica glass surfaces, viscous flow has been proposed as the transport mechanism responsible for smoothing [5]. Likewise, irradiation of bulk Si0 2 with electrons, neutrons and high energy ions has been observed to induce bulk relaxation of the solid, which has also been attributed to viscous flow [6,7]. More recently the development of atomic scale surface morphology on atomically smooth
surfaces has been investigated. Sputtering of single crystal graphite surfaces has recently been investigated in great detail by scanning tunneling microscopy [8,9]. Krim, et al, [10] have recently reported STM measurements of sputter induced roughening of epitaxial Fe films which exhibit self-affine fractal character.
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