Surface Modification Energized by FIB: The Influence of Etch Rates & Aspect Ratio on Ripple Wavelengths
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0960-N10-02-LL06-02
Surface Modification Energized by FIB: The Influence of Etch Rates & Aspect Ratio on Ripple Wavelengths Warren MoberlyChan CMS, Lawrence Livermore National Laboratory, L-367, 7000 East Ave., Livermore, CA, 945509234
ABSTRACT Ion beams have been used to modify surface topography, producing nanometer-scale modulations (and even subnanometer ripples in this work) that have potential uses ranging from designing self-assembly structures, to controlling stiction of micromachined surfaces, to providing imprint templates for patterned media. Modern computer-controlled Focused Ion Beam tools enable alternating submicron patterned zones of such ion-eroded surfaces, as well as dramatically increasing the rate of ion beam processing. The DualBeam FIB/SEM also expedites process development while minimizing the use of materials that may be precious (Diamond) and/or produce hazardous byproducts (Beryllium). A FIB engineer can prototype a 3-by-3-by-3 matrix of variables in tens of minutes and consume as little as zeptoliters of material; whereas traditional ion beam processing would require tens of days and tens of precious wafers. Saturation wavelengths have been reported for ripples on materials such as single crystal silicon or diamond (~200nm); however this work achieves wavelengths >400nm on natural diamond. Conversely, Be can provide a stable and ordered 2-dimensional array of 80°) creates "steps" [11, 13], and even at 90° some small steps form. These steps may trap impinging atoms, which crystallize (Fig. 4b and 4c). Ga ion etching also leads to small ripples on the surface, as viewed by HR-TEM. Wavy features can be common in a HR-TEM image due to astigmatism. The crystalline precipitates are fortuitous as they provide a location to make sure the stigmation is not present. Thus the ripples become visible everywhere between precipitates. The electron diffraction is more insightful of the long-range order of ripples, as it is acquired from a >2 micron area and exhibits an arc at a d-spacing of 380pm. (Diffraction rings of the polycrystalline SiC substrate provide an internal calibration in Fig. 4d to measure the ripple spacing, however, ripples only exist on the CVD-carbon region.) The polycrystalline beryllium surface is FIBed normal to the surface; however polishing irregularities mean the FIB ranges +/-3° in different locations of the surface. Because the low density of Be enhances ion channeling, the surface modulations that form are dependent on grain orientation. A well-ordered 2dimensional array of dots form on one grain, but a less-ordered set of ripples form on the next grain (Fig. 5a). A third grain exhibits "pits" appearing almost the geometric inverse of the dots. Wavelengths also change near a grain boundary. However, the different etch rate of each grain orientation means that a height difference now exists at the grain boundary, and ripple wavelength near walls may increase due to redeposition [13]. FFT processing (Fig. 5c) of SEM images acquired normal to the surface, quantifies long-range
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