Accurate focused ion beam sculpting of silicon using a variable pixel dwell time approach

This work demonstrates accurate focused ion beam sculpting of micron-scale curved shapes into initially planar solids. Sculpting is accomplished by varying the dose per pixel within individual boustrophedonic scans and accounting for the material-specific angle-dependent sputter yield and the ion beam spatial distribution. We refine this technique by demonstrating how a range of preferred dwell times leads to improved sculpting. An optimized dwell time range is delineated by two effects. Excessively large dwell times lead to enhanced deposition of ejected species, asymmetric milled features (when symmetric features are intended), and depths greater than intended values. These effects occur for dwell times such that the depth removed per pixel in a given scan is on the order of the width of the focused ion beam. On the other end of the dwell time range, inordinately low times lead to undesired ion milling outside targeted areas. Milling outside targeted regions, such as a circle or an ellipse, can occur because the ion beam is retraced to a rectilinear frame bounding the area. When dwell times are chosen to be on the order of the time to transit from the rectilinear frame to an outlined area edge, this leads to a significant dose over unintended areas, thereby producing a feature with irregular boundaries. Despite these two effects, a large range of acceptable dwell times (approximately three to four decades) can be established for milling most curved shapes. Hemispherical, parabolic, and sinusoidal features are demonstrated in Si(100). (c) 2006 American Vacuum Society.