Uniform ion beam milling of a non-flat surface at off-normal incidence and four azimuths

Atom removal from surfaces via bombardment by ion beams--ion beam sputtering or ion milling--is a widely employed technique to form geometric structures in materials. In this work we present and test a new method to achieving uniform material removal from an irregular surface. The method is based on ion milling at off-normal incidence under four consecutive perpendicular azimuthal specimen orientations. Mathematical analysis shows that uniform ion milling of an uneven surface is achieved at the polar angle of incidence theta where the quantity [formula: see text] has its maximum [Y(theta) is the number of removed atoms per incident ion]. The four-azimuth milling procedure is tested successfully for a 30 keV Ga+ beam on a suspended but wrinkled Si3N4/Au bilayer foil and on truncated Si pyramids with slopes up to 40 degrees. Despite the variations in local surface orientations, uniform milling is achieved at a polar incidence angle of 45 degrees and four consecutive azimuths of 0 degrees, 90 degrees, 180 degrees and 270 degrees.