Focused ion beam fabrication of spintronic nanostructures: an optimization of the millingprocess

Abstract

Focused ion beam (FIB) milling has been used to fabricate magnetic nanostructures(wires, squares, discs) from single magnetic layers (Co, permalloy) and spin-valve(permalloy/Cu/Co) multilayers (thicknesses 5–50 nm) prepared by ion beam sputteringdeposition. Milled surfaces of metallic thin films typically exhibit residual roughness, whichis also transferred onto the edges of the milled patterns. This can lead to domain wallpinning and influence the magnetization behaviour of the nanostructures. We haveinvestigated the milling process and the influence of the FIB parameters (incidence angle,dwell time, overlap and ion beam current) on the roughness of the milled surface. It hasbeen found that the main reasons for increased roughness are different sputter yields forvarious crystallographic orientations of the grains in polycrystalline magnetic thin films.We have found that the oblique ion beam angle, long dwell time and overlap < 1 are favourable parameters for suppression of this intrinsic roughness. Finally, we haveshown how to determine the ion dose necessary to mill through the whole thin film upto the silicon substrate from scanning electron microscopy (SEM) images only.