The performance metric of greatest interest to the user of a focused ion beam (FIB) system is generally its resolution. Because of the difficulty in defining and measuring the resolution of a FIB system directly, its performance is often assessed using a method related to the beam quality instead. This consists of the measurement of the rise distance of the beam current as the beam passes across an edge, which, for low currents where spherical aberration can be neglected, is closely related to the full width at half maximum (FWHM) of the current density of the ion beam. The edge, also known as the “knife edge,” corresponds to a sharp discontinuity in a specimen, as can be practically found on the surface of a graphite specimen. Because the rise distance can be used to obtain an idea of the dimension of the waist of a beam, it is, perhaps, an indication of the quality of an instrument. Because the rise distance depends on the quality of an edge, it is sometimes called edge sharpness. This concept bears similarities with the image sharpness method developed to assess the performance of SEMs, usually on gold nanoparticles on carbon specimen. Rise distance is actually a convolution of the current density distribution with the properties of the knife edge and depends strongly on the spatial distribution of the secondary electron yield of the edge. By using the rise distance, different systems can be compared in a quantitative way. To compare instruments, the identical specimen must be used and the measurements must be done in an identical way. This article discusses the method and some pitfalls in its application.
Far-Field Plume Characterization of a 100-W Class Hall Thruster