The imaging resolution of a focused ion beam (FIB) system is perhaps more difficult to measure even than that of a scanning electron microscope, especially with a heavy-ion (Ga, e.g.) FIB, because specimen erosion by sputtering is important [1,2]. Because of this beam size is sometimes used as measure of FIB beam quality instead of imaging resolution. A beam size measurement usually consists of a rise distance experiment where the beam is swept across a discontinuity such as a knife-edge; the change in current on the knife-edge is measured as a function of beam position and used to define a beam size. Although this is apparently a straightforward measurement there are numerous possibilities for errors, among which are statistical effects, specimen erosion, distortion due to implantation and other kinds of specimen damage. It is important to take these errors into account to avoid a misinterpretation of the result of the measurement and an incorrect estimate of the beam size. We consider some aspects and difficulties of the rise distance method, and some of the errors that can be encountered as a result of beam statistics and spatially varying secondary electron yields. Based on experiment and simulation, we find for a given beam and specimen that the result of a rise distance measurement can vary by a factor of three (3) due to statistical effects (beam noise) alone.
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