Sputtering limits versus signal-to-noise limits in the observation of Sn balls in a Ga+ microscope

Abstract

In principle, a scanning ion microscope can produce smaller probe sizes than a scanning electron microscope because the diffraction contribution is smaller. However, the imaging resolution is often severely limited by the sputtering damage. In this article, an experimental procedure to establish the limit of a focused ion beam system for imaging purposes is proposed. The procedure is based on the observation of the change in geometry (i.e., shrinking) of the features in a Sn-ball sample imaged with a Ga+ beam. Plots of the balls’ diameter versus the irradiation time give a straightforward visual evaluation of the time allowed for the observation of a single feature before the removal of material due to the ion bombardment becomes unacceptable. For each particle, the curve, together with the error band connected with the imaging process, gives the values of uncertainty/resolution due to the two competing processes, collecting of information (for example, from secondary electrons) and damaging of the target. A plot of the uncertainty that is derived from these two processes for different sampling times allows the determination of the limiting factor of the imaging mode in use, and, ultimately, the highest possible resolution obtainable with a given machine for the observation of a certain sample. Together with simulations and theoretical studies, the described procedure will be able to confirm the effectiveness of the new ion sources that are currently being developed.