A simple in situ method for optimizing settings for the Einzel lens elements in a focused ion beam.

Abstract

Ion beams have had an incredible impact on research in the past couple of decades. One major reason for this is the continued development of systems having optimal beam currents that allows one to image more clearly at different spot sizes to include higher currents that allow for faster milling. The advancements for Focused ion beam (FIB) columns have developed rapidly due to the computational optimization of lens designs. However, once a system has been produced, the optimal column settings for these lenses may change or simply become obscure. Our work involves regaining this optimization with the newly applied values through a new algorithm, requiring hours, rather than the days or weeks that existing methods require. FIB columns frequently utilize electrostatic lens elements (normally two, condenser and objective). This work presents a method to quickly determine the optimal lens 1 (L1) values for large beam currents (∼1 nA or greater), from a carefully acquired set of images without any detailed knowledge of the column geometry. Each set of images, acquired through a voltage sweep of the objective lens (L2) for a preset L1, is partitioned for its spectral content. The sharpest position at each spectral level is used to assess how close the preset L1 is to the optimal. This procedure is conducted for a range of L1 values, the optimal being the one having the smallest range in spectral sharpness. For a system that has suitable automation in place, the time to optimize L1 for a given beam energy and aperture diameter is ∼1.5 h or less. In addition to the technique for finding optimal condenser and objective lens parameters, an alternative peak determination method is presented.