Anticipated performance of achromatic quadrupole focusing systems when used with liquid metal ion sources

Abstract

Current focusing systems for producing finely focused ion beams from liquid metal ion sources (LMIS) using axially symmetric electrostatic lenses are limited by chromatic aberration due to the relatively high (∼10 eV) energy spread inherent in LMIS. The highest current densities achievable are in the range of 1–5 A/cm2 at energies above 20 keV. In this paper, we discuss the use of achromatic quadrupole lenses to overcome the limitation due to energy spread. These lenses require both magnetic and electrostatic focusing elements. Lens acceptance angles are limited by the electrode positions and by the necessary magnetic gradients. The coefficients relating image distance to lens excitation of some of the elements in the quadrupole multiplet are found to be much larger than in electrostatic systems. These factors combine to require excitation stabilities as high as 1 part per million in order to achieve current densities more than 10 A/cm2, although such performance can be expected at beam energies of 5 keV or below.