Micro-objective lens with compact secondary electron detector for miniature low voltage electron beam systems

Abstract

The low voltage scanning electron microscope appears to be attractive for the examination of ultrathin film such as surface contamination. However, achieving high resolution and high secondary electron detection efficiency at, say 100 eV landing energy, meets serious electron optical challenges. We describe a very low aberration micro-objective lens that is combined with an efficient secondary electron detector to address this issue. The objective lens comprises a mirco-einzel lens followed by a retarding region (from 10 000 to 100 V) and a final electrode just above the sample to minimize the electric field at the sample surface. The retarding field (a) lowers aberration of the objective lens and (b) accelerates the secondary electrons to improve the detection. Using computer modeling we have optimized the design to minimize the primary beam diameter and maximize secondary electron collection. For a landing energy of 100 eV the minimum beam diameter is about 10 nm at 96 μm working distance when the beam energy spread is 0.1 eV. About 50% of secondary electrons can be collected by the compact p-n junction detector micromachined to serve also as the bottom electrode of the einzel lens. The depletion region extends from 0.25 to 5 μm at zero bias and experimental results indicate a current gain of more than 2000.