Ultralow voltage imaging using a miniature electron beam column

Abstract

Miniature columns that are fabricated from silicon using advanced micromachining processes are a relatively new class of electron beam column. The defining characteristics of these columns are a thermal field emitter source, low voltage operation (typically <3 keV), simple design (two lenses, no crossover), microfabricated lenses, and all electrostatic components. Current production versions of miniature columns achieve <10 nm resolution at 1 keV, and have demonstrated <7 nm resolution at higher beam energies [Spallas et al., J. Vac. Sci. Technol., B 24, 2892 (2006)]. The resolution of miniature columns is limited by competing requirements to minimize optical aberrations, relax physical or geometric constraints (e.g., working distance and electrode–electrode distance), and maintain manufacturable mechanical tolerances of the column components (e.g., alignment, diameter, and placement). In this paper, the authors investigate imaging using a miniature electron beam column in a commercial field emission electron microscope equipped with a retarding field lens. Using a retarding field allows high-resolution imaging at landing energies lower than are possible by reducing the beam voltage because the aberrations are lower. The authors show that for the same landing energy, the resolution is better when using the retarding field. High-resolution imaging at landing energies down to 50 eV and imaging is down to 10 eV are demonstrated. Imaging nonconductive samples at landing energies less than the stable unity yield energy is not expected to be beneficial, yet the authors show that in some cases the ability to image nonconducting samples improves. Finally, the authors image a gas shale sample to demonstrate energy dependent contrast.