Strategies for low- and very-low-energy SEM

Motivations leading to lowering the primary beam energy in the SEM are summ arized and arguments are presented in favour of extending the operation ran ge down to tens and units of eV. Possibilities to produce beams at low ener gies below 1 keV and very low energies below 50-100 eV are reviewed, partic ularly the principle to keep the primary beam travelling fast within the co lumn and to decelerate it near the specimen. Both possible retarding field elements, an electrostatic immersion lens and a cathode lens, are compared in detail as regards important parameters of microscopes containing them. T hese include imaging parameters and aberrations, electric field on the spec imen surface, possible electrical and mechanical configurations :including the detection principles, and sensitivity to stray fields.