Low Energy Imaging of Nonconductive Surfaces in SEM

Abstract

If any not absolutely smooth and homogeneous nonconductive surface is observed in SEM, a locally variable charging appears which produces local electrical fields that destroy the image. The surface charge depends on the total electron yield σ, i.e. the number of outgoing electrons divided by the number of the incoming ones, on the primary current, resistance between the field of view and the earth, surrounding gas pressure and time. A charge balance is established between primary and emitted electron currents, the specimen current taking charges off the surface, and the charged particle bombardment from the atmosphere, which can be neglected in high vacuum, σ changes with the landing energy of primary electrons: near to zero energy, σ < 1, and the surfaces charge negatively. For nearly all materials, between two characteristic energy values E I and E II , σ exceeds unity and the charge is positive, while above E II , σ < 1 again and a negative charge appears. E I can be found usually between 80 and 300 eV and E II amounts to one or a few keV and depends on the electron impact angle. A novel method is proposed consisting in approaching one of the critical landing energies E I or E II where σ → 1 and the surface charge formation is suppressed to a level corresponding to the intensity of spontaneous discharging. The retarding field principle of low energy SEM realization, which minimizes the point resolution changes with the landing energy, makes both E I and E II equally well accessible. In order to find the critical energy without getting the specimen intensively charged, we recommend to measure, quickly enough from the point of view of the charging process duration, the time dependence of the signal in a single pixel by stepwise approaching the landing energy of the slowest change, i.e. the acceptably approximated critical energy. A model of the image signal vs. time development is presented, together with a description of algorithms and software for the critical energy measurement. The preliminary results presented confirm our basic ideas regarding the method.