Scanning electron microscopy of nonconductive specimens at critical energies in a cathode lens system

Abstract

Summary:A method for scanning electron microscopy imaging of nonconductive specimens, based on measurement and utilisation of a critical energy, is described in detail together with examples of its application. The critical energy, at which the total electron yield curve crosses the unit level, is estimated on the basis of measurement of the image signal development from the beginning of irradiation. This approach, concentrated onto the detected signal as the only quantity crucial for the given purpose of acquiring a noncharged micrograph, evades consequences of any changes in an irradiated specimen that influence the total electron yield curve and possibly also the critical energy value. Implementation of the automated method, realised using a cathode lens-equipped scanning electron microsope (SEM), enables one to establish a mean rate of charging over the field of view and its dependence on the electron landing energy. This dependence enables one to determine the energy of a minimum damage of the image of the given field of view. Factors influencing reliability and applicability of the method are discussed and examples of noncharged micrographs of specimens from both life and material science fields are presented.

Imaging of specimens at optimized low and very low energies in scanning electron microscopes

“…This optimisation method seems to have been proved in practice and even implemented in automatic procedure to a computer-controlled SEM (Frank et al 2001).…”

“…The beam is directed towards a pixel not illuminated before, and the time development of the pixel signal immediately begins to be registered. When this is performed for a span of energies, the search for energy with minimum signal change leads towards the higher energy of the critical ones (Frank et al 2001). The example given in Figure 10 demonstrates this method: Figure 10b is taken at the critical energy, so no significant charging appears.…”

Simulations and measurements in scanning electron microscopes at low electron energy

“…However, the method of reducing the beam energy does seem to have the effect of reducing charging and a method which measures the mean rate of charging and its dependence on landing energy was introduced by Frank et al . (). The method enables the energy for minimum damage within a given field of view.…”

Charging in scanning electron microscopy “from inside and outside”

“…In scanning electron microscopy, it has been known for a long time that imaging of insulating surfaces presents difficulties because of the so-called charging effects (Brunner and Schmid 1986, Frank et al 2000, Girard et al 1992, Joy 1989, Plies 1994, Reimer 1985, Ura 1998, among many others). To minimize these spurious effects and more generally to find the sign of charging, the approach called "total yield approach" is widely used.…”