A majority of the scanning electron microscopes (SEMs) now in use are probably employed as low voltage SEMs (LVSEMs), that is to say they are operated to produce beams with energies below 5keV. This trend away from the more conventional mode of operation at 20 or 30keV has gathered momentum over the past decade and has been driven by both theoretical and practical considera-tions.

Firstly, the distance travelled by an electron falls rapidly (in fact as about E1.6 ) as the incident ener-gy E is reduced. Images generated by low energy electron beams therefore contain enhanced surface information compared to those images recorded at higher energies. Since surfaces are of great inter-est in both the life sciences and in materials science this has been a persuasive factor. Secondly, both the secondary and the backscattered electrons now come from essentially the same interaction volume, rather than from volumes which are widely different in size and shape.