Use of electrostatic objective lens in an electron microscope

The invention relates to the use of a scanning electron microscope column equipped with a electron source for producing a beam (100) of electrons, an electrostatic immersion objective lens for focusing the beam on a sample (102), and a detector (116) at the source side of the objective lens for detecting electrons emerging from the sample, the detector showing a hole for passing the beam, the detector at least partly surrounding the beam, the lens equipped with a first electrode (110) through which the beam enters the lens; and a second electrode (106) through which the beam leaves the lens, the first electrode and the detector electrically biased with respect to the second electrode, the objective lens showing an angular magnification M ± for electrons emerging from the sample, a first part of the electrons travelling to the detector passing through the hole of the detector and a second part of the electrons travelling to the detector intercepted by the detector. Such a lens tends to show a low detection efficiency near the axis. Inventors found that, by applying a moderate positive bias to the sample, this could be remedied.