A CONDENSER OBJECTIVE LENS WITH ASYMMETRIC POLEPIECES TO FACILITATE THE EXTRACTION OF SECONDARY AND AUGER ELECTRONS

A new condenser objective lens for a scanning transmission electron microscope especially designed for through-the-lens extraction of secondary and Auger electrons is presented. The optimal axial field to extract the secondary and Auger electrons is a field that falls off as 1/z2. This field form has been realized by a new lens design with one sharp polepiece and a second, carefully shaped, polepiece. This polepiece consists of two magnetically separated parts. The bore in this polepiece measures 20 mm while the working distance is 10 mm. The spherical aberration on the condenser side is 2.5 mm for an acceleration voltage of 125 kV while, with the same lens excitation, the projector side spherical and chromatic aberrations are 1.1 and 1.5 mm, respectively. All (Auger) electrons with an energy up to 1000 eV emerging from the specimen can be concentrated in a disk of 4.2 mm and a maximum opening angle of 6-degrees for further energy analysis.