SEM – Comparision of Detectors

SEMs are frequently equipped with two detectors for secondary electrons: an in-lens detector and a conventional secondary electron detector.

The in-lens detector is located inside the electron column of the microscope and is arranged rotationally symmetric around the optical axis. Due to a sophisticated magnetic field at the pole piece, the secondary electrons are collected with high efficiency. In particular at low voltages and small working distances, images with high contrast can be obtained. Besides information about morphology and surface topography, the in-lens detector images differences in the work function (e.g., electronic variations) on the sample with high lateral resolution. On the other hand, in the images recorded by the conventional secondary electron detector, the topographic information is dominant.

Example

SE_inlens — Secondary electron images of graphite electrode material obtained with the conventional secondary electron detector (left) and the in-lens detector (right) (LEO 1530 Gemini, operated at 1 keV). While the contrast is rather homogeneous in the left image, pronounced differences are present in the in-lens image: the conducting graphite (low work function) appears with bright contrast whereas the separator, a fibrous polymer, appears black (high work functions; almost no electrons are leaving the surface there at the selected conditions). Sample: H. Buqa, P. Novak, PSI

SEM basics | SE imaging | Dependence on voltage