out-lens objective lens, conventional objective lens

The out-lens objective lens is a most widely used objective lens since the advent of SEM, which is rotationally symmetric with respect to the optical axis of an electron beam. As shown in Figure, A magnetic field is created concentrating at the opening of the yoke (indicated by yellow region).
Since a specimen is placed at a position about 10 mm below the lens or more, a large working distance is created. Thus a large specimen like a semiconductor wafer can be observed at large tilt angles. And an X-ray spectrometer (EDS, WDS), an EBSD detector, or a various-type of stage for heating, stretching or cooling can be installed. Magnetic materials can be observed and analyzed without the magnetic field of the lens. However, since the lens excitation is weak due to the long working distance, the probe size on the specimen is relatively large because lens aberration is not small. Thus, the objective lens is not suitable for high-resolution observation.

Fig. Schematic of the out-lens objective lens