Figure 1971 shows the structure of the electron probe-forming system in STEM mode in JEOL JEM-2010F TEMs. A setup, consisting of electrostatic gun lens and twin condenser lens system, controls the de-magnification of the Schottky field emission source.
Figure 1971. Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs.
Modern TEM systems typically have two condenser lenses. Operaters can normally control both lenses by adjusting the voltages of these lenses to alter the focal length of the beam crossover that follows each lens. The first condenser lens, CL1, lies close to the electron source with only the beam deflector coils separating them as shown in Figure 1971. The CL1 lens condenses, or demagnifies, a focused image of the electron source onto a plane somewhere before the second condenser lens CL2 (actually named CL3 in the JEOL interface) and condenser aperture. Excitation of the CL1 lens causes beam crossover above the aperture, and thus spreads the beam at the level of CL2 and causes a decrease in beam current and intensity.
Table 1971. Functions of CL1 and CL2 condenser lenses.
|Condenser lens 1
||Changes spot size
|Condenser lens 2
||"BRIGHTNESS" knob, which adjusts the
focal length of beam crossover to above or below the specimen