condenser mini lens
condenser mini lens
The condenser mini lens (CM lens), which is placed between the condenser lens and the objective lens, produces an electron beam with a convergence angle suitable for various observation modes (Fig. 1).
When the excitation of the CM lens is strong, parallel illumination of the electron beam is achieved for observations of bright- and dark-field images and HREM (high resolution electron microscope) images (Fig. 2).
On the other hand, when the lens excitation is weak, convergent illumination (small-area illumination) with a large convergence angle is achieved for observations of STEM (scanning transmission electron microscope) images, CBED (convergent beam electron diffraction) and EELS & EDS analyses (Fig. 3).
Fig. 1 Schematic of the positions of the condenser mini (CM) lens (green) and lenses in the vicinity of the CM lens.
Fig. 2 Ray paths of parallel illumination for the different excitation strengths of the CM lens.
The parallel illumination is used to observe transmission electron microscope (TEM) images.
(a) Ray path for low-magnification image observation. A large specimen area is illuminated by a weak excitation of the CM lens.
(b) Ray path for medium-magnification image observation. A smaller area is illuminated by increasing excitation of the CM lens.
(c) Ray path for high-magnification image observation. A small area is illuminated by a strong excitation of the CM lens.
Fig. 3 Ray paths of convergent illumination for the different excitation strengths of the CM lens.
The convergent illumination is used to form a small electron probe on a specimen.
(a) Ray path for STEM image observation, where a probe as small as possible is required. The CM lens excitation is set in the off state.
(b), (c) Ray paths for CBED pattern observation. The CM lens excitation is varied to change the convergence angle (α) of the electron beam or to adjust the size of the CBED disk.
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