Diplomarbeit Diplom-Ingenieur - Institut für Halbleiter

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The origin of the third contrast mechanism is interference of differently diffracted
e-beams. This contrast allows to resolve lattice fringes in High Resolution
Microscopy which is treated in section 3.4.
Figure 3.4: In regions with low mass thickness more electrons are forward scattered to
small angles and can be collected by the objective aperture. This causes a higher intensity
in the formed image. In higher mass thickness regions more scattering events occur
causing darker regions in the image. [32]
3.3.2 Dark Field and Bright Field imaging
Since the diffraction pattern of the crystal is reproduced in the focal plane of the
objective lens and the objective aperture is located in the same plane, a distinct
diffraction spot can be chosen in the DP mode for real image formation. To do that all
other spots are masked by the aperture and only the electrons of one beam are
collected by the intermediate lens. BF images are formed by collecting only the
electrons of the (000) peak by the objective aperture and show strong mass thickness
contrast.
On the other hand, collecting the electrons of any diffraction spot can show
contrast in the real image, whose origin are different lattice parameters or a different
structure form factor. Both of these properties are a part of the extinction distance ξg
(egn. 3.9) which is a determining factor of the amplitude of a scattered beam. This