Parallel Illumination in TEM
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Spatial coherency originates from the fact that the illumination is never perfectly parallel and is actually slightly convergent at the best imaging condition, which can be described as having a distribution of different illumination tilts. The effect of illumination tilting on the phase contrast transfer function (PCTF) is to cause a phase shift about the y-axis and, in the same way as for the temporal coherency, the PCTF is averaged over this tilt distribution.

The condenser lens system provides variable probe-convergence angles in STEM mode and adjustable parallel illumination in TEM mode. In principle, the diffraction spots become infinitely small when the specimen is illuminated by a parallel electron beam. The schematic illustrations in Figure 3694 show the convergent illumination configurations of various modes in TEMs. In the CTEM condition in Figure 3694 (a), the condenser mini-lens (CM lens) is strongly excited, and incident electrons are focused on the pre-focal point of the objective pre-field, resulting in a parallel illumination on a wide area on the specimen and providing highly coherent electron illumination. In the EDS condition in Figure 3694 (b), the CM lens is turned off and the incident electrons are focused on the specimen by the objective pre-field, resulting in a small-probe illumination. In this case, the illumination angle (α1) is large so that high beam intensity is obtained for a small area in the analytical EDS method. In the NBD mode in Figure 3694 (c), a smaller condenser aperture is used to form a smaller illumination angle (α2). Therefore, a small-diameter probe with relatively high coherence in the illumination is achieved. In the illumination condition in Figure 3694 (c), the illumination angle (α) with a constant probe size can be changed by changing the excitations of the condenser lenses and the CM lens to obtain the incident illumination to form ideal convergent beam electron diffraction (CBED) patterns.

CTEM mode EDS mode 2685a

Figure 3694. Convergent illumination configurations: (a) CTEM mode, (b) EDS mode and (c) NBD mode.

To have a camera length exactly as labeled in the instrument, some key points need to be satisfied as follows:
         i) The specimen should be placed exactly at the Eucentric height in the objective lens.
         ii) A parallel electron beam should be employed.
         iii) The focus of the objective lens should be exactly adjusted to the focus position.
         iv) The focus of the first intermediate (with “Diff focus” knob) should be adjusted at the back focal plane of the objective lens.

Theoretically, due to the large d (the diameter of the specimen over which the illumination is coherent, see page1708), parallel-beam TEM with α ≈ 0 mrad can be used to record lattice fringes with diffraction contrast (so-called HRTEM).

Note that in bright-field (BF) and dark-field (DF) TEM imaging, the specimen is imaged using a parallel incident electron beam(s).