Comparison between CTEM and STEM
- Practical Electron Microscopy and Database -
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This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.

 

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In general, a low voltage STEM is a hybrid instrument with the features of SEM and TEM with a convergent probe, while a high voltage TEM is a hybrid instrument with the features of TEM with both a parallel beam (for TEM function) and a convergent probe (for STEM function).

Table 4113a. Comparison between CTEM and STEM. 

 
CTEM
STEM
 ===============================  ===============================
Full name Conventional TEM  Scanning TEM
Basics Interference of coherently scattered electron waves Incoherent scattering
Electron source LaB6 (or FEG) FEG
Electron beam Wide-beam technique; a close-to-parallel electron beam Fine focused beam, formed by a probe forming lens
Positioning of electron beam Not controlled Highly controlled
Recording method Parallel  Serial
Recording time 0.5 – 2 s 5-20 s
Illumination area The whole area of interest Probe is scanned across the specimen
Problem due to sample stability and drift Less sensitive More sensitive
Main factor affecting image resolution The imperfections or aberrations in the objective lens The beam diameter generated by the probe-forming lens, also limited by aberrations
Benefit of aberration correction

Not significant

The best; most sensitive to spatial resolution
Typical point resolution 2 Å 2 Å
Obtainable information Atomic positions and elemental distribution Atomic positions and elemental distribution
Image interpretation Compared with simulations; atom columns dark at Scherzer defocus Direct interpretation: atom columns always bright; intensity ~Z1.7-2 (Z-contrast)
Imaging geometries CTEM Reversed illumination of TEM
     

Figure 4113a illustrates the comparison of the main contrasts in both CTEM and STEM modes.

Comparison of the main contrasts in both CTEM and STEM modes

Figure 4113a. Comparison of the main contrasts in both CTEM and STEM modes.

Figure 4113b shows the optical schematics of defocused TEM and STEM. The aberrations in STEM mode can be detected and corrected using the symmetry in the Ronchigram that includes the azimuthal and the radial circles of infinite magnification [1]. The solid lines represent the electron scattering direction at the radial circle in STEM mode, corresponding to the caustic curve in TEM mode. The azimuthal circle (broken line) is related to the angle of focusing on the specimen, i.e. part of the highly scattered electrons, which cross the optical axis on the specimen.

optical schematic of defocused TEM and STEM

Figure 4113b. The optical schematics of defocused TEM and STEM. Adapted from [3]

Figure 4113c shows the schematic comparison between contrast transfer functions (CTFs) for a parallel-beam CTEM and a STEM. For TEM imaging, the higher frequencies should be excluded by a proper objective aperture marked by the red arrow because they introduce contrast reversals. For STEM imaging, a proper probe-forming aperture should be applied to match the need of the spatial frequency marked by the green arrow.

contrast transfer function (CTF) for: (a) A conventional, parallel beam, CTEM and (b) A STEM

Figure 4113c. Schematic illustration of contrast transfer function (CTF) for: (a) A parallel-beam CTEM and (b) A STEM.

It can be considered that the coma-free alignment in TEM mode corresponds to STEM alignment with Ronchigram in STEM mode, that is the Ronchigram can be used to detect coma aberration in a probe-forming lens (the objective lens in STEM) [2]. Therefore, the STEM coma-free alignment is carried out by coinciding the aperture center with the center of the azimuthal- or radial-circle. Note that the Ronchigram locates on the diffraction plane.

Table 4113b. Equivalent functions in TEM and STEM modes.

CTEM
STEM
Caustic curve centering Aperture alignment with center of radial circle
Alignment of bright-field disk with aberration-free point Aperture alignment with center of azimuthal circle
Coma-free alignment based on caustic image Alignment using Ronchigram

 

 

 

[1] N. Dellby, O.L. Krivanek, P.D. Nellist, P.E. Batson, A.R. Lupini, J. Electron Microsc. 50 (2001) 177.
[2] J. A. Lin, J. M. Cowley, Ultramicroscopy 19 (1986) 31.
[3] Koji Kimoto, Kazuo Ishizuka, Nobuo Tanaka, Yoshio Matsui, Practical procedure for coma-free alignment using caustic figure, Ultramicroscopy 96 (2003) 219–227.

 

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