'Mini' Lenses in EMs 
- Practical Electron Microscopy and Database -
- An Online Book -

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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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The schematic illustrations in Figure 3785a show the convergent illumination configurations of various modes in TEMs. In the CTEM condition in Figure 3785a (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 3785a (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 3785a (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 3785a (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
(a)
(b)
(c)

Figure 3785a. Convergent illumination configurations: (a) CTEM mode, (b) EDS mode and (c) NBD mode.

In many EM systems, a strong objective pre-field lens or a 'mini' lens is used to assist to form a fine probe. In this case, the beam size is determined by the current of the 1st condenser lens (C1), and is affected by the convergence angle that is controlled mainly by the size of the C2 aperture and further adjusted by the objective lens prefield (or condenser mini-lenses).

Figure 3785b shows the structure of the electron probe-forming system in STEM mode in JEOL JEM-2010F TEMs. In the STEM mode, both condenser and objective mini-lenses are tuned off.

Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs

Figure 3785b. Schematic illustration of the probe-forming electron optics in STEM mode in JEOL JEM-2010F TEMs.

The mini-lens and the deflectors underneath the objective lens in Figure 3785c are introduced to decrease off-axis astigmatism and field curvature at low magnification. Another application of this mini-lens is that an enlarged diffraction pattern by ~3 times can be focused onto the SAD aperture.

Schematics illustrating the positions of the mini-lens and the deflectors underneath the objective lens in TEM

Figure 3785c. Schematics illustrating the positions of the mini-lens and the deflectors underneath the objective lens in TEM.

About the objective mini lens, Figure 3785d (a) shows the principle of magnifying an image (Normal-Mag mode). A transmitted image of the specimen is first formed and magnified by the objective lens, and then is magnified further by two to four lenses, including an objective lens, intermediate lenses, and a projector lens. As shown in Figure 3785d (b), at extremely low magnification (e.g. used for survey of interest), the image is formed by the OM (objective-mini) lens, intermediate lenses, and projector lens. The Diff mode in Figure 3785d (c)presents an electron diffraction pattern. In the Normal-Mag mode, the focus of the 1st intermediate lens is adjusted to the image plane of the objective lens where a selected area aperture is located. However, in the Diff mode, the focus of the 1st intermediate lens is adjusted at the back focal plane of the objective lens. Table 3785 lists the status of the lenses and apertures in different operation modes.

(a) Low-Mag mode, (b) Normal-Mag mode, and (c) Diff mode.

Figure 3785d. (a) Normal-Mag mode, (b) Low-Mag mode, and (c) Diff mode. The dashed-lenses are turned off in the relevant operation mode.

Table 3785. Status of the lenses and apertures in different operation modes*.

Lens
Normal-Mag mode
Low-Mag mode
Diff mode
Objective lens
On Off On
Objective aperture (back focal plane)
Normally use Normally not use Normally not use
Objective-mini lens
Off On Off
Selected area aperture
Normally not use Either use or not use Either use or not use
Focus of 1st intermediate lens
At the image plane of the objective lens - At the back focal plane of the objective lens
1st intermediate lens
On Off On
2cd intermediate lens
On On On
3rd intermediate lens
On On On
Projector lens
On On On
* "Normal-Mag mode" includes normal and high magnifications for HRTEM; "Low-Mag mode" includes very low magnifications, which is used for specimen survey; and "Diff mode" is for electron diffraction analysis.

 

 

 

 

 

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