Electron microscopy
 
Beam Tilt with Dark-Field Deflection Coils
- 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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Figure 3083a shows the principle of bright and dark field imaging in STEM. The scanning coils are normally incorporated into the objective lens. The convergent electron beam penetrating through the specimen forms a diffraction pattern. The deflection coil which tilts the electron beam for dark field imaging is indicated in the figure. Bright field imaging is performed by taking the transmitted electron beam only as shown in Figure 3083a (b). There are two ways to form dark field images:
        i) Deflect a specific diffracted beam, with beam tilt, into a centrally placed objective aperture as shown in Figure 3083a (c).
        ii) Displace the objective aperture so that only a specific diffracted beam passes through it as shown in Figure 3083a (d).

Beam Tilt with Dark-Field Deflection Coils

Figure 3083a. (a) The main parts of STEM systems for bright and dark field imaging, (b) Bright field imaging, (c) Dark field imaging with beam tilt, and (d) Dark field imaging with a displaced aperture.

In TEM mode, when we are studying the electron diffractions, the quality of images is less critical. Therefore, beam tilt performed by dark field deflection coils can be used. Table 3083 lists the procedures to excite some specific diffraction spots. For instance, if we want to excite the third-order reflection 3g in bright field condition, the following procedure can be performed:
        i) Tilt the beam using dark-field deflection coils to put –g on the optic axis ("0" in Figure 3083b (a)).
        ii) Tilt the specimen so that g is strongly excited.
        iii) Tilt the beam to put 0 back on the optic axis, resulting in 3g excited.

Table 3083. Examples of procedures to excite some specific diffraction spots.

Step i
Tilt the beam to set optic axis
-g -2g -3g -4g -5g -7g
Step ii Tilt the specimen to excite g spot
Step iii Tilt the beam to put 0 back on the optic axis
Results
Excited diffraction spot
3g 5g 7g 9g 11g 13g

procedures to excite some specific diffraction spots

Figure 3083b. (a) Beam tilt, and (b) Results with 3g excited.

 

 

 

 

 

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