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Electron Microscopy Facility
Subsidiary of the Research and Graduate Studies Office of the AVP

Tecnai F20 TEM Alignment

By Paul Minson,
May 05, 2023 02:34 PM

Tecnai F20 Basic Alignment Procedures

The checklist is meant as a reminder. If you need more than a reminder, read the sections that follow the procedure checklists. FEEL FREE TO ASK FOR HELP! The Lab manager is here to assist and train users as well as keep the instrument in working condition, so he is willing to help.

1. Move Specimen to Eucentric Height

The eucentric height is important in the microscope. It is not only convenient that the area of interest stays centered when tilting around the tilt axis (if you never tilt anyway, this feature may be of little interest), but it also defines a reference value for the objective-lens current. The normal changes made to the objective for focusing have little effect, but stronger changes (changes in focus by several tens of micrometers or more) can have considerable effect on:

  • The effective magnifications and camera lengths. (The objective lens not only focuses the image but also contributes the largest magnification of any lens in the system; strong changes in objective lens current change this magnification and thereby also the final magnification and camera length.) 
  • Proper alignments. 

Finding Eucentric Height Using the α−Wobbler

  1. First do a coarse find of the eucentric height.

    1. Find and center an easily recognizable feature in the specimen, at ~2kX to 3kX magnification. 
    2. Activate the α-wobbler of the CompuStage using the L2 button on the hand panel. For magnetic specimen it may be advisable to reduce the wobbler angle (because of the effect of the specimen’s magnetism on the objective-lens field). 
    3. Minimize the sideways motion of the image with the Z-axis height control. 
    4. Switch the wobbler off using the L2 button. 

  2. Then do a fine alignment of the eucentric height.

    1. Find and center an easily recognizable feature in the specimen, at ~10kX to 30kX magnification. 
    2. Activate the α-wobbler of the CompuStage using the L2 button on the hand panel. 
    3. Minimize the sideways motion of the image with the Z-axis height control. 
    4. Switch the wobbler off using the L2 button. 

2. Center the Condenser 2 Aperture (“C2 Aperture Centering”)

  1. Press the Eucentric Focus.
  2. Select a suitable magnification (between 10kX and 50 kX).
  3. Remove the objective and SA apertures (‘Objective’ and ‘Selected Area’ buttons on the Apertures control panel).
  4. Condense the beam to the crossover point using the Intensity knob (or to be just larger than the small circle).
  5. Center the beam, as necessary, on the phosphor (viewing) screen using on the Tune workset the Beam Shift direct alignment.
  6. Expand the beam to be just larger than the large circle (4 cm diameter) the viewing screen (intensity knob).
  7. The illuminated area should be centered on the phosphor screen. If not, center the expanded beam: on the Aperture control panel press the Adjust button for the C2 aperture, then use the multifunction X and Y knobs on the hand panel to center the illuminated area on the phosphor screen. 
  8. Repeat steps 3 through 6 until the illuminated area remains centered when expanded.

Condenser Aperture InformationThe condenser aperture is the illuminating-beam limiting aperture in the column. The alignment of the aperture is important for two aspects:

  • For convenience, to make the expanding (defocused) and contracting (focused) beam stay centered on the screen (thereby making it unnecessary to adjust the beam position continuously). 
  • To ensure reproducible illuminating conditions. Misalignment of the condenser aperture leads to a beam tilt (and thus a change in the rotation center). In principle, it is possible to adjust the rotation center whenever the aperture is changed. In practice, it is much easier to make sure the aperture centering is reproducible - and thus the rotation center stays the same. 

Note 1: Because of the latter aspect, it is much more important to align the aperture in a reproducible manner (always following the same procedure; that is, use the same magnification and defocus the beam by the same amount each time) than having a ‘perfectly’ aligned aperture (by whatever criterion).

Note 2: There typically is a difference between the aligned condenser aperture in the microprobe and nanoprobe modes (requiring adjustment when switching between these modes). This mechanical misalignment is a consequence of the difficulty of mechanical alignment of the minicondenser lens. 

3. Correct Condenser 2 Astigmatism (“Stigmate C2”)

  1. Remove the selected area and the objective apertures from the beam, and move to a clear portion of the specimen or a hole in the specimen. 
  2. Condense the beam close to crossover. If the beam is not circular as it is adjusted through crossover, then the astigmatism in the Condenser lens needs to be minimized.
  3. Select a tab (Tune tab) containing the Stigmator Control panel and press the Condenser stigmator button. 
  4. Adjust the beam roundness using the Multifunction-X and Y control knobs. Astigmatism is corrected when the focused beam remains as circular as possible when going through beam focus or crossover (Intensity). 

Direct Alignment Procedures

The following is a checklist for performing the direct alignments. Before proceeding, you should have already performed the above alignment procedures. Also note that some steps will cause the condenser stigmation to need readjustment. Make those adjustments as needed to continue with the direct alignments. The microscope should be in the same state in which you will be working (i.e. spot size, gun lens, extractor voltage, and magnification range-Mh, SA, M, or L.)

4. Align the Beam Tilt Pivot Points

  1. Change magnification to 125kX.
  2. Set the focus to eucentric height using the Eucentric Focus button on the right hand panel. Keep the focus at eucentric height for rest of alignments until doing Objective Astigmatism.
  3. Condense the beam (using the intensity knob) to the smallest possible size.
  4. On the Tune workset, in the Direct Alignments control panel, Click on “Beam tilt ppX”.
  5. Use the MF knobs (Beam tilt) to place the blinking spots in the same position.
  6. Click “done.”
  7. Click on “Beam tilt ppY” in the Direct Alignment control panel.
  8. Use the MF knobs (Beam tilt) to place the blinking spots in the same position.
  9. Click “done.”

5. Align the Beam Shift

  1. Condense the beam (intensity knob) to just larger than the small circle (still at 125kX magnification).
  2. Click on “Beam shift” in the Direct Alignment control panel.
  3. It may be necessary to decrease magnification to locate the beam.
  4. Use the MF knobs (Beam shift) to center the beam on the viewing screen.
  5. Click “done.”

6. Align the Rotation Centering (“Objective Lens Centering”)

  1. Expand the beam (intensity knob) to fill the viewing screen (still at 125kX mag.)
  2. Find an easily recognizable feature in the specimen and center it in the beam and on the screen.
  3. Focus the image (“Focus” knob).
  4. Click on “Rotation center” in the Direct Alignment control panel.
  5. Use the MF knobs (Rotation center) to minimize the lateral movement of the image. The 'focus wobble' can be made smaller or larger with the Focus Step Size knob.
  6. Click “done.”

Direct Alignments Information

The Direct Alignments Control Panel offers access to the microscope's direct alignments. In addition to the alignment procedures where the operator is taken through a set of alignments in a structured way, the Tecnai microscope provides (rapid) access to a restricted set of direct alignments. In contrast with the procedures, direct alignments do not switch the microscope to predefined states but instead are applicable to the current microscope state. The direct alignments are shown in a list. Which alignments are available depends on the microscope mode.

A few Direct Alignments notes:

  • You do not change any alignment by activating it. Only when you change a setting with the Multifunction knobs, do you change alignments. Changes become operational immediately. They are stored when the alignment is exited (or by clicking Done). 
  • Activate a direct alignment by clicking on its title. Stop an alignment by pressing Done. Activating another direct alignment also stops the current one. 

7. Correct the Objective Astigmatism (“Objective Stigmation”)

The astigmatism in the objective lens is most easily detected by performing a Fourier transform on an image of amorphous material at high magnification. The Fourier transform image should appear as circular concentric rings or one big circular diffuse spot, depending on the focus. Remember that the objective astigmatism is affected by the size and location of the objective aperture.

Prepare the image

  1. Find a region of the sample that is amorphous OR mostly amorphous
  2. Select a suitable magnification (340 kX for work at or below that magnification, or the highest magnification you will use if it is 380kX or higher).
  3. Focus the image.

Prepare the CCD camera

  1. Go to the camera software (Digital Micrograph) on the computer desktop.
  2. Click on Start View on the camera tools window to start a viewing window and adjust the exposure time (0.5 seconds), and beam intensity so that light pixels read ~8k-12k counts.
  3. Open the live, reduced FFT image of the active search image (Process🡪Live🡪Reduced FFT).

Adjust the Objective Stigmators

  1. On the Tune workset, in the Stigmator control panel, select the “Objective” stigmator button.
  2. While viewing the FFT of the Search image, adjust the objective stigmator so that the rings in the FFT image appear circular. 
  3. You can also adjust the size of the concentric rings in the FFT image by adjusting the objective focus. Rings expand as you get closer to focus.