Tecnai T20 TEM - STEM Setup and Alignments

STEM Setup & Alignments

1. Preparation:

   1. Load sample and move sample to Eucentric height in the standard way as for parallel imaging
   2. Remove Objective and Selected Area apertures if they were left inserted

   3. Either load a STEM FEG register for your desired application, or do the following:

      1. Select the desired C2 aperture size and spot size for the required resolution and signal level for your application/technique
      2. Center the C2 aperture at ~30Kx using the normal TEM parallel beam procedure for centering the C2 aperture.
      3. Turn on STEM mode in the STEM control panel (CP) in the user interface
2. Select Search mode on the STEM Control Panel (STEM CP) and select a low magnification so that you can see the sample features in the image
3. Select Diffraction alignment in the Direct Alignments CP and use MF knobs to center the probe in the aperture of the HAADF (around the 9 o’clock position on the small 5mm circle)
4. Move the sample to a hole or to the amorphous carbon support film
5. Stop Search mode in the STEM CP so the beam is aimed at the hole or amorphous carbon support film 

6. Align the beam:

   1. Switch to Probe mode (turn off diffraction mode by pressing the ʻdiffractionʼ button)

   2. Center the C2 aperture

      1. Center the beam by selecting ʻbeam shiftʼ in the Direct Alignments CP and adjust using the MF knobs
      2. Increase magnification to 380kX, re-center if needed using beam shift
      3. Correct astigmatism using the objective stigmator if needed
      4. Condense the beam (using the focus knob set to step 4 or 5) to crossover and center beam using the beam shift track ball
      5. Expand the beam larger than the 4cm circle (focus knob)
      6. Center the beam by clicking on “adjust” next to the C2 aperture in the motorized aperture CP then adjust using the MF knobs
      7. Condense the beam to crossover; if it’s not at the center of the screen repeat steps iv-vii
   3. Adjust the beam tilt pivot point X in the Direct Alignments CP using the MF knobs
   4. Adjust the beam tilt pivot point Y in the Direct Alignments CP using the MF knobs

   5. Check the Rotation Centering in the Direct Alignments CP

      1. Condense the beam
      2. Select ʻRotation Centerʼ in the Direct Alignments CP
      3. Adjust the focus step ring counterclockwise so that the illuminated area is not oscillating anymore. Use the beam shift track ball to move the beam toward you off the small circle a little bit (this prevents the black circle from hindering your view).
      4. Turn the focus knob a bit past crossover on the Gaussian side so that the beam has a bright caustic in the interior (if the beam perimeter is a bright ring then you are on the wrong side of crossover). The beam should be around 3-5 millimeters in diameter on the phosphor screen.
      5. Adjust the Rotation Centering using the MF knobs to move the bright caustic to the center of the beam. Use the beam shift trackball as needed to bring the beam back to the same position if it moves too far.

   6. Adjust the Objective lens astigmatism

      1. Make the beam ~3 mm in diameter on the non-Gaussian side of crossover
      2. Use the Objective stigmators to make the disk circular (and—if visible—create the symmetrical ʻthree-pointed starʼ at the center of the disk)
      3. Repeat steps b through f as needed
7. Center and condense the probe using ‘beam shift’ and the focus knob
8. Return to Diffraction mode (press the ‘diffraction’ button), and start the scan (press ‘search’ button)

9. Refine the sample height

   1. Using the Z-axis buttons, move the sample in Z until the image appears to be in focus
   2. Increase Magnification and repeat the above step a
10. Refine the stigmation 

Method #1 using focus window:

1. While in ʻsearchʼ mode, move the sample onto a fine-featured region of the sample.
2. Increase magnification to slightly higher than the highest magnification at which you intend to take images
3. Activate the focus window and place over area with fine features.
4. Check for astigmatism by running in and out of focus, looking for directional ‘stretching’ or ‘smearing’ of the image.
5. Adjust astigmatism by adjusting focus to ‘best focus’ where blur of edges is equal in all orientations, then select Condenser stigmators and use MF X button to find the sharpest image, followed by MF Y button to find the sharpest image.
6. Run in and out of focus to see if astigmatism is gone. If not, repeat step e

Method #2 using the Ronchigram (this is advanced, and many samples are not suitable for it):

1. While in ʻsearchʼ mode, move the sample onto an amorphous region of the sample.
2. Stop ʻsearchʼ mode
3. Focus the Ronchigram to see diffuse fringes inside but around the perimeter of the Ronchigram
4. Use the Condenser stigmators and the MF knobs to make the fringes circular and symmetrical inside the Ronchigram.

1. If you made adjustments to the Condenser stigmators 

   1. Go back into Probe mode and adjust the beam roundness again using the Objective stigmators (as in step 7.f above), then return to Diffraction mode and do step 11 again. Iterate adjusting the stigmation in both modes until they stabilize.
   2. End the stigmation process in diffraction mode.
2. Locate your area of interest, focus, and start imaging.

Ending a STEM Session

1. Put microscope into stand-by state:

   1. Switch out of STEM mode (into parallel beam mode) by pressing the ‘STEM’ button on the STEM tab
   2. Switch microscope mode to ‘Microprobe’ if in Nanoprobe mode using the ‘Microprobe’ button on the Tune workset.
   3. Reduce magnification to ~10,000x
   4. Center the beam on the phosphor screen using beam shift, and spread it to cover the whole screen using the intensity knob.
2. CLOSE THE COLUMN VALVES
3. Reset the holder by pressing the ‘Holder’ button on the Search tab in the flap-out of the Stage panel.
4. Unload the holder as normal.