Manual Calibration of Lens Aberrations

Manual Calibrations
To avoid radiation damage to the sample, we use the Standard Specimen (Au particles) for lens aberration calibration before inserting our samples. The lens aberrations of STEM are corrected by (i) Manual Correction and (ii) Software Correction iteratively. Manual Correction deals with the lower-order aberrations, mainly A1 and B2. Software Correction handles the higher-order aberrations which would otherwise be almost impossible with manual efforts, e.g., C3, A3, S3, A4, D4, B4.

• PL alignment

1. Insert Beam stopper (F5) >> Mark the screen with a circle (left click the mouse) >> Remove Beam stopper (F5).
   The black dot on the fluorescence viewing screen is the absolute reference, i.e., the centers of the beam stopper and BF detector (OneView) are centered to it.

2. Center the CL1 aperture to the concentric origin spot by the “Aperture Controller”.

3. Manually wobbling the electron beam by the “OBJ Focus” knob to find the concentric spot.
   You may use HT wobbling instead of "OBJ Focus" to find the concentric origin spot.

4. Activate “PL align” in course mode and use DEF-X and DEF-Y to shift the circle to the concentric spot.
   You cannot perform PL alignment correctly if there is a tilt (B2) of the electron beam. Hence, PL alignment and B2 should be performed iteratively.

• z-height focus

1. Double click "STD focus".
   Double pressing “STD focus” is to command the system to go back to the pre-set value in the objective lens of STEM.

2. Press Z up or Z down to bring the Ronchigram into the focus position.
   The Ronchigram will be zoom in if the Z direction is pressed correctly. Stop Z adjustment when the image is just about blown up. When Z height is close to the focus plane (within 10 nm), we may conveniently use the "Obj Focus" to zoom in and out.

• Coarse A1 correction

1. Press the “COR CRS STIG” button >> Use DEF-X or DEF-Y>> Adjust A1 in the Fine mode
   Remember to disable the “CRS” COARSE BUTTON

2. Use DEF-X and DEF-Y in sequence until the streaks in Ronchigram disappear.
   Underfocus a bit, adjust the streak lines until it disappears; overfocus a bit, adjust the streak lines (on the opposite) until it disappears. The fine A1 can be corrected using the "COND STIG" in the coarse mode.

• B2 correction

1. Press the “Beam Align” button >> Use DEF-X or DEF-Y >> Adjust B2 with a FINE knob
   Remember to disable the “CRS” COARSE BUTTON

2. Use DEF-X and DEF-Y in sequence until the streaks in Ronchigram disappear.
   Underfocus a bit, bring the “flat” region to the center; overfocus a bit, bring the “flat” region to the center.
   There could be a few flat regions, bring the closer one to the center. The "flat" region usually is at the center of a triangle (A2) or spherical shape (C3).

• A2 Correction (optional)

1. Ensure that the 3-fold astigmatism is present in a slight overfocus/underfocus condition.

2. Access the software button with appropriate stepsize e.g., stepsize 9, for A2 corrections.
   If A2 correction is not easy for you, you may rectify it later in the software correction. For fine A2 correction use stepsize 6.

Important Notes
If you are adjusting A1 and B2 correctly, the Ronchigram is getting closer and closer to you (zooming In); if you are adjusting A1 and B2 incorrectly, the Ronchigram is getting further and further from you (zooming out). As you continue to turn the DEF-X or DEF-Y knob, the zoomed-in Ronchigram starts to rotate; we should stop just before the image starts rotating. The final manual calibration is very subtle with just a few clicks. Insert and center the CL2 aperture before going to the software correction.