Pivot points

Before we begin to scan the probe to make a STEM image, it is essential to check the pivot points. (Sadly, this is not possible on all makes of machine because it is regarded as 'too difficult' by the manufacturers. However, you can usually obtain access to the control indirectly via the computer.) Think of the following diagram, where the pivot points are incorrectly adjusted:

In an ideal world, the probe continues to point in exactly the same direction as it is scanned over the specimen by the beam shift coils. However, if the rocking point of the scan coils is wrong, the beam tilts as well as shifts (see introductory guide if you have forgotten the meaning of tilt and shift). This has two consequences: the Ronchigram moves relative to the detectors as the probe is scanned, and the magnification of the STEM image will be uncalibrated. At worst, the beam may be only tilting, and not moving laterally at all, in which case the apparent magnification of the STEM image will be much higher than the nominal value. Note that this pivot point adjustment is quite different to the pivot point values in TEM mode. In the latter, we adjust for a stationary probe as a function of tilt; here we adjust for stationary tilt as a function of shift. We are aiming for shift purity.

There are different ways of making this adjustment on different machines. It is essential to correct it for each spot size and/or objective/condenser combinations of setting. The way the beam shift interacts with the objective pre-field means that the degree of tilt can be strongly correlated to strength of the objective.

Ask the demonstrator: How do I adjust the pivot points in STEM mode?

If in doubt, one way of doing this is to assume the objective aperture is truly in the back focal plane of the objective lens (usually, it is not quite in the back focal plane). If we pretend it is in the back focal plane, then a way of testing the pivot points is to select diffraction mode and observe the Ronchigram. Insert a small objective aperture (smaller than the Ronchigram), which should cast a shadow over the Ronchigram, and switch on the pivot point adjustment.

If two objective apertures become visible, the diffraction lens is not focussing on the back focal plane; focus the diffraction lens until only one objective aperture is visible. Now remove the objective aperture and adjust the STEM pivot points until the two Ronchigrams are superposed. Note that in STEM mode, focussing the diffraction lens in diffraction mode does not mean making the beam make a sharp point (as in TEM diffraction), because there a very large range of angles present in the conical beam coming out of the specimen

Copyright J M Rodenburg