STEM Mode on the FEI CM20

Note: The last time I used an FEI machine was several years ago. The machine I was using was commissioned in 1993, so all of this is probably very much out of date. Also, the following was developed for a particular CM20 which was rather temperamental, so take all this with a pinch of salt...

To select scanning mode, go to the 'modes' page and press 'scanning bf and df' twice. Note that in the bottom left- hand corner of the scanning screen there is toggle selection between 'CRT' and 'TEM'. Also, the diffraction knob can either be on or off: by default, it is selected as 'on' when you go into scanning mode, and it is best to leave it on.

It is essential to understand that CRT/TEM sub-modes change all the usual knobs like the magnification, the focus knob, the shifts, the intensity knob, and the defaults selected in the 'align' page, e.g. for the rotation centre.

'CRT'(cathode ray tube) is essentially STEM mode: The magnification knob will now adjust the amplitude of the scan of the probe. Large scans result in low STEM magnifications (think about it). When CRT is selected, the magnification displayed on the main screen is the STEM magnification - the inverse of the scan amplitude. The electrical shifts will now either shift the probe or, if the scan unit is scanning the beam, shift the cross-hairs on the display consoles. The shifts also move about the 'reduced area scan' when appropriate. Control of C2 is disabled in this mode, but the spot size can still be changed. To enable control of C2, you have to select 'uudiff' mode, also on the left-hand side of the screen.

The main use of 'TEM' mode on the 'scanning mode' page is to allow you to change the camera-length of the projector system, via the magnification knob, and to align the detector. This is important in STEM for getting the detectors to appear to be the correct size relative to the beams coming out of the specimen. The focus knob changes the diffraction lens, provided the diffraction button is on.

Select a stationary beam by pressing 'scan stop' on the scanning unit. You can also stop the beam by pressing the cross-wire button twice, which is what the manual suggests. The former automatically puts the beam onto the centre of the scan field of view; the latter stops the probe at a particular point indicated by the cross-hairs.

Always align the Ronchigram with the probe at the centre of the scan range. To ensure this happens, go to maximum magnification in 'CRT' mode, and centre the cross-hairs on the screens.

It is also worth understanding how to align the diffraction pattern (i.e. the detector plane: what you see on the phosphor screen in STEM mode with the diffraction button lit up) is aligned. On the alignment page (when selected from STEM mode) there are three variables that control the same shift coils, and which shift around the what you see on the phosphor screen: DET ALIGN (detector align), DIFF SHIFT (diffraction shift) and DIFF ALIGN (diffraction align). DIFF ALIGN and DET ALIGN have different values for each setting of the camera length. DIFF SHIFT is a constant shift which is added to all DET ALIGN and DIFF ALIGN settings. Don't bother to get these perfectly right until you are confident that the Ronchigram is well lined up. However, you might have to change them to keep the illumination on the screen as you adjust the Ronchigram. If in doubt, change DIFF ALIGN. By the way, DET ALIGN only works for long camera lengths: the computer squeals at low camera lengths.

In STEM mode, the C2 control is disabled. To activate it while still seeing the Ronchigram, select the 'uudiff' toggle on the left-hand side of the 'scanning' mode screen. If you oscillate between selecting and de-selecting uudif, you can adjust C2 in uudif mode so that it has the same setting (i.e. the Ronchigram is identical) as when the microscope is in ordinary STEM mode. The computer automatically selects the setting of C2 in STEM mode, and this variable cannot be changed. However, it is important to align C2 (which has to be done in uudiff mode) at or around the same setting that the microscope will select when it returns to STEM mode. You can change the objective lens focus by turning the ordinary 'focus' knob in STEM/CRT mode.

What you actually do:

Before going into scanning mode, roughly align for normal TEM, select and align a large condenser aperture.

1) Get to see the Ronchigram properly

1.1) select 'scanning mode' with 'CRT' toggled on the bottom
left of the screen, the diffraction knob 'on', and 'uudif'
not selected.
1.2) select spot size 6.  Higher spot sizes increase
resolution slightly, but decrease beam current.  Low spot
sizes give rather poorer resolution.
1.3) select the highest STEM magnification,
1.4) press the cross-hair button, centre the cross-hairs,
press the cross-hair button again
1.5) select 'TEM' on the bottom left toggle, make sure the
diffraction knob is 'on'.
1.6) change the camera length (MAGNIFICATION) through all
its settings, observe the phosphor screen.  You may find you
lose the illumination at high camera lengths.  Don't panic.
Don't adjust the DIF ALIGN until you have checked the
objective rotation centre.  To do this:
     1.7)  select 'CRT', still with the diffraction knob on.
     1.8) select 'align' and the 'rotation centre'
     1.9)  the multifunction knobs will have a dramatic
     effect on the position of the beam, as the tilt sweeps
     the beam across the detector plane.  If you need to
     find the beam at this stage, do it this way.
     1.10)  exit 'align'
1.11)  select 'TEM' on the scanning page
1.12)  select a suitable camera length so that you can see
the Ronchigram clearly.

2)  Align the Ronchigram

2.1)  Select CRT on the scanning page, uudif not selected.
2.2)  Roughly focus the objective (FOCUS) so that the
Ronchigram has a section of it 'blowing up.'
2.3)  Select 'align' and 'rotation centre'
2.4)  Adjust multi-function knobs until the Ronchigram
expands and contracts in the centre of the aperture (not the
centre of the screen).
2.5)  Deselect align and select 'astigmatism'
2.6)  Focus the objective (FOCUS - make sure you are in CRT
mode) and adjust the condenser stigmators so the intensity
at focus is as flat as possible.
2.7)  Go to 2.4 and repeat.
2.8)  Focus the Ronchigram (FOCUS), select 'uudif' on the
scanning screen, and now refocus the Ronchigram with C2
(INTENSITY).  The setting of C2 should be low: not above the
cross-over.
2.9)  Go through focus with C2 (INTENSITY) and check the
centre of Ronchigram corresponds the centre of C2.  If it
doesn't, both the rotation centre and the condenser aperture
are out of line.  There may be different centres above and
below C2 focus.  Mark the centre of both centres by moving
this onto the centre of the phosphor screen (DIF ALIGN under
'align'), and then adjust the rotation centre to this new
axis.  Note that this step is a refinement, and only
necessary for very good resolution.
2.10)  Insert a small condenser aperture that just covers
the central flat region of the Ronchigram.  Re-check the
rotation centre and get the aperture exactly on the line of
centre of image expansion.

3)  Check the pivot points

3.1)  Insert an objective aperture: one that appears smaller
than the Ronchigram
3.2) Select CRT on the scanning page, with 'uudif'
unselected, and focus the Ronchigram (FOCUS).
3.3)  Select TEM on the scanning page, with 'uudiff'
unselected.
3.4)  Select 'align' and 'pivot points'
3.5)  Adjust the diffraction lens focus (FOCUS in the TEM
scanning sub-mode) until only one objective aperture is
apparent.
3.6)  Remove the objective aperture.
3.7)  Adjust the multi-function knobs until the Ronchigrams
are co-incident: repeat for both x and y pivot points.
3.8)  Check DIF ALIGN and DET ALIGN for all camera lengths
(MAGNIFICATION).  In DET ALIGN, the Ronchigram should be at
about '2 o'clock' on the outer circle marked on the phosphor
screen.
3.8)  Exit Align, re-select CRT on the scanning page.

4) Set up monitors

4.1)  From the scanning page, select 'detectors'
4.2)  From the detectors page, select 'greyscales'
4.3)  Adjust brightness and contrast on the monitors (not
the detectors) until all stripes are visible and evenly
saturated.

5) Start scanning and align detectors

5.1)  Make sure the scanning page is selected, with CRT
selected and uudiff unselected.
5.2)  Put the Ronchigram in focus (FOCUS), and lift the
phosphor screen.
5.3)  Press the wide scan button (large square), ensure
'scan stop' is deselected.
5.4)  Go down in STEM magnification (MAGNIFICATION but not
as far as LM mode, low magnification).
5.5)  Adjust DET ALIGN in align until the dark-(right) and
bright- (left)field images are as they should be.  If
necessary, adjust the camera length (select TEM on the
scanning page, then adjust MAGNIFICATION).
5.6)  Focus image with the objective (CRT mode with FOCUS
knob) and refine condenser stigmators in 'stig' mode.
5.7)  If super-keen, re-iterate from 1.1.

Copyright J M Rodenburg