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probably very much out of date.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
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
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'
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
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'
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
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
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
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
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