Alan Nicholls
Research Resources Center
University of Illinois at Chicago
So How does
the JEM-2010F
STEM work?
and how do you
get good data
out of it?
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Optimizing STEM performance
• Specimen considerations
•JEM-2010F STEM optics & aberrations
• STEM Detectors
STEM (solid state & PMT); CCD (TEM imaging & EELS); EDX
•FasTEM and STEM
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Specimen Considerations
• Specimens for STEM need to be contamination free!
• Do not touch any part that is used inside a vacuum system with
your bare hands (I.e. Ion Mill holders etc)
• IN PARTICULAR no part of the specimen holder, support or
tools should ever be touched by ungloved hands.
• Keep use of acetone down to a minimum - this is notoriously
dirty and a primary source of hydrocarbon contamination.
• Specimens that are glued will always need Plasma cleaning in
the holder. 20 min Ar, 10 min O2 at 10W is recommended if the
specimen is not damaged by oxygen.
• Specimens can be plasma etched at 100W BUT must be in the
Gatan duomill holder supplied NOT the specimen holder.
Approximately 1nm/minute is removed.
• DO NOT Plasma Clean any holder without a specimen in.
• DO NOT EVER Plasma Clean heating or cooling holders!
• Specimens on Carbon Films can be plasma cleaned or use an
Infra Red Lamp with the specimen on a slide or filter paper at
setting 6 for 30 minutes.
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STEM - the
important parts
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Characteristics of different electron sources
Thermal emission
Field emission
W
LaB6
Shottky ZrO/W
~5x105
~5x106
50m
Energy Width (ev)
Operating Vacuum (Pa)
Condition
s
Temperature (K)
Emission
Brightness (A/cm2/sr) at 200kV
Electron Source Size
Current (A)
~5x108
Thermal FE
W (100)
~5x108
Cold FE
W (310)
~5x108
10m
0.1-1m
10-100nm
10-100nm
2.3
1.5
0.6-0.8
0.6-0.8
0.3-0.5
10-3
10-5
10-7
10-7
2800
1800
1800
1600
300
~100
~20
~100
20-100
5-20
1%
1%
1%
7%
5%
2%
20%
10%
Short term
stability
Long term
stability
Maintenance
1%/hr
3%/hr
1%/hr
6%/hr
Not necessary
Not necessary
Start-up takes
time
Price & Operation
Low & simple
Low & simple
High & easy
Build up
necessary
after change
High & easy
3 months
1 year
JEM-100CX
JEM-3010
>4 years
(UIC 8 years +)
JEM-2010F
Lifetime
UIC instruments
?
NA
10-8
10-9
Flash every
few hours
High &
complicated ?
?
1 year
NA
HB601
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TEM>STEM
Microscope should be
aligned in TEM mode
before entering STEM
mode. If EELS spectra
to be collected this
should include GIF
alignment.
If you need to adjust A2
to optimize the probe
this should be done
before TEM alignment.
A2 adjustments are only
necessary for ultimate
imaging resolution and
should be done slowly!
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Nomenclature
TEM
STEM
Gun
N/A
N/A
(Virtual Objective Aperture)
X
(Condenser Stigmator)
Cond
Condenser Aperture
Condenser Stigmator
Objective Aperture
X
Objective Stigmator
Specimen
Obj
Objective Aperture
N/A
Objective Stigmator
N/A
X
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NB Increasing Condenser
lens strength increases
the demagnification
Decreasing A2 increases
the demagnification :- the
electrostatic focussing
effect is the difference
between A2 and A3
(voltage applied to each
stage of accelerator
(~55kV))
But increasing
demagnification lowers
beam current as defining
aperture after condenser
lenses
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STEM - How much demagnification do
we need for atomic resolution?
Source size
~170nm
C1=6V; C2=4.7V; Demag 570
C1=7.07V; C2=4.64V; Demag 860
OPTIMUM
C1=8V; C2=4.61V; Demag 1185
To get to 0.13nm use A2 to increase gun
demagnification - reduce 7.3>6.8
Beam size at specimen
0.2nm
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Total demagnification
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TEM mode
v
STEM mode
TEM - Image is readout in parallel from the whole illuminated area.
All pixcels in the image are exposed at the same time.
Specimen
Detector
STEM - Image is read out in serial from area scanned on the
specimen. Intensity from each pixcel is read out and displayed
independently in order.
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HADF
EELS
BF
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Effect of Cs on Ronchigram
Increasing Objective lens
strength
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Stigmating using the Ronchigram
a
b
c
Ronchigram from amorphous area.
Select Scan mode Spot 1 and a
magnification above 100Kx.
a) underfocus astigmatic
Red circle marks
unaberrated part of
Ronchigram that
should be selected
by Objective aperture
b) underfocus stigmated
c) Gausian focus stigmated (almost!)
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< Au contact layer on
GaAs - 0.2nm probe,
(C1 6.06; C2 4.65; A2 7.3)
Si dumbells resolving
0.136nm 004 spacing
with 0.13nm probe
>
(C1 6.06; C2 4.65, A2 6.8)
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JEOL HADF Detector
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Photomultiplier Tube
Radiation causes photoelectrons to be generated by
cathode. These are multiplied by the dynode chain
(typically 8 elements) giving a 108 amplification of the
signal.
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Bright Field
Annular Dark Field
Schematic of Gatan STEM Detector
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So which STEM detector is better?
Solid state
or
• Easy to fabricate
• Cheap to replace
• Can be cut into any shape
PMT based
• Gain of system is high with a
DQE of 0.9
• Noise level is low
• Good at TV rate or low signal
BUT
• Large dark current
BUT
• DQE poor for low intensity
• scintillator not as robust as SSD
• Electron beam damage
• more expensive and bulky
• Insensitive to low energy
electrons
For Z contrast STEM PMT is best!
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Gatan Imaging Filter
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CCD detector components
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CCD Readout
Read out rates can be as fast as
0.01s per frame. CCD can be reexposed once read out.
Repeat until all rows are read out
CCD arrays have:low noise and good DQE when
cooled.
High dynamic range
BUT
they are expensive
($250K for 4Kx4K CCD)
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Electron Energy Loss Spectrum from Graphitic Carbon
EELS Spectrum from Graphitic Carbon
In STEM mode, with 0.2nm, probe use 0.1s for zero loss region and, as a
starting point, 1s per 100eV to look at higher losses (eg O at 532eV - 5sec)
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XEDS Spectrum from BSCCO
Typically need a > 0.25nA to collect a statistically significant spectrum
in 100sec (I.e. 0.5-1nm spot size 50-70m CA). Can collect spectra with
0.2nm probe for ID of Major (>10%) components.
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Spectrum Imaging
EELS only
XEDS & EELS
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XEDS Spectrum Imaging
• Acquire spectrum at
each point in image
typically using 1nm
probe to get sufficient
X-ray signal.
• Short acquisition
time at each point and
multiple scans.
128x128 SI usually
takes 30-45 minutes
to get significant data
• Data can be
interrogated
afterwards to
generate spectra, new
maps and linescans
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XEDS Spectrum Imaging
LINESCAN
example.
For planar defects
data can be, post
acquisition,
integrated parallel
to the interface
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FasTEM
Start Up
Programs must be started in the order listed.
• FasTem Server and GIF are usually left running
• You MUST Login to FasTem server before opening the Client
(Control window).
• FasTem Video Server does not need to be running.
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Simple Knobset
Image Save
Q.Beam Selector
Multi Function
WOB/DEF/STIG
MAG/DIFF Select
Detector Select
TEM
Image Shift Y
TEM
Image Shift X
Brightness
Focus
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FasTEM GUI
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FasTEM Client
Selector
• Projector alignment settings
are stored for the different
detectors. Please make sure
that you center the illumination
in probe size 1-3 at 100Kx
• CAM TOP is not used
• MSC is not used (used for above
GIF CCD camera for TEM)
• CAM BOT is the off axis camera (or
other TV rate camera)
• SCRN is the fluorescent screen
• STEM is for Scanning mode
• GIF is for the Gatan Imaging Filter
(EELS & Filtered TEM imaging)
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FasTEM Client
STEM Selector
Mode
EM - TEM;
AL - Alignment - center caustic figure
with Condenser Def then illumination
with Condenser Shift
SM - STEM monitor;
DM - STEM Digital Micrograph
Active Detector
TEI - JEOL HADF Detector
EXT - Gatan STEM detector
Scan Mode
PIC/RDC - Full Frame/ Reduced
Frame
SPOT/SPOT1/SPOT2 - Spot modes,
Spot1 should be used for
Ronchigram
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FasTEM Client
STEM Selector
Probe Size & Camera Length -
choose from dropdown list.
NB Spot size on ASID unit affects probe size selected through FasTEM - Make
sure Small is selected.
Alignment Function
Default is “Projector” for centering beam on detector.
Change to “Condenser 2 (Beam Tilt)” for electronic specimen shift - be careful
miss-aligning column!
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STEM set up
• Choose appropriate probe size
Probe
Size
A2 Condenser
Aperture
0.14nm
0.2nm
0.5nm
1.0nm
6.8
7.3
7.3
7.3
30mm
30mm
50mm
70mm
For
Z Contrast HADF at highest resolution
EELS + Z Contrast (>20pA)
XEDS, EELS (>100pA)
XEDS, EELS (>500pA)
• Choose appropriate camera length
Camera
Length
2cm, 4cm
8cm
10cm
12cm
15cm
15cm off axis
Inner
Angle
85mrad
62mrad
52mrad
40mrad
For
EELS + HADF using Gatan detector
JEOL HADF
JEOL HADF
JEOL HADF
JEOL HADF
15cm CL centered on off-axis camera for Ronchigram
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Digital Micrograph
main window
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Digiscan STEM controls
Scan Setup
• Select - 256, 512 and 1024 are set up for phase
locked imaging. Use Default for fast scanning
(Pixcel time can be altered)
• Waveform Monitor - Allows brightness and
contrast to be optimized.
• Control Beam - Beam position tool only visible
when not scanning image.
•NB Esc key stops scan immediately, Stop
button only at the end of a frame. Record only
acquires one frame
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Z-contrast STEM - summary
• Align microscope in TEM mode at desired A2 (inc. GIF for EELS)
• Enter STEM mode with largest CA (150m). Go to Spot1 with
magnification greater than 100Kx - choose spot size and appropriate
camera length for Ronchigram (NB 1nm up may be too bright for camera - for
large probe sizes put aperture in, wobble Objective lens and minimize wobble using X&Y
aperture controls)
• Correct for Astigmatism (preferably on amorphous region) move to
area for imaging. If fringes are visible in Ronchigram put appropriate
aperture on axis. If fringes are not visible specimen is too thick, too far
of axis or has too thick amorphous surface layer - choose another
area.
• Choose appropriate camera length for STEM detector - insert
detector change back to PIC mode.
• Increase magnification to x4M and focus to get atomic resolution
image!
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