Transcript Glaeser_TEM Instrumentation2008

INSTRUMENTATION FOR
HIGH-RESOLUTION
DATA COLLECTION
ROBERT M. GLAESER
Electron Crystallography Workshop
UC Davis
Sept. 7-12, 2008
Basics of high-end TEM vacuum
Ion pump for gun
The better electron guns do
require quite good vacuum
anticontaminator
Ion pump for specimen
The trend now
is to use turbo- molecular
pumps rather than oil-based
roughing pumps and
diffusion pumps
Diffusion pump for camera & viewing chamber; oil-based roughing
pump for specimen airlock and to “back” the diffusion pump
CLEAN vacuum at the specimen is more important than very high vacuum
- Water vapor; Pump-oil or other organics
Vacuum points to remember
• At ~10-6 Torr, one monolayer of residual gas atoms hits
a surface in one second
– “Sticking coefficient” is ~1 at low temperature
• Diffusion pumps easily reach 10-6 - 10-7 Torr
– and are good at pumping water
• Ion pumps reach 10-8 -10-10 Torr
– but are overloaded by water
• Turbo pumps good to <10-7 T, also good for water
• Cryo-traps probably good to 10-10 T, subject to
saturation
• Roughing pumps and diffusion pumps pose danger of
backstreaming of oil
Schematics of electron sources
Standard tungsten hairpin
Large emitting area (10 µm?)
Hardy as a too-bright light bulb
(note Ken’s ironic humor)
Lanthanum hexaboride (LaB6)
~100x brighter than a hairpin
Small emitting area (3 µm)
Needs good vacuum, lasts a year or more
Field Emission Gun (FEG)
~1000x brighter than a hairpin
Very small emitting area (0.1 µm)
Needs very good vacuum, lasts multiple years
What is the best voltage to use?
•
Higher accelerating voltage is always better
– EXCEPTION: CCD performance becomes MUCH worse as the voltage increases
•
Worries about too little contrast at higher voltage are myths based on
– Not understanding the physics of electron scattering
– Not accounting for the need to defocus more or to use a smaller objective aperture
ADVANTAGES OF HIGHER VOLTAGES
Some are incremental, some are fairly major
•
•
•
•
•
Reduced loss of signal due to inelastic + elastic (multiple) scattering
Increased depth of field/Ewald sphere is more planar
Weak-phase approximation becomes more accurate
Greater gun brightness
Shorter wavelength results in improved spatial coherence envelope
function
• Smaller DE/E is equivalent to smaller Cc – i.e. better temporal coherence
envelope function
The need for the best possible
electron gun
For good coherence, use high C1 excitation
and small C2 aperture.
But this produces very low illumination intensity so need a brighter electron source
“Low-dose” technique is used to limit
radiation damage
(a)
(c)
(b)
(d)
Minimum-dose systems
(MDS) are a normal part of
the microscope software
• Search for promising specimen
areas with a very low exposure, at a
very low magnification
• Peak at the electron diffraction
pattern (if applicable)
• Focus (if applicable) with an
Images must thus be recorded with “safe”
electron exposures
< 10 e/A2 at 100 keV and liq. N2-cooled
< 20 e/A2 at 300 keV “ “
“
Diffraction lasts ~7X longer at low temp.
intense beam at high magnification,
but to one side of the area used to
record data
• Record data as images or as
electron diffraction patterns with the
desired electron exposures
Helium temperature often provides no
improvement over nitrogen temperature
helium
nitrogen
15 e/Å2
15 e/Å2
90
90
105
105
Comolli and Downing (2005) J. Struct. Biol. 152:149-156
In fact, there are many disadvantages when using helium cooling
Specimen contamination from condensed N2 gas & O2 gas
Electrical conductivity of carbon film is MUCH worse
Liq. He is more expensive and less convenient to use than N2
Most labs with He-stages now use only liq. N2 cooling
Spot-Scan illumination minimizes
beam-induced movement
Spot size 300 - 1000 Å:
Close-to-crossover => larger
convergence angle
But the effect of beam
divergence is no worse than
averaging data from a large
area with a “spread” beam
Cannot be used for self-supported
specimens over open holes due to
severe charging!
Beam-induced movement, not EM technology
limits image quality
ALL images should be this good, but they never are …
• Strong diffraction spots
out to 1.86A
– Even two very faint
ones at 1.75A
Henderson & Glaeser,
unpublished
• F30 at
LMB/Cambridge
• Standard Gatan stage
(liq. N2)
• Paraffin monolayer
crystals on continuous
carbon film
• Spot-scan
• Photographic film
• Raw Fourier
transform; no
unbending
Alternative detector-technologies
THIS TOPIC IS COVERED BY COMING SPEAKERS
Yifan Chang – Film/CCD for
electron diffraction
Stumpf & Booth – Commercial CCD solutions
Downing – Limitations of current technologies
with respect to DQE & MTF, and
candidate technologies for the future
Electron detector
ACKNOWLEDGEMENT
Several slides in this presentation
were modified from those prepared by Ken Downing
for his lecture on the same topic,
2006 Davis Workshop