HighResolution TEM

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High-Resolution TEM
Fumiyasu Oba
1/23/2003 TUG meeting
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Outline

• Basics (phase contrast)
• Image simulation
• Microscope conditions alignments
• Important parameters
• Image recording on CCD

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Imaging in TEM

• Contrast (intensity difference) on image plane
• Mass-thickness contrast (BF imaging)
• Diffraction contrast (BF, DF imaging)
• Phase contrast (HR imaging)

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Phase Contrast (1)

• Phase (and amplitude) change by an object
  (potential).

Objective lens
Diffraction pattern
Image
Plane wave
Spherical wave
Back focal plane (reciprocal space)
Specimen (real space)
Image plane (real space)
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Phase Contrast (2)
Si lt110gt
q(x,y) A(x,y) exp(isjt(x,y))

• Specimen

jt(x,y)
s interaction constant
jt(x,y) projected potential
Image plane
y(x,y) F F q(x,y)T(u)
T(u) contrast transfer function
Intensity (image) I(x,y) y (x,y)2
In practice, phases and amplitudes are modulated
when passing through objective lens. This effect
is given by T(u) .
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Contrast Transfer Function

• (Phase) contrast transfer function

T(u) exp (i c(u))
Or, its imaginary part T(u) sin(c(u))
c (u) p ( Df l u2 0.5 CS l3 u4 )
u spatial frequency
Df defocus value (lt0 underfocus)
l wave-length of electron beam
CS spherical aberration coefficient
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Effective CTF

• Effective contrast transfer function
• Teff(u) T(u) Ec (u) Ea (u)
• Envelope damping functions
• Ec (u) Chromatic aberration envelope
• Electron beam energy spread
• Objective lens current spread
• Ea (u) Spatial coherence envelope
• Convergent angle

T(u)
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Defocus Dependence of CTF

• Teff(u)E(u) sin(c(u)), c (u) p ( Df l u2
  0.5 CS l3 u4 )

Df Dfsch
Df 1.6 Dfsch
Df 0.33 Dfsch
Df 0
CTF significantly depends on defocus value.
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Point Resolution
CTF at Scherzer defocus

• lt point resolution
• Phase contrast images are directly
  interpretable.
• gt point resolution
• Difficult to interpret because of the
  oscillation of CTF.

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CTF of Tecnai F30 S-TWIN
CTF at Scherzer defocus (EMS software package)
1 / u
Information limit
- Teff(u)
Point resolution
u

• Point resolution 0.2 nm
• Information limit 0.14 nm

Cs1.2mm, Cc1.4mm Energy spread 0.7eV
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Outline

• Basics (phase contrast)
• Image simulation
• Microscope conditions alignments
• Important parameters
• Image recording on CCD

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Image Simulation (1)

• Why image simulations necessary?
• Specimens are not ideally thin.
• Multiple scattering complicates resultant
  images.
• Images significantly change with defocus values.
• If it is not close to Scherzer defocus, images
  are not directly related to atomic positions.

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Image Simulation (2)

• Simulated images for Si lt110gt (Multi-slice, EMS)

Microscope parameters Tecnai F30 S-TWIN
Df (nm)
0
-120
-20
-40
-60
-80
-100
15
12
9
t (nm)
6
3
1.7Dfsch
Dfsch
0.33Dfsch
- Teff(u)
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Outline

• Basics (phase contrast)
• Image simulation
• Microscope conditions alignments
• Important points
• Image recording on CCD

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HRTEM Recommended Conditions (1)

• Mode TEM BF (microprobe)
• C1 aperture 2 mm (largest)
• C2 aperture 100/50 um (second/third largest)
• Objective aperture 100 um (largest)
• or, second largest ? (slightly larger than the
  point resolution)
• Extractor 3.5 - 4 kV
• Emission lt100 mA
• Gun Lens 1 - 3
• Spot size 1
• Magnification 500 kx to 750 kx
• gt 750kx for quantitative analysis of
  CCD-recorded images.
• Exposure time 3 - 5 seconds with emulsion
  setting 5.6
• lt 1.5 seconds to minimize the effect of
  specimen drift.

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HRTEM Recommended Conditions (2)

• The spatial coherence is maximized while
  minimizing the energy spread in the beam.
• A beam is bright enough to allow a reasonable
  exposure time at high magnification.
• C2 aperture
• Smaller aperture ----- smaller beam size, higher
  coherency
• Effective when the specimen charges.
• (The total beam current on the specimen is
  reduced.)
• Larger aperture ----- larger beam size, lower
  coherency
• Effective when the specimen contaminates.
• (The edge of the beam can be far from the
  area of interest.)

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Alignment Procedures
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Astigmatism Correction (1)

• FFT pattern of amorphous area
• (Real-time processing on DigitalMicrograph)

Well aligned
Misaligned
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Astigmatism Correction (2)
Defocus dependence of images from amorphous area
?f
Well aligned
LaB6
Misaligned
For FEG, astigmatism correction is difficult
without real-time FFT.
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Beam Tilt Alignments

• Methods to locate the beam on (near) the
  optical axis.
• Current rotation center ----- by wobbling the
  objective lens current and minimizing image
  displacements.
• High voltage rotation center ----- by wobbling
  the high tension and minimizing image
  displacements.
• (N/A on Tecnai)
• Coma-free alignment (accurate) ----- by wobbling
  the beam tilt angle and minimizing focus
  difference.
• (Defocus values should be the same when beam is
  tilted with respect to the optical axis by the
  same angle with opposite sign.)

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Current Center

• 1. Go to an area which has some feature.
• 2. Click Rotation center.
• Objective lens current is wobbled, and the image
  go through focus.
• 3. Make the sideways movement of the image as
  small as possible.
• The focus wobble can be made smaller or larger
  with the Focus Step Size knob.

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Coma-Free Alignment (1)

• 1. Go to an amorphous area.
• 2. Click Coma-free alignment x or y.
• The beam is wobbled in the x or y direction by
  tiling beam.
• 3. Adjust the beam tilt until the images for both
  tilts have the same apparent defocus.
• Preparations
• Coma-free pivot points to minimize the movement
  of the beam under coma-free alignment conditions.
• Coma-free amplitude to adjust the tilt angle used
  for coma-free alignment.
• Astigmatism correction.

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Coma-Free Alignment (2)

• Carbon foil images obtained at two different
  angles of the incident beam, before and after
  coma-free alignment.

Before
After
a
a
-a
-a
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Effect of Beam Tilt on Diffractograms

• Diffractograms
• (FFT patterns of amorphous regions)
• Similar to the effect of astigmatism
• Astigmatism correction may be required after the
  alignment of beam tilt.

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Suggested Alignment Procedures

• 1. Align beam tilt based on rotation center.
• 2. Correct astigmatism of objective lens.
• 3. Align beam tilt based on coma-free alignment.
• 4. Correct astigmatism (if necessary).

Readjustment of zone axis may be required after
the beam tilt alignment.
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Outline

• Basics (phase contrast)
• Image simulation
• Microscope conditions alignments
• Important parameters
• Image recording on CCD

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Important Parameters

• Microscope alignment
• objective lens astigmatism, beam tilt
• Thin Foil Specimen
• Thin area with minimal ion-thinning damage
• Zone axis
• Alignment for the area of interest

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Thin Foil Specimen

• Make thin area with minimal ion-thinning damage.

hole
Damaged

• Finishing with low gun energy may be effective
• PIPS lt 2.5keV
• DuoMill lt 3keV ( liquid Nitrogen cooling stage)

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Zone Axis

• Precise alignment for the area of interest is
  important when we are interested in atomic
  structure.

On zone axis
ZnO 0001
Off zone axis
5nm
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Outline

• Basics (phase contrast)
• Image simulation
• Microscope conditions alignments
• Important parameters
• Image recording on CCD

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Image Recording on CCD (1)

• CCD (charge-coupled device)

1 x 1 1024 x 1024 pixels (pixel size 25 x 25
mm)
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Image Recording on CCD (2)
As recorded at 900kx
Resolution halved ? 450kx
InP lt110gt
Cu2O lt100gt
Image automatically smoothed on PowerPoint. See
raw image on another software.
1nm
For quantitative analysis, relatively high
magnification (900kx or 1.35Mx) is required.