notes 13 spring2014
TRANSCRIPT
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Transmission Electron Microscopy (TEM)
In a typical TEM a static beam of electrons at 100-200kV accelerating voltage illuminate a region of an electron transparent specimen which is immersed in the objective lens of the microscope.
The transmitted and diffracted electrons are recombined by the objective lens to form a diffraction pattern in the back focal plane of that lens and a magnified image of the sample in its image plane. A number of intermediate lenses are used to project either the image or the diffraction pattern onto a fluorescent screen for observation.
The uniqueness of TEM is the ability to obtain full morphological (grainsize, grain boundary and interface, secondary phase and distribution,defects and their nature, etc.), crystallographic, atomic structural andmicroanalytical information such as chemical composition (at nm scale),bonding (distance and angle), electronic structure, coordination numberdata from the sample.
A simple analog
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An alternative comparison
JEOL 2010F
Electron gun
Probe forming lenses - Cond.
Specimen holder
Magnifying lenses - Int. & Proj.
Objective Lens
HAADF Detector(high angle annular dark-field)Viewing Chamber
Camera Chamber
STEM Detector &/or EELS
XEDS Detector
Basic features ofan analytical electron microscope
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FEI Titan
VacuumThe electron microscope is essentially a series of connected vessels separated by valves.
The vacuum near the specimen is around 10-7 Torr. The vacuum in the gun depends on the type of gun, either around 10-7 Torr (tungsten or LaB6) or 10-9 Torr (for a Field Emission Gun).
The pressure in the projection chamber was usually the worst. The projection chamber holds the negatives used to record images. These negatives can outgas, limiting the ultimate vacuum. [digital recording eliminates this!]
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The Lenses in TEMCondenser lenses(two)-control howstrongly beam is focused (condensed) onto specimen. At low Mag. spreadbeam to illuminate a large area, at highMag. strongly condense beam.
Objective lens-focus image (imageformation) and contribute most to the magnification and resolution of the image.
Four lenses form magnificationsystem-determine the magnificationof the microscope. Whenever themagnification is changed, the currentsthrough these lenses change.
Image Formation in TEM
Ray Diagram for a TEM
Control contrast
Control brightness,convergence
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Why Electrons?
Improved Resolution!In the expression for the resolution
(Rayleigh’s Criterion)
r = 0.61/nsin-wavelength,V-accelerating voltage, n-refractive index
-aperture of objective lens, very small in TEM
sin so r=0.61/ ~0.1 radians (5.5o)Green Light 200kV Electrons~400nm ~0.0025nmn~1.7 oil immersion n~1 (vacuum)r~150nm (0.15m) r~0.02nm (0.2Å)
1/10th size of an atom!unrealistic!
Resolution Limited by Lens Aberrations
point is imagedas a disk.
Spherical aberration is caused by thelens field acting inhomogeneously onthe off-axis rays.
point is imaged
Chromatic aberration is caused by thevariation of the electron energy (PSvoltage) so electrons are notmonochromatic.
rmin0.91(Cs3)1/4
Practical resolution of microscope. Cs–coefficient of spherical aberration of lens (~mm)
as a disk.
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Specimen Holder
a split polepieceobjective lens
holder
beam
Heating and strainingTwin specimen holder
Double tilt heating
Rotation, tilting, heating, cooling and straining
Specimen Holder with Electrical Feedthroughs
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Beam and Specimen Interaction
(EDS)
(EELS)SAED & CBED
diffraction
BFDFHREM
Imaging
Scanning Transmission Electron Microscopy (STEM)
In STEM, the electron beam is rastered (scan coil) across the surface of a sample in a similar manner to SEM, however, the sample is a thin TEM section and the diffraction contrast image is collected on a solid-state detector.
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Imaging in the TEM
• Two principal kinds:
– Diffraction contrast imaging.(bright field / dark field)
Use either a non-diffracted or diffracted beam and remove all other beams from the image by the use of an objective aperture.
– Phase contrast or high resolution imaging.(HREM)
Use all of the diffracted and non-diffracted beams (by using a large objective aperture or none at all) and add them back together (phase and intensity) to form a phase contrast image
Silicon <100> zone axispattern.
Selected Area Diffraction
Parallel Electron Beam
Sample
Diffraction Plane
Selected Area DiffractionParallel Illumination.Lens Aberration limitsresolution to ~1 µm.
Objective Lens
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BF & DF Imaging – Diffraction Contrast
Objective aperture
C-filmamorphous
crystal
D
T
BF image
C-film
crystal
D
T
C-film
crystal
DF image
Diffraction + mass/thickness= Contrast
Objective aperture
DDF CDF
Beam tilt
T-transmittedD-diffracted
Hole in OA
OA OA
DDF – displacive DF; CDF – centered DF
Bright Field (BF) and Dark Field (DF) Imaging
Incident beam
specimen
transmitted beam
diffracted beam
objective aperture
hole in objectiveaperture(10-100m)
BF imaging-only transmitted beam is allowedto pass objective aperture to form images.
BF
DF
DF
DF imagingonly diffractedbeams areallowed to passthe aperture toform images.
Particles in Al-CuAlloy.
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Phase Contrast ImagingHigh Resolution Electron Microscopy
(HREM)
Use a large objectiveAperture (get both beams). Phases and intensities of diffracted and transmitted beams are combined to form a phase contrast image.
TD
Si
Objectiveaperture
Electron diffraction pattern recordedFrom both BN film on Si substrate.
BN
Electron Diffraction
Specimen foil
T D
e-
L 2
r
dhkl
[hkl] SAED pattern
L -camera lengthr -distance between T and D spots1/d -reciprocal of interplanar distance(Å-1)SAED –selected area electron diffraction
Geometry fore-diffraction
Bragg’s Law: = 2dsin
=0.037Å (at 100kV)=0.26o if d=4Å
= 2dr/L=sin2as 0r/L = 2
r/L = /d or
r = Lx 1d
hkl
Reciprocal lattice
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Why are there so many spots?
Reciprocal lattice
k – wave vectorlkl = 1/ – wavelength of electron
SAED Patterns of Single Crystal, Polycrystalline and Amorphous Samples
a b c
a. Single crystal Fe (BCC) thin film-[001]b. Polycrystalline thin film of Pd2Sic. Amorphous thin film of Pd2Si. The diffuse
halo is indicative of scattering from anamorphous material.
r1 r2200
020
110
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Diffraction Spot Intensity
Spot intensity: Ihkl lFhkll2
Fhkl - Structure Factor
Fhkl = fn exp[2i(hu+kv+lw)]N
n=1
fn – atomic scattering factor
fn Z, sin/
h,k,l are Miller indices and u,v,w fractional coordinates
Specimen Preparation-DestructiveDispersing crystals or powders on a carbon film on a grid
3mm
Mechanical Thinning
Grind, Lap
Machine & Slice
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Cross-Sections
Substrate
Four pieces of a specimenformed from thin film(s) on a
substrate.
Thin Film
The four pieces areglued together ( face-to-face ,
and face-to-back ) to forma cross section.
Glue
Cross-Sections...
A 2.8mm diameter pieceis drilled from the
cross section.
The 2.8mm diameter rodis placed within a 3mm
external diameter metal tube.
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Cross-Sections...
Thin slices are cut from the rod,and mechanically thinned to
~100 m.
~100 m
3.0mm
Mechanical Thinning
Planar Thinning Dimple GrindingHolderHolder
DimpleWheel
HolderBeveled Plate
Specimen
ThinRegion Planar Thinning
DiscSpecimen
V
PumpedElectrolyte Jet
PumpedElectrolyte Jet
Electro-Chemical Thinning
Ion Gun
Ion Gun
Specimen
Ion Milling
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Focused Ion Beam (FIB) MillingFIB beam
Sample epoxied to grid
Grid
Sample milled by FIB beam
TEM beam
Focused Ion Beam (FIB) System
• focus a Ga ion beam to a few tens of nanometers to mill the specimen
• interaction of Ga ion beam with the specimen also generatessecondary electrons that can be used for SEM imaging. So wecan observe the area under milling during the milling process.FIB permits selected area milling.
• high specimen milling rates as well as high positional accuracy for milling of the area(s) of interest.
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FIB procedure
a. Select an area of interest,
Coat with a layer of metal (Pt)
b. Make “trench” using
high current ion beam
c. Thinning the wall d. Cut the wall and removehttp://www.labcompare.com/623-Videos/139165-AURIGA-Laser-FIB-SEM-Microscope-from-ZEISS/
Applications of TEM
TEM
Conventional TEMMicrostructure, morphology (grain size, orientation), phase distribution and defect analysis (point defects, dislocations and grain boundaries)
In situ TEMIrradiation and deformation experimentsEnvironmental cells (corrosion)Phase transformations(hot- and cold-stage, electric field)
Analytical TEM (Z-contrast imaging)Chemical composition-EDS, EELS, ELNES, EXELFS, Z-contrast imagingCBED-lattice strain, thickness, charge density
HRTEMLattice imaging, structure of complex materialsand atomic structure of defects (interfaces)
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Sub-Nanometric EDS Analysis (JEOL-2010F Field-emission TEM)
MBE-grown InGaAsP/InPMulti-quantum well structure
EDS spectra taken with a 5ÅProbe. A.1nm InGaAsP layerB.~3nm away from interfaceWithin InP matrix.
A
B
A
B
InGaAsP
InP
HREM
In-situ nano-indentation
See movie
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Limitations of TEM
• Sampling• Interpretation of image
• Beam damage
• Specimen preparation