Overview of Scanning Electron Microscope, Transmission …bart/627/P627_S13_L05_08Feb2013... ·...
Transcript of Overview of Scanning Electron Microscope, Transmission …bart/627/P627_S13_L05_08Feb2013... ·...
Overview of Scanning Electron Microscope,Transmission Electron Microscope,
Scanning Transmission Electron Microscope,Low Energy Electron Microscope
P.E. Batson
with help from K.A. Mkhoyan, U. Minnesota
• Why electrons rather than light?• What physical processes do we use for obtaining image contrast?• How are the instruments above related?• What are the results like?
KA Mkhoyan
rest mass kinetic energy
Total Energy
Wavelength of light -> 3800 – 7500 Å -> not very interesting for materials characterizationwhere regions of materials are less than 10 nanometer (100 Å) or smaller,
and atomic distances, where much new physics occurs, are smaller than 1 nanometer.
KA Mkhoyan
1
0.1V -> 38 Å1.0V -> 12 Å100V -> 1.2 Å1kV -> 0.38 Å10kV -> 0.12 Å
Now, what happens when electrons interact with the specimen and howdo we collect them?
Large angle, relatively chaotic processes
Small angle, relatively well behaved processes
Image plane, or detector
Source
Optics
Apertures optimized for signal
http://www.purdue.edu/rem/rs/sem.htm#2
SEM System islargely a probe formingoptical system that imagesby scanning and collectingserial, time varyingsignals that are fed to a TV-like display
http://www.tasc.infm.it/research/tem/images/semimage.jpg
Electron Column
Gun
Optics
Specimen
Pumps
Knobs and displays
Detectors
Computers!
The ORION™ Helium ion Microscope from Carl Zeiss SMTCurrently one of these is being installed at Rutgers:
Torgny Gustaffson - Len Feldman
50μm
Anthophyllite asbestos
Size of fibers – about 50μm
Alveoli Size 140 μmCell Size 10-100 μm
Result: Cancer
http://usgsprobe.cr.usgs.gov/picts2.html
Topographical Contrast
ProbeForming
The first 50 Years of Electorn Microscopy
The first Next 50 Years of Electorn Microscopy
7’6”
Yao Ming
Many signals, one microscope: Nion UltraSTEM™
Aberration corrector 1
Aberration corrector 2
Described in: Krivanek et al. Ultramicroscopy 108 (2008) 179-195 and Dellby et al. EPJAP 2011. More info at www.nion.com.
UltraSTEM200* UltraSTEM200*
*instrument shown:CNRS Orsay, France*instrument shown:CNRS Orsay, France
UltraSTEM200*
*instrument shown:CNRS Orsay, France
Fully modular (all lenses, the corrector, etc., are independent modules, with identical mechanical interfaces) and thus very flexible.
Ultra-stable, friction-free sample stage.
Operating voltage range 20-200 kV.
Efficiently coupled EELS.
UHV at the sample (<10-9 torr; <10-7 Pa).
http://www.specs.de/cms/upload/PDFs/SPECS_Prospekte/LEEM.pdf and IBM Corp (Ruud Tromp)
Probe forming
Image forming
Low Energy Electron Microscopy
http://www.research.ibm.com/leem/
2nm resolution today
Deceleration
http://www.specs.de/cms/front_content.php?idcat=209
Electron Column
Gun
Optics
Specimen
Pumps
Knobs and displays
DetectorsComputers!
Specimen
Source
SEM TEM/STEM LEEMEnergy 2-10 KeV 50-200 keV 0-10 eVProbe Size 1-10 nm 0.1-1 nm 2-5 nm
Probe FormingOptics
Imaging Optics
Variations on a Theme!
Energy width -> 0.8 – 3 eV 0.25-0.40 eV
++--
Tungsten thermionic total current -> 0.1mA/srField Emission total current -> 1.0 μA/sr
so where small probe is not needed, tungsten wins.
Cylindrical Electron Lens
Lorentz ForceF =(-e/c) v x B
so Fθ = (-e/c) voBr
producing a spiral: vθ
finally
Fr = (-e/c)vθBz
so that electron beam is deflectedtowards axis
Br
Bz
V
Cylindrical Lens Focusing
Specimen
Source
SEM TEM/STEM LEEMEnergy 2-10 KeV 50-200 keV 0-10 eVProbe Size 1-10 nm 0.1-1 nm 2-5 nm
Probe FormingOptics
Imaging Optics
Reiterating variations on a theme.
Now, what happens when electrons interact with the specimen and howdo we collect them?
First: THICK Specimens
Large angle, relatively chaotic processes
Small angle, relatively well behaved processes
Image plane
Source
Optics
Apertures optimized for signal
With thick specimens, most of the signal comes out the front side.
0 Eo – Incident beam Energy
Primary Imaging Mechanism for the SEM and HIM: Secondary and Backscattered Electrons
Fred Cosandey
http://upload.wikimedia.org/wikipedia/commons/8/82/SEM_blood_cells.jpg
Why does Secondary Electron Imaging look like such a “natural” rendering of the specimen?
Secondary electron image of red blood cells shows surface topography in a very easilyunderstood image.
Unlit areas in the common situation on the right are dark in the scanning electron case. The image looks “natural” because it matches a situation we encounter nearly every moment of the day.
Important result: If we reverse the ray paths, and exchange the source and detectorswe will get the same contrast.
eye
Surface Surface
LightSource
ElectronSource
detector
ShadowShadow
Illustrates a general principle: Reciprocity. If we interchange the source and detector and reverse the particle paths, we get identical results.
http://upload.wikimedia.org/wikipedia/en/0/00/SEM_SE_vs_BE_Zr_Al.png
Secondaries BackscatteredBackscattered electrons are higher energies:penetrate deeper, see less surface contrast
Fred Cosandey
Suppose your specimen thickness is much less than a few times λ ?
Then the large plume of scattered electrons below the surface does not exist !!
λ
The secondary electron resolution can then be as good as the probe size.
Zhu, Y., Inada, H., Nakamura, K. & Wall, J. Nature Mater. 8, 808–812 (2009).
Secondary Electron Image using Hitachi STEM at Brookhaven
Annular DarkField
Backsideparticle
Backsideparticle
Zhu, Y., Inada, H., Nakamura, K. & Wall, J. Nature Mater. 8, 808–812 (2009).
Secondary Electron Image using Hitachi STEM at Brookhaven
Atomic resolution with secondary (and backscattering) electrons
Backside atomvisible in AnnularDark Field
Backside atommissing in secondaries
Now, what happens when electrons produce xrays?
Large angle, relatively chaotic processes
Small angle, relatively well behaved processes
Image plane
Source
Optics
Apertures optimized for signal
Background due to Bremstrahlung (deceleration) radiation from beam electrons
Light elements
Typical Energy Dispersive X-Ray Spectrum
Multiple linesfrom singleelement
Apollo SDD - Silicon Drift Detector
Older design required liquid nitrogen for high performance, because the charge amplifierwas located away from the Si detector. This particular picture is actually of a new detector designed for performance under hard radiation.
In the new design, the charge amplifier is located on the silicon detector, lowering noise and allowingoperation using only a Peltier cooler. No liquid nitrogen is needed.
These detectors work by collecting electron hole pairs generated by incoming x-ray photons. The number of carriers is proportional to the energy ofthe x-ray, so a pulse is generated for each x-raywith a height proportional to the x-ray energy.A preamp and pulse height analyzer producesthe spectrum.
http://gsc.nrcan.gc.ca/labs/ebeam/images/sem6.jpg
We can use a single peak in the EDX spectrum
To produce maps for different elements.Here is a result for a natural mineral.
This can also be a high resolutiontechnique if the specimen is made verythin.
Specimen
Source
SEM TEM/STEM LEEMEnergy 2-10 KeV 50-200 keV 0-10 eVProbe Size 1-10 nm 0.1-1 nm 2-5 nm
Probe FormingOptics
Now look at theImaging Optics
Now, what happens when electrons interact with the specimen and howdo we collect them?
Large angle, relatively chaotic processes
Small angle, relatively well behaved processes
Image plane
Source
Optics
ConjugatePlanesConjugate
Planes
What changes hereis the intermediatelens
Diffraction Imaging
Crystallites in the material which are slightlyrotated.
Patterson Function Inversion Model
Recombined to form interferenceFred Cosandey
Reciprocal lattice RODS Making Satellitespots
Imaging a small step at the surface
Fred Cosandey
Spots for types a,b,c layers can be apertured to reveal thedomain behavior of that surface structure
Imaging using surface satellite spots
Fred Cosandey
Small Angle elastic scattering makesinterference patterns
Specimen
Source
SEM TEM/STEM LEEMEnergy 2-10 KeV 50-200 keV 0-10 eVProbe Size 1-10 nm 0.1-1 nm 2-5 nm
Probe FormingOptics
Imaging OpticsNow moving to the LEEM
http://www.research.ibm.com/leem/
The LEEM does typicalTEM things, but using <10eV electrons at the specimensurface. Here is an example of Dark Field Imaging
Si 2x1 reconstruction has 2 orientationsGreen Spot Red Spot
Scanning Transmission Electron Microscopy
What’s different: Strong dependence on
Analytical Information
Two types of signals:Elastic vs Inelastic Scattering Large angle,
relatively chaotic processes
Small angle, relatively well behaved processes
Source
Optics
Electron Energy Loss Spectroscopy
Annular Dark Field STEM
Atomic structure, EELS in nanoscale devicesBonding = 2 - 5eV changesConduction bands = 0.2-1eV changesStrain = 0.05-0.25eV changes P.E. Batson, Jan. 2004
Electron Energy Loss Spectroscopy
Bright Field (BF) and Annular Dark Field (ADF) Imaging
Small Angle Phase Contrast Large Angle Scattering Rutherfordand Thermal Diffuse Scattering
BF-ADF pair using same defocus – optimized for ADF
Interference Patterns Positive Definite Guassian Images
Aberration Correction!! Silicon [110]
Before After
120kV 25 year old instrument
SJ Pennycook, M. Chisholm, A Lupini, M Varela, K van Benthem, A Borisevich, M Oxley, W Luo, and S Pantelides, In Advances inImaging and Electron Physics - Aberration-Corrected Electron Microscopy, 153 327-384. Elsevier (2008).
The number of accessible latticereflections increases from 2-3 at 2 Angstromto as many as a dozenat 0.8 Angstroms
GaAs Example fromAR Lupini
[111]
[211]
[321][110]
Scherzer Defocusto balanceSpherical Aberrationwith Defocus
CTEM: Bright Field Phase Contrast
Jia, C.L. & Urban, K. (2004). Atomic-resolution measurement of oxygen concentration in oxide materials. Science 303, 2001.
Uniformity ofcontrast is resultof high pass filter
Aberration Corrected
Defocus delocalization
CTEM: Bright Field Phase Contrast
Now, we look at EELS analytical signalsusing transmitted electrons.
Large angle, relatively chaotic processes
Small angle, relatively well behaved processes
Source
Optics
Electron Energy Loss SpectroscopyAnnular Dark Field STEM
Gatan GIF/Enfina/Tridiem/Quantum/HREELS
Enfina:
CCD Array
HREELS:
HREELS: 60-250meV, depending on exposure timeHigh throughput P.E. Batson, Jan. 2004
Multipole correction at entrance
x500
x5000
Typical results for cold field emission system – 0.3 eV. Intensity at the Si L23 edge is larger than that in the direct interband region. P.E. Batson, Jan. 2004
Core Loss: composition,chemistry, electronic structure
Optical Excitations
Direct Band Gap HARD!!
Gatan/Phillips Tecnai Performance
Si L23 edge at ~100eV2 nm probe
Zandbergen Lab in Delft
These are very exciting results, because they are obtained from commercial equipment.In the past all high resolution results were on home built machines.
P.E. Batson, Jan. 2004
Processing required to convert measured data
into 2p3/2 intensity for comparison with theory :
1) Background subtraction2) Deconvolution 3) Removal of 2p½ part
Allows reproducible comparisons
Si L2,3
P.E. Batson, Jan. 2004
Using DOS defined
A(E-Eth)1/2(Emax-E)
And a scattering theory That includes the
core exciton,We can fit the data
In terms of the model DOS
P.E. Batson, J. Electron Microscopy 45, 51–58 (1996).P.E. Batson, Jan. 2004
ELNES Si L3 Interpretation
Dipole selection rules are important
Features in EELS corespond to Regions of high DOS
Brillouin Zone Center is mostly p-like and so is dipole forbidden and does not show strongly in data
P.E. Batson, Jan. 2004
GeSi Alloy SeriesMost conduction band BZ special points canbe followed as we go from Si to Ge
P.E. Batson, Atomic Resolution Electronic Structure in Silicon-Germanium Alloys, J. Electron Microscopy 45, 51-58 (1996).
P.E. Batson, Jan. 2004
We can use thisCB information tofollow band offsetin special cases:
In GeSi alloys, theSi core loss is constant and is usedas a zero in energyto get CB position.
ħ γ = ħ vf / R
Damping follows the form:
where vf is the “Fermi” velocity for Sigives vf = 2.05x108 cm/s
Size LimitedConductivity
Drude form for the Conductivity:
σ = n e2 τ / m
with τ= 1/γ
Providing a link between theLocal conductivity and
damping parameter.P.E. Batson, Jan. 2004
J. Nelayah, et al., "Mapping surface plasmons on a single metallic nanoparticle," Nature Physics 3, 348-353 (2007).
Mapping Surface Plasmon Scattering Using EELS Signals
This is the supporting materialfor the result of the next slide.
Spectra were acquired from 400-850 eV.
Ti L23, Mn L23 and La M45 absorptionedges were acquired for each pixel of the area shown in (B).
The Ti L23 shows pronounced finestructure changes as you go into theLaxSr(1-x)MnO3 from the SrTiO3 .
ADF imaging shows disorder at interfaces,
FFT of image shows 0.98A spot.
Quantification for elements, result in the mapson the previous sklide.
Example of Core Loss Imaging:EELS atomic-resolution chemical mapping
D. A. Muller, L. Fitting Kourkoutis, M. Murfitt, J. H. Song, H. Y. Hwang, J. Silcox, N. Dellby, O. L. Krivanek, Science 319, 1073-1076 (2008).
Example of Core Loss Imaging:EELS atomic-resolution chemical mapping
Pulsed laser-depositedLa0.7Sr0.3MnO3/SrTiO3multilayer
40 mr illum. half-angle0.7 nA beam current~1.2 Å probe>70% eff. EELS coupling
64x64 pixel map7 msec per pixel, i.e. 29 sec total acquisition time 10 sec additional processing time
i.e., <1 min total time5 Å
D. A. Muller, L. Fitting Kourkoutis, M. Murfitt, J. H. Song, H. Y. Hwang, J. Silcox, N. Dellby, O. L. Krivanek, Science 319, 1073-1076 (2008).
Ti (L)La (M)
RGBMn (L)
Kazu Suenaga & Masanori Koshino, Nature, 2010.
MAADF imaging of single layer BN with impurities
60 keV MAADF image, 6 x 106 e- / Å2, with probe tails and high spatial frequency noise removed.Image courtesy Matt Chisholm, ORNL, image processing by olk.B and N atoms are readily identifiable by their MAADF intensities.C and O substitutional impurities are identifiable in the line profiles.
O.L. Krivanek, M.F. Chisholm et. al., Nature 464 (2010 ) 571-574.
Histogram analysis of MAADF image (in which probe tails have been removed)shows that B, C, N and O can be identified unambiguously in monolayer BN.
The experimentally worked out dependence of image intensity on Z goes as Z1.64.
BN monolayer with impurities: histogram analysis
Result of DFT calculation overlaid on an experimental image
Cx6
Na adatom
O
N
Longer bonds
C ring is deformed
B C
C
O
MAADF imaging of single layer BN with impurities