AMCWorkshop-2008 SEM Ver02 Handouts

8/12/2019 AMCWorkshop-2008 SEM Ver02 Handouts

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Supported by the U.S. Department of Energy under grants DEFG02-07-ER46453 and DEFG02-07-ER46471 2008 University of Illinois Board of Trustees. All rights reserved.

Advanced MaterialsCharacterization Workshop

Scanning Electron MicroscopyScanning Electron Microscopy(SEM)(SEM)

Wacek SwiechJim Mabon

Vania Petrova

Mike MarshallIvan Petrov

2

Basic Comparison to Optical Microscopy

1

2 1/20 0 0

1/20

2 1 1/20 0

:

/ 2 (2 )

: (2 )

[2 (1 (2 ) )]non rel

relat

de Broglie h p

eU m v p m v h m eU

hence h m eU

h m eU eU m c

=

= = =

=

= +

OpticalOptical SEM (secondary electron)SEM (secondary electron)The higher resolution and depth of focus available with the SEM are c learly observed,electrons have small mass (m 0).

From Scanning Electron Microscopy and X-Ray Microanalysis , Joseph I. Goldstein et al. Plenum Press

9.84 ( c/3!)0.7030,000

5.851.2210,000

4.161.735,000

1.873.881,000

1.335.48500

v (x10 7 m/s) relat (x10 2 nm)U (V)

P a r a

l l e

l A c q u

i s i t i o n

S e q u e n

t i o a

l A c q u

i s i t i o n


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3

What does the SEM do?

An instrument for observing andanalyzing the surface microstructureof a bulk sample using a finelyfocused beam of energetic electrons

An electron-optical system is usedto form the electron probe which isscanned across the surface of thesample (raster pattern).

Various signals are generatedthrough the interaction of this beamwith the sample. These signals arecollected by appropriate detectors.

The signal amplitude obtained ateach position in the raster pattern isassembled to form an image.

Many Applications:Most widely employed microscop ytechnique other then optical microscopy

Surface morphology (SE, BSE, FSE)

Composition analysis (X-EDS, WDS)Crystallography (electron diffraction andchanneling techniques)

Optical properties (cathodoluminescence CL)Many other more specialized applications

Animation from A Guide to X-Ray Microanalysis ,Oxford Microanalytical Instruments

4

Secondary and Backscattered Electrons

Backscattered electronsare primary beamelectrons scattered backout of the sample.

Secondary electrons arelow energy electronsejected from the specimenatoms by the energeticprimary beam

Adapted From L. Reimer, Scanning Electron Microscopy ,2nd edition, Springer Verlag

Energy Distribution of Emitted Electrons


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Electron Beam / Specimen InteractionsIncident

BeamSecondaryelectrons (SE)

Characteristic X-rays

UV/Visible/IRLight

BremsstrahlungX-rays

Backscatteredelectrons (BSE)

Auger electrons

EDS/WDSEDS/WDSImagingImaging

CLCL

Heat

Specimen Current

Isc = Ib Ib Ib = Ib 1 +))

Elastic ScatteringInelastic ScatteringMicrometer-size Interaction Volume

ImagingImagingIsc

, E =0.5-30 keV, = 0.2-1 o

6

5 keV

15 keV

15 keV

15 keV

15 keV

25 keV

W

C

Al

TiTiTi

Monte-Carlo simulations of electron scattering

PMMA @ 20kVEverhart et.al. (1972)

Animation from A Guide to X-Ray Microanalysis ,Oxford Microanalytical Instruments

Monte Carlo Calculations, CASINO

Simulations are very useful fortesting if a measurement ispossible or interpreting results

Determine effective lateral or depthresolution for a particular signal in adefined sample

Simulate X-ray generation / X-rayspectra in a defined sample

Simulate image contrast

1 m


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Sequential Image Acquisition in SEM

The scan of the electronbeam and the screen rasterare synchronized withintensity proportional to thecollected signal

Magnification is given bythe ratio of the length of the

line on display device tolength scanned on the realsample

Figure from Scanning Electron Microscopy and X-Ray Microanalysis , Joseph I. Goldstein et al. Plenum Press

M = L display /L specimen

Animations from, The Oxford Guide to X-Ray Microanalysis ,Oxford Instruments Microanalysis Group

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Typical SEM Column / Vacuum ConditionsJEOL 6060LV

Courtesy JEOL USA

An SEM specimen chamber typicallyoperates at high vacuum conditions:Vacuum


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Thermionic Sources- Tungsten Filament / LaB 6

Schematic of a generalizedW-Hairpin electron source

LaB 6W


http://www.feibeamtech.com/pages/schottky.html

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Cold Field Emission Electron SourceSharp Single Crystal (310) Tungsten Tip

(310)

Courtesy Hitachi Instruments From Scanning Electron Microscopy and X-Ray Microanalysis , Joseph I. Goldstein et al. Plenum Press

Courtesy Hitachi Instruments


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Major Electron Beam Parameters


Four electron beam parameters define

the probe which determine resolution,contrast, and depth of focus of SEMimages:

Probe diameter d p Probe current I p Probe convergence angle p Accelerating Voltage V o

These interdependent parameters mustbe balanced by the operator to optimizethe probe conditions depending onneeds:

Resolution Depth of Focus Image Quality (S/N ratio) Analytical Performance

Electron optical brightness, , is a constantthroughout the column, thus is a veryimportant electron source parameter

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Origin of Depth of Focus

Adapted from Scanning Electron Microscopy and X-Ray Microanalysis , Joseph I. Goldstein et al. Plenum Press

Quantifying Depth ofFocus

For an observer it is takenthat image defocusbecomes detectable whentwo image elements fullyoverlap, where an imageelement is given by theresolving power of thehuman eye (~0.1mm).

The depth of focus canthen be describedgeometrically by:

D ~ 0.2 mm / M

Distance above andbelow plane of focusthat beam becomesbroadened to anoticeable sizeblurring in theimage


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Objective

Aperture

Function of Condenser Lens

Ray diagram forlens weaklyexcited

Longer focallength, Small 1, Larger d 1

More beamaccepted intoobjectiveaperture

Higher probecurrent atspecimen

Larger focal

spot atspecimen Lower

resolution Higher Signal

Levels

Ray diagram for lensstrongly excited

Short focal length,Small 1, Smaller d 1

Less beam acceptedinto objectiveaperture

Lower probe currentat specimen

Smaller focal spot atspecimen

Higher Resolution

Lower Signal Levels


De-magnify the beam obtained from the source to enable a small spot to beobtained on the sample. Multiple lenses are more typically used in the condenserlens system.

1 1 1u v f

+ =Object Image

u v / M v u=

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Objective Lens / Working Distance

Focus the electron beam onthe specimen with minimallens aberrations

Short Focal Lengths(W 1) > smaller d 2, larger 2 ->better resolution

Longer Focal Lengths(W 2) > larger d 2, smaller 2 ->better depth of field

Smaller Apertures > smaller d 2, smaller 2 ->

better resolution & better depth of focus

Correction coils are used tocorrect asymmetries in lens(correct astigmatism)

Adapted from Scanning ElectronMicroscopy and X-Ray Microanalysis , Joseph I. Goldstein et al.Plenum Press


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Lens Aberrations / Optimum Aperture Angle


Aperture diffraction

causes a fundamentallimit to the achievableprobe size

Spherical Aberration

Chromatic Aberration

Optimum apertureangle determinedby combined effectof sphericalaberration andaperture diffraction

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Lens Aberrations / AstigmatismLens Aberrations Astigmatism


Astigmatism is caused by imperfections in the lens or other interference.It can be corrected using additional elements called stigmatorscontained inside the objective lens

Octupole lens stigmator

Magnetostaticquadrupole lens isbasis of a stigmator


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Secondary Electron Detector / Imaging

Faraday Cage (collector) is usuallybiased a few hundred volts positive(for collection efficiency)

Scintillator is biased +10kV toaccelerate electrons to sufficientenergy to efficiently excite scintillatingmaterial

Amplified output level is directly usedto set brightness (offset) and contrast(gain) in corresponding pixel in image

Contrast from predominatelyangular dependence ofsecondary electron yield andedge effects.

Everhart-Thornley SE detector

Reactive ion etching of Al/Si(001)

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Secondary Electron YieldDependence of SE yield with angle(local) of incidence with surface

Escape probability forSEs as a function of depthof generationimage resolution for SEimaging will approach theprobe size



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Edge Effect on SE Yield

Adapted From L. Reimer, Scanning Electron Microscopy ,2nd edition, Springer Verlag

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Analogy to Oblique and Diffuse Optical Illumination

Secondary electronyield is stronglydependent on localangle of incidencewith beam

Backscatteredelectrons are alsodirectly and indirectlydetected (image is notpure SE)

Together this, alongwith the high depth offocus of the SEM,gives the familiar SEMimages with a goodperceptive sense ofsurface topography



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Exceptionally High depth of Focus

Carbon NanotubeElectrodeposited Gold DendriticStructure

100,000X original magnification10,000X original magnification

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Extremely Wide Range of Magnifications

Sputtered Au-Pd on Magnetic TapeMiniature Sensor Device -Calorimeter

12X original magnification 500,000X original magnification


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Backscattered Electron Detectors and Yield

Solid State 4 quadrantBackscattered Electrondetector placed annular tobottom of objective lens

Composition image electronically sum signalfrom all 4 quadrants

BSE topographic images differencing various detectorquadrants

Backscattered electron yieldis a strongly dependent onsample mean atomic number

Graphic from, The Oxford Guide to X-Ray Microanalysis ,Oxford Instruments Microanalysis Group

Typical 4 quadrant solid state BSE detector

Objective lens pole piece

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Backscattered Compositional Contrast

Secondary Electron Image Backscattered Electron Image

SnBi alloy most useful on multi-phase samples can be sensitive to < 0.01 average Z differences flat-polished specimens essential


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Non-Conductive Specimens - Charging

=0Isc = Ib Ib Ib = Ib 1 + ))

Upper cross-over energy, E 2,for several materials

Total emitted electron coefficient+ as a function of beam energyWhen +=1 charge balance


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Variable Pressure (VP / LV / Environmental) SEM A different and simpler solution to specimen charging of un-coated non-

conductive samples is to introduce a gas (air, etc.) into the specimen chamber.

The high energy electrons ionize the gas, thus positive ions are available todynamically neutralize any charge on the sample.

Available in both Schottky and Thermionic (Tungsten) instruments

Uncoated Dysprosium Niobium Oxide Ceramic@ highvacuum

@ 20 Paair


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Energy-Dispersive Spectroscopy (EDS) solid state detector simultaneously measures allenergies of X-ray photons

Wavelength Dispersive Spectroscopy (WDS) sequentially measures intensity vs X-raywavelength (energy). Superior energy resolution and detection limits (P/B ratio).

Electron Backscattered Diffraction (EBSD) acquires electron diffraction informationfrom surface of highly tilted bulk sample with lateral resolution of 10s of nm

Cathodoluminesence (CL) optical emission spectrometer and imaging system for 300-1,700nm. Liquid He cooled stage module.

EDSEDS

WDSWDS

CLCL

JEOL JSM-7000F Analytical Scanning Electron Microscope

EBSDEBSD

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Characteristic X-ray Generation A scattering event kicks out

an electron from K,L,M, or Nshell of atom in specimen

An electron from an outershell falls to fill in thevacancy

Energy difference results inrelease of an X-ray ofcharacteristicenergy/wavelength or anAuger electron

Atomic Number

X-ray vs. Auger Generation

Graphic from, The Oxford Guide to X-Ray Microanalysis ,Oxford Instruments Microanalysis Group


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Energies of Characteristic X-rays to 20 keV


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Energy Dispersive X-ray Detector: Si(Li)

Collimator

MagneticElectronTrap

X-ray Window

Si(Li)Detector

FETchargesensitiveamplifier

Copper Rod(at Liq. N 2Temperature)

Graphics from A Guide to X-Ray Microanalysis ,Oxford Microanalytical Instruments


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Mechanism of X-ray Energy Determination

X-ray loses energy through inelasticscattering events creating electron /hole pairs

High voltage bias keeps generatedpairs from re-combining

Charge sensitive amplifier countspairs generated by X-ray

Spectrometer calibration effectivelymultiplies by energy/pair (3.8 eV) todetermine X-ray energy

V o l t a g e

Time

Charge

restore

Voltagestep

X-ray event

Animations from, The Oxford Guide to X-Ray Microanalysis ,Oxford Instruments Microanalysis Group

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EDS Spectral Resolution and Count RatesPulse processing timeconstants are used toadjust available count rateversus spectral peakresolution

Long time constants are best for singleacquisition analysis for best energy resolution.

Short time constants are best for fastacquisition of X-ray elemental maps(elemental distribution images) or line-scans(intensity or concentration profiles).

Compromise is often needed.


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X-ray EDS Microanalysis in the SEM

Fast Parallel Detection

Qualitative elementalanalysis From beryllium up on

periodic table Sensitivities to


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Parallel Beam WDS

X-ray Optical Systems, Inc.http://www.xos.com/index.php/?page_id=71&m=2&sm=3

Comparison of EDS (SiLi) toParallel Beam WDS(Thermo Instruments MaxRay)

Hybrid X-ray Optic containing both apolycapillary optic (up to ~12 keV) and aparabaloidal grazing incidence optic(up to ~ 2.3 keV)

Courtesy Thermo Instruments

Ex. Identification

of sub-micron WParticle on Si

Electron Backscattered Diffraction in the SEM (EBSD)

Courtesy HKL Technology (Oxford InstrumentsMicroanalysis Group)

70(202)

(022)

(220)

Silicon


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EBSD of GaAs Wafer

LT

TN

LN

N

L (RD) Rolling

N (ST) Normal

T (LT) Transverse

Microtexture in Al-Li Alloys for Future Aerospace ApplicationsInvestigation of crystallographic aspects grain morphology and delaminations

Complimentary toXRD texturedetermination

- gives local texture& misorientationsTrue Grain ID, Size and Shape Determination


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Channeling / Diffraction Contrast with a Forward Scatter detector

316L Stainless Steel

Before deformation: equiaxedgrains with no deformation

evident and lots of annealingtwins

Deformed to uniform elongation@ 200 C, considerable strain

contrast, individual grains nolonger discernable, highlydeformed structure

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Limit of Lateral Resolution for EBSD

5 m step size = 0.01 m Deformed Copper

Literature generallyreports a lateralresolution limit (x-direction) of 5 50nm.Resolution in the y-direction is somewhatless due to high tilt ofsample.

Actual resolutionobtained depends on

beam voltage, probesize, sample material(Z), specimenpreparation, etc.


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Phase Discrimination / Phase IDForward Scatter image

Phase Image (Stainless Steel)Red = FCC ironBlue = BCC iron

Z-projected Inverse Pole FigureImage

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Cathodoluminesence (CL)Emission of light from a material during irradiation byan energetic beam of electrons

Wavelength (nm)Collection of emitted light

Parabaloidal mirrorplaced immediatelyabove sample (samplesurface at focal point)

Aperture for electron beam

Collection optic in position between OL lens and sample


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Cathodoluminesence Imaging and Spectroscopy

Optical spectroscopy from 300to 1700 nm

Panchromatic andmonochromatic imaging (spatialresolution - 0.1 to 1 micron)

Parallel Spectroscopy (CCD)and full spectrum imaging

Enhanced spectroscopy and/orimaging with cooled samples(liq. He)

Applications include: Semiconductor bulk materials Semiconductor epitaxial layers Quantum wells, dots, wires Opto-electronic materials Phosphors Diamond and diamond films Ceramics Geological materials Biological applications

(fluorescent tags)

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Pan-Chromatic CL imaging of GaN

Defects (dislocations) areobserved as points or lines ofreduced emission


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Monochromatic CL imaging of GaN Pyramids

SEM composite

550 nm

The strongest yellow emissioncomes from the apex of theelongated hexagonal structure.

CL Image

550 nmCL imaging ofcross-sectionalview

SEM

Results courtesy of Xiuling Li , Paul W. Bohn, and J. J. Coleman, UIUC

Typical CL spectra

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The Dual-Beam Focused Ion Beam (DB-FIB)Electroncolumn

Ioncolumn

Ptdoser

The FEI Dual-Beam DB-235 FocusedIon Beam and FEG-SEM has a highresolution imaging (7nm) Ga + ion columnfor site-specific cross-sectioning, TEMsample preparation, and nano-fabrication. The Scanning ElectronMicroscope ( SEM ) column provides highresolution (2.5 nm) imaging prior to,during, and after milling with the ionbeam. The instrument is equipped withbeam activated Pt deposition and 2 in-situ nanomanipulators: Omniprobe forTEM sample preparation and Zyvex formultiprobe experiments.

site specific cross-sectioning and imaging* Serial sections and 3-D reconstruction are an extension of this method

site specific preparation of specimens for Transmission Electron Microscopy (TEM) site specific preparation of specimens for EBSD nano-fabrication (micro-machining and beam-induced deposition) modification of electrical routing on semiconductor devices failure analysis mask repair


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Ion Microscopy: Ions andElectrons

The gallium ion beam hits the specimenthereby releasing secondaryelectrons , secondary ions and neutralparticles (e.g. milling).

The detector collects secondary electronsor ions to form an image.

For deposition and enhanced etching:gases can be injected to the system.

Layout of the

Focused Ion Beam column

Liquid Metal Ion Source

The Focused Ion Beam (FIB) Column

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Ion/Electron Beam Induced Deposition

Precursor moleculesadsorb on surfacePrecursor isdecomposed by ionor electron beamimpinging on surfaceDeposited film is lefton surfaceVolatile reactionproducts arereleased

IBID and EBID

Similarly, reactive gases, can beinjected for enhanced etching inmilling and improving aspect ratio formilled features

Pt, W, and Au are common metalsSiO x can be deposited as an insulator


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The DB-FIB Specimen Chamber

Pt Injection Needle

FIB Column

CDEM detector

Electron Column

ET & TLDelectron detectors

Specimen Stage(chamber door open)

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Polished CrossSection of Semiconductor 1. Locate area of interest

2. Deposit Pt ProtectionLayer

3. Mill stair step trench(20nA), face (1nA)cross-section, andpolish (0.1nA) - all withion beam

4. Image features with

SEM or Ion beam


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TEM Sample Preparation w/ Omniprobe

Step 1 - Locate the area ofinterest (site specific)

Step 2 - Deposit a protectiveplatinum layer

Step 3 - Mill initial trenches

e-beam view after Step 3

Step 5 - Perform frame cutsand weld" manipulator needle tosample

Step 6 Mill to release fromsubstrate and transfer to grid

Step 7 "Weld" sample to a CuTEM half-grid and FIB cutmanipulator needle free

Step 8 - FIB ion polish toelectron transparency

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Pre-Thinned TEM Sample prepared by FIBDiced Wafer with Thin Film Prethinned Section

Prethinned TEM Sample on Half Grid After Thinning

TEM Direction

TEM Direction

Ion Beam DirectionPt ProtectionLayer

Half Grid

Grind to 30 MicronsDice to 2.5 mm


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Pre-Thinned TEM Sample prepared by FIB

Drawing of typicalpre- thinnedspecimen for FIBTEM samplepreparation

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Software for Controlled Patterning in FEIDB235

Developed by Jim Mabon


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Precisely controlled etching and deposition

500 nm

Pt Flower

30 nm Pt dot array

Etched or deposited structures using grey-scale bitmaps (more complex,parallel process) or scripting language (sequential, unlimited # of points).

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Photonic Array: A seed layer for photoniccell crystal growth nucleation fabricated(etching) with the FIB.J. Lewis, P. Braun

Pt Dot: This Platinum dot is used as an etchmask in porous silicon experiments.P. Bohn

FIB patterning by etching and/or depositionBitmap Script


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5m

[110]

[110]

Step 1 :Micro-structures by FIB fabrication

: 5.5 : 13.7

1m

: 75

[110]

Step 2 :

Ge growth on extra large-miscut Si

Step 3 :Isolate specified orientation

Exploration of novel orientations in Si crystalline structure:Ge nanostructure growth on Si

: 28.2

FIB & AFM

K. Ohmori, Y.-L. Foo, S. Hong, J.-G. W en, J.E.Greene, and I. Petrov, Nanoletters, 5 369 2005

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Ge nanowires on Si(173 100 373)

Long straight Ge nanowires on Si(173 100 373)period: 60 nmwidth: ~40 nm

azimuthal direction d : 114

500 nm

[110]d

Z-contrast STEM image20 nm

[121]Si substrate

(517)(113) (111)

Hill-valley structure composed of(113)&(517) facets

Plane index: (k 100 200+k)

4 m

: 75 p

: 28.2 [110]

K. Ohmori, Y.-L. Foo, S. Hong, J.-G. W en, J.E. Greene, and I. Petrov,Nanoletters, 5 369 2005


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Summary: Scanning Electron Microscopy

Remarkable depth of focus Imaging from millimeters to a nanometer Chemical composition with 0.1-1 m resolution Crystallography using electron EBSD Optical properties on a micrometer scale (via CL)

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Site specific cross-sectioning imaging and EBSD Serial sections and 3-D reconstruction are an

extension of this method Site specific preparation of specimens for

transmission electron microscopy (TEM) Nano-fabrication (micro-machining and beam-induced

deposition)

Modification of the electrical routing on semiconductordevices

Summary: Focus Ion Beam Microscopy


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References

Scanning Electron Microscopy and X-ray Microanalysis by Joseph Goldstein, Dale E.Newbury, David C. Joy, and Charles E. Lyman (Hardcover - Feb 2003)

Scanning Electron Micros copy: Physics o f Image Formation and Microanalysis (SpringerSeries in Optical Sciences) by Ludwig Reimer and P.W. Hawkes (Hardcover - Oct 16, 1998)

Energy Dispersive X-ray Analysis in the Electron Microscope (Microscopy Handbooks) by DCBell (Paperback - Jan 1, 2003)

Physical Principles of Electron Microscopy: An Introducti on to TEM, SEM, and AEM by RayF. Egerton (Hardcover - April 25, 2008)

Adv anced Scann ing Elect ron Micr osc opy and X-Ray Micr oanalysi s by Patrick Echlin, C.E.Fiori, Joseph Goldstein, and David C. Joy (Hardcover - Mar 31, 1986)

Monte Carlo Modeling for Electron Microscopy and Microanalysis (Oxford Series in Opticaland Imaging Sciences) by David C. Joy (Hardcover - April 13, 1995)

Electron Backscatter Diffraction i n Materials Science by Adam J. Schwartz, Mukul Kumar,David P. Field, and Brent L. Adams (Hardcover - Sep 30, 2000)

Introduction t o Texture Analysis: Macrotexture, Microtexture and Orientation Mapping byValerie Randle and Olaf Engler (Hardcover - Aug 7, 2000)

Electron backscattered diffraction: an EBSD system added to an SEM is a valuable newtool in the materials characterization arsenal : An article from: Advanced Materials & Processesby Tim Maitland (Jul 31, 2005)

Cathodolumin escence Microscopy of Inorganic Solids by B.G. Yacobi and D.B. Holt(Hardcover - Feb 28, 1990)

Introduction to Focused Ion Beams: Instr umentation, Theory, Techniques and Practice by

Lucille A. Giannuzzi and Fred A. Stevie (Hardcover - Nov 19, 2004)

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Acknowledgements

Frederick Seitz Materials Research Laboratory issupported by the U.S. Department of Energy

under grant DEFG02-07-ER46453 and DEFG02-07-46471

Sponsors:

Documents

AMCWorkshop-2008 SEM Ver02 Handouts