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David Muller 2008
Scanning Electron Microscopy
David Muller 2008
10 4
10 5
10 6
10 7
10 8
0 100 200 300 400 500 600 700
Inte
nsity
(arb
. uni
ts)
Energy Loss (eV)
O-K edge
Si L edge
Incident Beam
Valence Excitations
Electron Energy Loss Spectrum of SiO2
Most likely energy transfer is ionization of valence electrons
David Muller 2008
Path of the Electron Beam
SE1SE1SE2SE2BS2BS2
David Muller 2008
Kanaya-Okayama Depth Penetration Formula
RR= = ____________________________0.890.89(Z (Z ρρ))
0.0276 A E0.0276 A E 1.671.67μμmm
R= Depth PenetrationR= Depth PenetrationA= Atomic Weight (g/mole)A= Atomic Weight (g/mole)E= Beam Energy (KV)E= Beam Energy (KV)Z= Atomic numberZ= Atomic numberρρ = density (g/cm )= density (g/cm )22
David Muller 2008
The Affect of Accelerating Voltage
30KV30KV 15KV15KV 5KV5KV 1KV1KV .5KV.5KV
3.1 3.1 μμmm
.99 .99 μμmm
.16 .16 μμmm.01 .01 μμmm(100A)(100A)
35 A35 A
Depth Penetration in IronDepth Penetration in Iron
Primary BeamPrimary Beam
(predictions from the KO formula)
David Muller 2008
Interaction Volume vs Accelerating Voltage
25 kV25 kV
15 kV15 kV5 kV5 kV
Better control of where SE, BSE and x-rays are produced at lower beam voltages
David Muller 2008
Interaction Volume –Sample Composition
SilverSilver
CarbonCarbon
IronIron
(20 kV incident beam in all 3 cases)
Pear to apple-shaped
David Muller 2008
Secondary Electrons
SE1SE1SE2SE2
SE3SE3
final lensfinal lens
specimenspecimen
BSEBSE
David Muller 2008
Electron Interactions (Between Primary Beam and Sample)
• SE1- at point of primary interaction• SE2- away from initial interaction point• SE3- by BSE outside of sample• BSE1- at point of primary interaction• BSE2- away from initial interaction point
David Muller 2008
Lateral Distribution of SE
SE1SE1
SE2ASE2A
SE2BSE2B
SE Escape DepthSE Escape Depth
Total Beam Penetration VolumeTotal Beam Penetration VolumeSE1 > 100KXSE1 > 100KXSE2ASE2A-- 50KX50KXSE2B< 15KXSE2B< 15KX
David Muller 2008
Lateral Distribution of BSE
BS2ABS2A
BS2BBS2B
BS1BS1
BS2A Escape DepthBS2A Escape DepthBS2B Escape DepthBS2B Escape Depth
David Muller 2008
One Primary Electron In Can Create Several SEs Out at Low Accelerating Voltages
100 100 AngstromsAngstroms
inPEoutSE
=δSecondary Electron Yield Coefficient
David Muller 2008
Energy Distribution of Emitted Electrons
00 50 50 eVeV 2 kV2 kV EEPEPE
SESE BSEBSE
AugerAuger# of
ele
ctro
ns#
of e
lect
rons
colle
cted
colle
cted
electron energyelectron energy
inPEoutSE
=δSecondary Electron Yield Coefficient
David Muller 2008
As-received samples are all coated with a carbon contamination layerOverall scaling factor is from the different backscattering responses of the substrate
Secondary Electron Yields
Cleaned in-situCarbon-contaminated
David Muller 2008
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0KV
EC1
EC2
KV
Charge Neutralizationδ
Incident Beam Voltage
Electron Yield δ= # SE out / # Inc e- in
Sample charges +ve(increases landing energyOf incident electrons)
Sample charges –ve(reduces landing energyOf incident electrons)
Q: If sample charges, does it get brighter?
David Muller 2008
Voltage Contrast with SE
The floating end of the via chain is bright because of trapped negative charge causes secondary electrons to be repelled.
The remainder of the chain is neutral, and thus
darker.
(http://www.acceleratedanalysis.com/hepvc.html)
(SE have low energies so are easily deflected by small voltages)
David Muller 2008
High Vacuum E.T. Secondary Electron Detector
Photomultiplier
Light guide
glass target
Phosphorousscreen (Al-coated)(10 kV)
Faraday cage(-150 - +300 V)
Scintillator
David Muller 2008
Secondary Electron Detectors
Specimen
PMT
E.T. SED
TLD
InternalLens
David Muller 2008
Field-Free Operation Immersion Lens
Small area, high resolutionLarge area, lower resolution
Lens Modes of a Modern SEM
David Muller 2008
TLD in BSE Mode
Specimen
• Within-the-lens detector ispart of the final lens
• Bias voltage down to -150V SE
SE3
BSE
David Muller 2008
Topography Affects Secondary Electron Emission (Angle of Incidence)
David Muller 2008
Scanning Action of the Electron Beam in a 3-D
Specimen
David Muller 2008
Location of Detector Leads to Shadowing
AABB
CC
+300 V+300 V
SESE--detectordetector
Increasing the detector bias will wash out the shadows
David Muller 2008
What Is “Reality” in the SEM ?
David Muller 2008
What Is “Reality” in the SEM ?
Previous image turned upside down. We need to know where the detector is to tell bumps from pits!
David Muller 2008
Why Edges Appear Brighter
David Muller 2008
Edge Effect at Lower Voltage
David Muller 2008
Edge Effects on a Sphere
A
B
50
100
150
200
250
0 200 400 600 800 10002nd
e- In
tens
ity
Distance (microns)
David Muller 2008
Example of Sample charging in a Secondary Electron Image
Charging is worseOn this faceAs more secondariesescape
David Muller 2008
A Line Profile on a Semi-conductor Line
One Micron SiO2 in SiOne Micron SiO2 in Si
The EThe E--Beam line profile of the specimenBeam line profile of the specimen
Where do you measure “One Micron” ?Where do you measure “One Micron” ?
David Muller 2008
A Line Profile on a Semi-conductor Line
One Micron SiO2 in SiOne Micron SiO2 in Si
A 1 KV bean has minimal beam penetration and can giveA 1 KV bean has minimal beam penetration and can givean image that is closer to ‘reality’.an image that is closer to ‘reality’.
1KV1KV
.99 .99 μμmm
David Muller 2008
A Line Profile on a Semi-conductor Line
One Micron SiO2 in SiOne Micron SiO2 in Si
A 5KV beam penetrates deep into the specimenA 5KV beam penetrates deep into the specimenwhich gives the appearance of the peakswhich gives the appearance of the peaks
being closer togetherbeing closer together
5KV5KV
.74uM.74uM
David Muller 2008
Line Profiles on the Same Sample Can Change with Accelerating Voltages
1KV1KV
3KV3KV
2KV2KV
5KV5KV
.99 .99 μμmm .92 .92 μμmm
.85 .85 μμmm .74 .74 μμmm
This was a 1 This was a 1 μμm linem line
David Muller 2008
Secondary Electrons
SE1SE1SE2SE2
SE3SE3
final lensfinal lens
specimenspecimen
BSEBSE
David Muller 2008
Angular Distribution of BSE
• Normal angle of incidence
• Greater angle of incidence
David Muller 2008
Angular Distribution of BSE
• Contrary to SE images, BSE images can have dark edges
David Muller 2008
Electron Emission Coefficient Vs. Atomic Number at 20 KV
• Electron Emission Coefficient
• Atomic Number
TotalTotal
BSEBSE
SESE
2020 8080
11
.2.2
David Muller 2008
SE Electron Emission Coefficient Vs Atomic Number at Various KV
• SecondaryElectron Emission Coefficient
• Atomic Number
2KV2KV
5KV5KV
10KV10KV
15KV15KV20KV20KV
2020 8080
11
.2.2.2.2
David Muller 2008
SE Emission Coefficient Vs. KV at Various Atomic Numbers
• Secondary Electron Emission Coefficient
• Accelerating Voltage in KV
55 1515 2525
11
.2.2.2.2
AUAUALAL
CC
David Muller 2008
Z Dependence of BSE
From “Scanning Electron Microscopy and X-Ray Microanalysis”, Goldstein et al, 3rd ed. Chap 3
David Muller 2008
Tilt Dependence of BSE
From “Scanning Electron Microscopy and X-Ray Microanalysis”, Goldstein et al, 3rd ed. Chap 3
David Muller 2008
Reverse Biased S.E.D. Repulses Secondary Electrons
--150 V150 V
SESE--detectordetector
AABB
CC
David Muller 2008
Backscatter Electrons Ignore the Bias
--150 V150 V
SESE--detectordetector
BSEBSE
AABB
CC
David Muller 2008
A Solid State BSD Can Image Two Ways
• Elemental backscatter images are acquired by adding detectors A+B.
• Topographical backscatter electron images can be acquired by subtracting b from a (A-B)
David Muller 2008
Solid State BSD
From http://www.jeol.com/sem_gde/bkscat.html
David Muller 2008
-ve Biased E-T Noisy Backscattered Signal
-ve Biased E-T Secondary Electron Signal
Grains in a Polished Fe-Si Alloy imaged by Different SEM methods
Backscattered A-B “Topographic” SignalBackscattered A+B “Composition” Signal
David Muller 2008
Electron Channeling in a Crystal
Electron wave fields within a crystal for incident electron directions close to the Bragg angle qB. The vertical lines are the position of the Bragg reflecting atomic planes. From H. Niedrig, “Electron backscattering from thin films”, Journal of Applied Physics -- April 1982 -- Volume 53, Issue 4, pp. R15-R49
David Muller 2008
Electron Channeling in a Crystal
Electron Backscatter Diffraction Pattern of Germanium. Right –automatic indexing software matches the high symmetry zone axes and spacing between them to identify the crystal type and orientation. (University of Queensland, http://www.uq.edu.au/nanoworld/xl30_anl.html)
David Muller 2008
Electron Backscattering Diffraction Patterns (EBSD or EBDP) for Orientational Imaging
Orientation Imaging Map (color shows grain orientation)
Boundary – Color shows angle of grain boundary
(From http://www.edax.com/technology/EBSD/OIM/intro6.html)
David Muller 2008
Sample Prep for EBSD
No pattern visible3 micron diamond polish
Pattern Image Quality (IQ) = 251 micron alpha alumina
Pattern IQ = 17710 minutes colloidal silica
Pattern IQ = 22430 minutes colloidal silica
http://www.edax.com/TSL/support/EBSD_Sample_Prep.html
Damage layer must be much less than the range of the electrons
