Secondary electron detector
description
Transcript of Secondary electron detector
Secondary electron detectorelectron strikes scintillator and converted to light pulse - Amplified and displayed
Raster the beam over sample and display at the same time and get image (basically an intensity map)
Scan smaller and smaller areas to increase magnification
Object: Convert radiation into an electrical signal which is then amplified
SelectSecondary electrons
Backscattered electrons
X-rays
Auger electrons
Photons from Cathodoluminescence
Absorbed electron current
Incident beamLight
(cathodoluminescence)
BremsstrahlungSecondary electrons
Backscattered electrons
heat
Elastically scattered electrons
Transmitted electrons
Specimen current
Auger electrons
Characteristic X-rays
Sample
Any of the collected signals can be displayed as an image if you either scan the beam or the specimen stage
5 m
Signal Detection
Electron Detectors
Scintillator – Photomultiplier system (Everhart-Thornley, 1960)
1) Electron strikes scintillator
plastic
Li-glass
CaF2 (Eu)
P47
Photons produced
2) Light conducted by light pipe to photomultiplier
3) Signal passes through quartz window into photomultiplier
4) Photons strike electrodes – emit electrons (photoelectric effect)
5) Electrons cascade through electrode stages
output pulse with 105 – 106 gain
Up to 300V potential to collect secondary electrons
Deflect – does not require line-of-sight geometry
Collection efficiency ~ 50% SE
~ 1-10% BSE
Backscattered Electron Detectors
Usually solid state devices
Annular – thin wafer (Si semiconductor)
Extrinsic p-n junction
p-type = positive charge carriers (holes) dominant
n-type = negative charge carriers (electrons) dominant
Use Li as donor
Use B as acceptor
1) Backscattered electron strikes semiconductor
2) Valence electron promoted to conduction band – free to move
Leaves hole in valence band
3) No bias → recombination
Forward bias → current
~ 3.6 eV expended per electron / hole pair
Current of 2800 electrons flows from detector if 10keV electron enters
4) Amplify signal
5) Display
Energy-filtered electron detectorsIn lens detectors
EsB = Energy selective backscatteruses filtering grid
AsB = Angle selective backscatteruses angle
INLENS SE image from a sectionedsemiconductor. Clearly visible: No BSE contrast!
The same section but seen with the LL-BSE; detected with the INLENS EsB at 1.27 kV
Si
Ti
TiN
Si3N4
Simultaneously acquired In-lens SE (left) and EsB image (right) from a fuel cell showing the outer electrode. We see doped ZrO2 and different phases of Ni-oxide.
Gold particles seen with the In-lens SE and AsB detector. We see surface contrast with the In lens SE and crystalline contrast from single elastic scattered BSE electrons (Mott scattering).
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Beam deceleration: enhancing resolution and contrast
If Bias=0 (no BD):Landing V = HV
What is beam deceleration?New optics mode enabling high resolution imaging and high surface sensitivity at very low kV
BD specifications:• Landing energy range: 30 keV down to 50 eV• The deceleration (Bias) can be continuously adjusted
by the user
Benefits:• Enhances the resolution • Provides additional contrast options• Greatest benefit at 2kV and below
Bias
HVLanding V
Beam
vCD
Sample
TLD
2-mode final lens
Gold on carbon1kV1.75MX imaging, <0.9nm resolution
Gold on carbon2kV2.8MX imaging, <0.8nm resolution
Deprocessed IC1kV600KX imaging
Pt catalyst nanoparticles2kV1.0MX imaging
Low voltage-high contrast detector with beam deceleration
Through-the-lens detector with beam deceleration
Through-the-lens detector without beam deceleration
Pt sample. Landing energy 2keV, Beam deceleration=4kV.
Image Formation
Scanning
Signals are produced as beam strikes sample at single location
To study an area, must scan either beam or sample stage
For beam scanning, there are 2 pairs of scan coils deflecting the beam in X and Y
located in bore of objective lens
Produce a matrix of points – a map of intensities
Output displayed on screen or collected digitally
Each point on specimen corresponds to point on screen
Scanning is synchronized
Emission characteristics produce contrast in resulting image
Topography
Atomic # differences
Etc.
Magnification
Ratio between size of display screen (or recorded image) and size of area on specimen
M = L / l
L = length of scan line on screen
l = length of scan line on specimen
L is fixed, so magnification changed by
changing area scanned on specimen
Mag Area on Sample
10X 1 cm2
1000X 100 μm2
100,000X 1 μm2 specimenscreen
1X10X
Picture Element
Region on specimen to which beam is addressed and from which information is transferred to screen
High resolution screen spot size ~ 100μm diameter
Corresponding picture element depends on magnification
Picture Element size = 100 μm / magnification
= L / N
L = length of scan line on specimen
N = Number of picture elements along the scan line (lines / frame)
Mag Picture Element Size 10X 10 μm 1000X 0.1 μm100,000X 1.0 nm
True focus: area sampled is smaller than picture element size
If beam sampling area extends to at least 2 picture elements
= blurring = “hollow magnification”
No additional information gained by increasing magnification
Depth of Field
Determined by distance where beam broadening exceeds one picture element
Beam broadening due to divergence angle
Depth of field
Plane of focusD
Region of image in effective focus
Long working distance
Sample surface
Short working distance Insert smaller objective aperture to improve D
Depth of Field (D)
Aperture radius( μm)
Mag. 100 200 600
10X 4 mm 2 mm 670 μm
1000X 40 μm 20 μm 6.7 μm
100,000X 0.4 μm 0.2 μm 0.067 μm
Must choose between two modes of operation
1) High resolution = short working distance
2) High depth-of-field = long working distance and / or small aperture
Compared to light microscopes at the same magnification
SEM 10 – 100 X greater depth-of-field
Contrast origins
Compositional differences
Different emitted current intensities for scanned areas of different average atomic #
BSE intensity is a function of Z
1) Regions of high average Z appear bright relative of low Z areas
2) The greater the Z difference = greater obtainable contrast
3) High Z = high η, so z contrast not as high for adjacent pairs of elements higher in periodic chart
Electron Backscatter
Backscattering more efficient with heavier elements
Can get qualitative estimate of average atomic number of target
Image will reveal different phases
Brighter = higher average Z
Topography
Backscattered electrons
If ET detector not biased, or negatively biased
If no SEs are detected, then only those BSEs scattered directly into detector will be counted (line-of-sight geometry)
Those surfaces facing detector will be bright
As if viewing specimen with light source in direction of detector
Topography
Secondary + Backscattered electrons
ET detector positively biased
Collect secondary electrons emitted from all surfaces, more where incidence angle is high
Entire surface appears illuminated
Always some contribution of BSEs
high Z areas
surfaces oriented toward detector
Solid-state detection system - application of the p-n junction diode
Take p-type Si
Apply Li to surface
Diffuses to form p-n junction
Apply reverse bias at high temp (room temp)
expands intrinsic region
Must keep cold (LN2 = 77K) or Li will diffuse
++++++
Depletion width W
Space-charge layers
Direction of built-in field
-------
p n
X-Ray spectrometryEDS: Energy dispersive spectrometry
Inelastically scattered – absorbed
Number of charges created:
N = E / Є
E = photon energy
Є = 3.8 eV for Si
5 KeV photon →
1300 electrons (2 X 10-16 C)
4) Potential sweeps electrons and holes apart
-500 to -1500 V
1) After passing through isolation / protection window (Be, BN, C, etc.) X-ray absorbed (photoelectric absorption) by Si
2) Inner shell ionization of Si → electron ejected with energy = 1.84 eVPhotoelectron creates electron-hole pairs (elevating electrons to the conduction
band)
3) Relaxation of the Si back to the ground state → SiK X-ray or Auger electron
To preamplifier
X-rays
p-type region (dead layer ~ 0.1μm)
Li-drifted, intrinsic region
n-type region
Gold contact surface (~2000Å)
Gold contact surface (~200Å)
Electrons
holes
X-Ray spectrometryEDS: Energy dispersive spectrometry
6) Leads to output pulse (convert charge to voltage in preamplifier)
→ linear amplifier
7) Sort by voltage in a multichannel analyzer
→ voltage histogram
EDS
Resolution ~ 150 eV
If separation < 50eV, very difficult to resolve
If looking for a minor element in the presence of major elements, need even more separation (200eV or more)
Fe – Co
Ti – V
Cr – Mn
Pb – S
Ba – Ti
Si – Sr
W - Si
EDS detector
Silicon Drift Detector (SDD)
Conventional diode = homogeneous electric field between layers
SDD = radially gradient potential field in active volume
Electrons guided toward center readout node
Can process very high count rates (up to 1,000,000 cps)
No LN2 cooling
Wavelength Dispersive Spectrometry (WDS)
Bragg Law:
θ
nλ = 2d sinθ
d
At certain θ, rays will be in phase,
otherwise out of phase = destructive interferencecambridgephysics.com – Bragg’s Law demonstration
d is known - solve for λ by changing θ
Move crystal and detector to select different X-ray lines
Si Kα
S Kα
Cl Kα
Ti Kα
Gd Lαsample
Crystal monochromator
Proportional counter Maintain Bragg condition = motion of
crystal and detector along circumference of circle (Rowland circle)
Spectrometer focusing geometry
Curve crystal to improve collection efficiency
Crystal bent to 2R Crystal bent to 2R, then ground to R – All rays have same angle of incidence and focus to detector
VLPET
Only small areas of the sample will be “in focus” for vertical spectrometers
In focus region = elongate ellipsoid on sample
For vertical spectrometers –
Shortest axis of focus ellipsoid coincides with stage Z (parallel to electron optic axis)
Stage focus extremely important
Light optical system = very short depth of field
Advantageous for focusing X-ray optics
MonochromatorsUse different crystals (or synthetic multilayers) with different d-spacings to get different ranges in wavelength
Smaller d = shorter λ detection and higher spectral resolution
synthetic crystals
pseudocrystals (e.g., stearate films on mica)
layered synthetic microstructures (multilayers) - LSM
“crystal” 2d(Å)LIF Lithium flouride 4.0
PET Pentaery thritol 8.7
TAP (TlAP) Thallium acid phthalate25.76
Ge Germanium 6.532
LAU Lead laurate 70.0
STE Lead stearate 100.4
MYR Lead myristate 79.0
RAP Rubidium acid phthalate 26.1
CER Lead cerotate 137.0
LSM W / Si W / C 45
60
80
90
98
Lowest Z diffracted Resolution Count Rates
Kα LαLIF K In high medium
LLIF high high
PET Al Kr medium high
LPET medium very high
VLPET medium ultra-high
TAP O V low medium
LTAP low high
STE B low medium
LSM Be low very high
1 5 10 50 100
Wavelength (Å)
LIFPET
TAPSTE
Resolution can be improved somewhat with use of collimating slits
Accelerating voltageMonochromator
(“crystal”)
Spectrometer number
Diffraction order
K lines also available on PET
K lines also available on LIFCr, Mn, Fe usually prefer LIF for high spectral resolution
Crystal Comparison
0
100
200
300
400
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
length / PET
Inte
nsi
ty (
cps/
nA
)
PbMa
UMb
PET
LPET
VLPET
Detectors for WDS analysisUsually gas filled counter tubes
1) X-ray enters tube and ionizes counter gas (Xe, Ar)2) eject photoelectron3) photoelectron ionizes other gas atoms4) Released electrons attracted to + potential on anode
wire – causes secondary ionizations and increases total charge collected
5) Collect charge and convert to output pulse – the energy of this pulse will be proportional to the energy of the X-ray - → count
Gas proportional counters
Use Ar, Xe, Kr…
1-3 kV on anode wire
windows
Be
Mylar
Formvar
Polypropylene
“softer” X-rays = thinner windows
Can be sealed, or gas - flow.
Low energy detection: low pressure flow (Ar – 10%CH4 = P-10)
Higher energy : sealed Xe (low partial pressure Xe + CH4) or high pressure P-10
For P-10
28 eV absorbed / electron – ion pair created
MnKα = 5.895 KeV
210 electrons directly created
Increase signal by increasing bias and # of secondary ionizations = gas amplification factor
Gas type
Shift P-10 peak to lower λ by increasing pressure
High pressure
Low pressure
Xe, low pressure
X-ray pulse must be processed by electronics resulting in dead timeAnother X-ray may enter during this time = not counted
Correct for (usually a few microseconds)
N = N’ / (1 – Τ N’)
T = dead time
N’ = measured count rate
N = actual count rate
raw
Pulse Height Analysis
Used to separate energies of overlapping lines (recall: nλ = 2d sinθ)
Variables: bias
baseline
window
Al in chromite FeCr2O4
λ Al Kα = 8.339 Å
λ Cr KβIV = 8.34 Å
E Al Kα = 1.487 KeV
E Cr KβIV = 5.946 KeV
Apatite
λ P Kα = 6.157 Å
λ Ca KβII = 6.179 Å
E P Kα = 2.013 KeV
E Ca KβII = 4.012 KeV
baseline
In integral mode the pulse height analyzer accepts all counts above the baseline
In differential mode, an energy acceptance window is employed to select a particular line
In some cases, the overlap in energy and wavelength is impossible to resolve – must use overlap corrections
V in ilmenite (FeTiO3)
V Kα = 2.5036 Å
Ti Kβ = 2.51399 Å
Sr in feldspar
Sr Lα = 6.8629 Å
Si Kβ = 6.753 Å generally use TAP at this wavelength
baseline
Pb M3-N4
Pb M
Pb M
WDS – background measurement
S sK
S K
S KS K absorption edge
Increasing spectrometer efficiency
WDS – background measurement
Comparison of EDS and WDS
LaPO4
Comparison of EDS and WDS
Comparison of EDS and WDS
Comparison of EDS and WDS
Comparison of EDS and WDS
La Lα1,2
Comparison of EDS and WDS
La Lα1,2
Th interferences on U-M region
Th absorption edges significant for high Th monazite
Brabantite
ThO2
Monazite (LIF monochromator) in wavelength region of NdL
EDS spectrum
EDS vs. WDS
WDS EDS
Element range ≥4 ≥10 (Be) (≥ 4 thin window)
Resolution to 5eV ~150eV
Instant range = eV resolution entire range
Max. count rate 50,000 cps <2000cps (SDD ~ 1,000,000)
Data collection time minutes minutes
Artifacts rare lots
Sensitivity at least 10X EDS
Pk/bkg vs. voltage