Signal Detection

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Signal Detection
Object: Convert radiation into an electrical
signal which is then amplified
Select
Secondary electrons
Backscattered electrons
X-rays
Auger electrons
Photons from Cathodoluminescence
Absorbed electron current
Incident
Light
beam Auger
(cathodoluminescence)
electrons
Secondary
electrons
Bremsstrahlung
Characteristic
X-rays
Backscattered
electrons
Sample
heat
Any of the collected signals can be
displayed as an image if you either scan
the beam or the specimen stage
Specimen
current
Elastically
scattered
electrons
Transmitted
electrons
Secondary electron detector
electron 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
5 mm
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 detectors
In lens detectors
EsB = Energy selective backscatter
uses filtering grid
AsB = Angle selective backscatter
uses angle
Si3N4
Ti
Si
INLENS SE image from a sectioned
semiconductor. Clearly visible: No BSE
contrast!
TiN
The same section but seen with the
LL-BSE; detected with the INLENS
EsB at 1.27 kV
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).
Beam deceleration: enhancing resolution and contrast
Beam
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
HV
• Provides additional contrast options
Landing V
• Greatest benefit at 2kV and below
2-mode final lens
TLD
vCD
Sample
Bias
If Bias=0 (no BD):
Landing V = HV
11
Deprocessed IC
1kV
600KX imaging
Pt catalyst nanoparticles
2kV
1.0MX imaging
Gold on carbon
1kV
1.75MX imaging, <0.9nm resolution
Gold on carbon
2kV
2.8MX imaging, <0.8nm resolution
Low voltage-high contrast
detector with beam
deceleration
Through-the-lens
detector with beam
deceleration
Pt sample.
Landing energy 2keV, Beam deceleration=4kV.
Through-the-lens
detector without beam
deceleration
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
10X
1X
screen
specimen
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
10X
1000X
100,000X
Picture Element Size
10 μm
0.1 μm
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
Short
Long working
working distance
distance
Depth of field
Insert smaller
objective aperture
to improve D
Sample surface
D
Plane of focus
Region of image in
effective focus
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
X-Ray spectrometry
EDS: Energy dispersive spectrometry
Solid-state detection system - application of the p-n junction diode
p
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
Direction of built-in field
Space-charge layers
+
+
+
+
+
+
n
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 eV
Photoelectron 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
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
X-Ray spectrometry
EDS: Energy dispersive spectrometry
To preamplifier
Electrons
holes
Gold contact
surface (~2000Å)
n-type region
Li-drifted, intrinsic
region
p-type region (dead
layer ~ 0.1μm)
X-rays
Gold contact
surface (~200Å)
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:
nλ = 2d sinθ
θ
d
At certain θ, rays will be in phase,
otherwise out of phase = destructive interference
cambridgephysics.com –
Bragg’s Law demonstration
d is known - solve for λ by changing θ
Move crystal and detector to select different X-ray lines
Crystal
monochromator
Si Kα
S Kα
Cl Kα
Ti Kα
Gd Lα
sample
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
Monochromators
Use 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”
LIF
PET
TAP (TlAP)
Ge
LAU
STE
MYR
RAP
CER
LSM
2d(Å)
Lithium flouride
Pentaery thritol
Thallium acid phthalate
Germanium
Lead laurate
Lead stearate
Lead myristate
Rubidium acid phthalate
Lead cerotate
W / Si W / C
4.0
8.7
25.76
6.532
70.0
100.4
79.0
26.1
137.0
45
60
80
90
98
Lowest Z diffracted
Kα
LIF
K
LLIF
PET
Al
LPET
VLPET
TAP
O
LTAP
STE
B
LSM
Be
Resolution
Count Rates
high
high
medium
medium
medium
low
low
low
low
medium
high
high
very high
ultra-high
medium
high
medium
very high
Lα
In
Kr
V
Resolution can be improved somewhat with use of collimating slits
LIF
PET
TAP
1
5
10
Wavelength (Å)
STE
50
100
Spectrometer
number
Monochromator
(“crystal”)
Diffraction order
Accelerating
voltage
K lines also available on PET
K lines also available on LIF
Cr, Mn, Fe usually prefer LIF for high spectral resolution
Crystal Comparison
VLPET
400
Intensity (cps/nA)
LPET
300
200
PET
PbMa
100
UMb
0
0.0
0.5
1.0
1.5
2.0
length / PET
2.5
3.0
3.5
Detectors for WDS analysis
Usually gas filled counter tubes
1)
2)
3)
4)
X-ray enters tube and ionizes counter gas (Xe, Ar)
eject photoelectron
photoelectron ionizes other gas atoms
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
Xe, low
pressure
Low
pressure
X-ray pulse must be processed by electronics resulting in dead time
Another 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
λ
Cr KβIV = 8.34 Å
E
Al
E
Cr KβIV = 5.946 KeV
Kα
Kα
= 8.339 Å
= 1.487 KeV
baseline
Apatite
λ
λ
E
E
P Kα = 6.157 Å
Ca KβII = 6.179 Å
P Kα = 2.013 KeV
Ca KβII = 4.012 KeV
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
baseline
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
WDS – background measurement
Pb Ma
Pb Mb
Pb M3-N4
WDS – background measurement
S K absorption
edge
S Ka
S sKa3,4
S Kb
Increasing spectrometer efficiency
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
ThO2
Brabantite
Monazite GSC 8153 U-region (PET)
UMb
2.30
UMa
Th-M5 edge
Th-M4 edge
Ar-K edge
Th M2-N1
I (cps/nA)
1.80
Th M5-P3
Th M4-O2
1.30
ThMg
0.80
Ca Ka1,2
0.30
37500
ThMb
Dy Lb 1 Tm La Gd Lb2,15
Pm Lg Ho La
Th M3-N4 Dy La1
1
Er La1
Tm La2
Eu Lb2,15
Ho La2 Sm Lb2,15 Eu Lb1
Tb
Lb
4
Tb Lb 1
Eu Lb3
Gd Lb1
Er La2
1
38500
39500
40500
41500
42500
43500
Wavelength (sin-q * 105)
44500
45500
46500
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
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