Signal Detection. Incident beam. 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. Light (cathodoluminescence). - PowerPoint PPT Presentation
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 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 Inciden t beam Light (cathodoluminescenc e) Bremsstrahlung Secondary electrons Backscattere d electrons heat Elastically scattered electrons Transmitted electrons Specimen current Auger electrons Characterist ic 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 mm Signal Detection
Transcript
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 electronsBackscattered electronsX-raysAuger electronsPhotons from CathodoluminescenceAbsorbed 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 mm
Signal Detection
Electron DetectorsScintillator – Photomultiplier system (Everhart-Thornley, 1960)1) Electron strikes scintillator
plasticLi-glassCaF2 (Eu)P47
Photons produced
2) Light conducted by light pipe to photomultiplier3) Signal passes through quartz window into photomultiplier4) 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 electronsDeflect – does not require line-of-sight geometryCollection efficiency ~ 50% SE
~ 1-10% BSE
Backscattered Electron DetectorsUsually solid state devicesAnnular – thin wafer (Si semiconductor)
1) Backscattered electron strikes semiconductor2) Valence electron promoted to conduction band – free to move
Leaves hole in valence band3) No bias → recombination
Forward bias → current
~ 3.6 eV expended per electron / hole pairCurrent of 2800 electrons flows from detector if 10keV electron enters
4) Amplify signal5) 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).
11
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 userBenefits:
• 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 FormationScanning
Signals are produced as beam strikes sample at single locationTo 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 Ylocated 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 imageTopographyAtomic # differencesEtc.
MagnificationRatio between size of display screen (or recorded image) and size of area on specimen
M = L / lL = length of scan line on screenl = length of scan line on specimenL 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 ElementRegion on specimen to which beam is addressed and from which information is transferred to screenHigh resolution screen spot size ~ 100μm diameterCorresponding picture element depends on magnification
Picture Element size = 100 μm / magnification= L / NL = length of scan line on specimenN = 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 sizeIf beam sampling area extends to at least 2 picture elements
= blurring = “hollow magnification”No additional information gained by increasing magnification
Depth of FieldDetermined by distance where beam broadening exceeds one picture elementBeam 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
Must choose between two modes of operation1) High resolution = short working distance2) High depth-of-field = long working distance and / or small aperture
Compared to light microscopes at the same magnificationSEM 10 – 100 X greater depth-of-field
Contrast originsCompositional differencesDifferent emitted current intensities for scanned areas of different average atomic #
BSE intensity is a function of Z1) Regions of high average Z appear
bright relative of low Z areas2) The greater the Z difference = greater
obtainable contrast3) 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
TopographyBackscattered electrons
If ET detector not biased, or negatively biasedIf 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 brightAs if viewing specimen with light source in direction of detector
TopographySecondary + Backscattered electronsET detector positively biasedCollect secondary electrons emitted from all surfaces, more where incidence angle is highEntire surface appears illuminatedAlways some contribution of BSEshigh Z areassurfaces oriented toward detector
Solid-state detection system - application of the p-n junction diode
Take p-type SiApply Li to surfaceDiffuses to form p-n junctionApply reverse bias at high temp (room temp)
expands intrinsic regionMust 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 – absorbedNumber of charges created:
N = E / ЄE = photon energyЄ = 3.8 eV for Si5 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)
Electronsholes
X-Ray spectrometryEDS: Energy dispersive spectrometry
6) Leads to output pulse (convert charge to voltage in preamplifier)
→ linear amplifier7) Sort by voltage in a multichannel analyzer
→ voltage histogram
EDSResolution ~ 150 eVIf 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 – CoTi – VCr – MnPb – SBa – TiSi – SrW - Si
EDS detector
Silicon Drift Detector (SDD)
Conventional diode = homogeneous electric field between layersSDD = radially gradient potential field in active volumeElectrons guided toward center readout nodeCan process very high count rates (up to 1,000,000 cps)No LN2 cooling
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 θMove 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 geometryCurve 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 sampleFor vertical spectrometers –
Shortest axis of focus ellipsoid coincides with stage Z (parallel to electron optic axis)
Stage focus extremely importantLight 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 wavelengthSmaller d = shorter λ detection and higher spectral resolution
synthetic crystalspseudocrystals (e.g., stearate films on mica)layered synthetic microstructures (multilayers) - LSM
25.76Ge Germanium 6.532LAU Lead laurate 70.0STE Lead stearate 100.4MYR Lead myristate 79.0RAP Rubidium acid phthalate 26.1CER Lead cerotate 137.0LSM W / Si W / C 45
60809098
Lowest Z diffracted Resolution Count RatesKα Lα
LIF K In high medium LLIF high highPET Al Kr medium high LPET medium very high VLPET medium ultra-highTAP O V low medium LTAP low highSTE B low mediumLSM Be low very high
1 5 10 50 100
Wavelength (Å)
LIFPET
TAPSTE
Resolution can be improved somewhat with use of collimating slits
LSMs
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
nsity
(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 wirewindows
BeMylarFormvarPolypropylene
“softer” X-rays = thinner windowsCan 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-1028 eV absorbed / electron – ion pair createdMnKα = 5.895 KeV
210 electrons directly createdIncrease 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 countedCorrect for (usually a few microseconds)
N = N’ / (1 – Τ N’)T = dead timeN’ = measured count rateN = actual count rate
raw
Pulse Height AnalysisUsed to separate energies of overlapping lines (recall: nλ = 2d sinθ)
λ P Kα = 6.157 Åλ Ca KβII = 6.179 ÅE P Kα = 2.013 KeVE Ca KβII = 4.012 KeV
baseline
In integral mode the pulse height analyzer accepts all counts above the baselineIn 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 correctionsV in ilmenite (FeTiO3)
V Kα = 2.5036 ÅTi Kβ = 2.51399 Å
Sr in feldsparSr Lα = 6.8629 ÅSi Kβ = 6.753 Å generally use TAP at this wavelength
baseline
Pb M3-N4
Pb Ma
Pb Mb
WDS – background measurement
Th Mα
Th Mβ
High net intensity (pk-bkg) and large pk/bkg
Major element analysis is primarily characterization of peaks…
Measured net Int.20% error, ~350ppm error
Correct net Int.
Trace element analysis is primarily characterization of background…
S sKa3,4
S Kb
S KaS 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 regionTh absorption edges significant for high Th monazite
