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					   Signal Detection

Object: Convert radiation into an electrical signal which is then amplified
Select
SE                                                    Incident
                                   Light              beam
BSE                                                            Auger
                                   (cathodoluminescence)       electrons
X-rays
                                                                         Secondary
Auger electrons                                                          electrons
                                    Bremsstrahlung
Photons from Cathodoluminescence
Absorbed electron current     Characteristic                                  Backscattered
                                 X-rays                                       electrons



                                          Sample
                                                                       heat



                                                                    Elastically
                                                                    scattered
                                                                    electrons
                                          Specimen
                                          current            Transmitted
                                                             electrons
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



                                                                              TiN
                                              Si


INLENS SE image from a sectioned               The same section but seen with the
semiconductor. Clearly visible: No BSE         LL-BSE; detected with the INLENS
contrast!                                      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

What is beam deceleration?                                            Beam
New optics mode enabling high resolution imaging and high
surface sensitivity at very low kV

BD specifications:
                                                                             2-mode final lens
   • Landing energy range: 30 keV down to 50 eV
   • The deceleration (Bias) can be continuously adjusted
       by the user
Benefits:                                                                       TLD
   • Enhances the resolution
                                                            HV
   • Provides additional contrast options                                             vCD
   • Greatest benefit at 2kV and below                    Landing V
                                                                                Sample
                                                                 Bias


                                                              If Bias=0 (no BD):
                                                              Landing V = HV

  11
Deprocessed IC                      Pt catalyst nanoparticles
1kV                                 2kV
600KX imaging                       1.0MX imaging




Gold on carbon                      Gold on carbon
1kV                                 2kV
1.75MX imaging, <0.9nm resolution   2.8MX imaging, <0.8nm resolution
Low voltage-high contrast         Through-the-lens     Through-the-lens
detector with beam                detector with beam   detector without beam
deceleration                      deceleration         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                    1X
                                                                10X
                10X                     1 cm2
           1000X                      100 μm2
        100,000X                        1 μm2         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                     Picture Element Size
            10X                          10 μm
          1000X                         0.1 μm
        100,000X                        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
                         Short working distance
                          Long working distance    Insert smaller
    Depth of field                                 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 Detection and Analysis

 Energy Dispersive spectrometry (EDS)
          Solid-state detection system - application of the p-n junction diode




                                                 -
                                                                         W         +
                                                 -     Depletion width
                                                                                   +
                                                 -
                                         p                                         +   n
                                                 -   Direction of built-in field
                                                                                   +
                                                 -
 Take p-type Si                                                                    +
                                                 -
                                                                                   +
 Apply Li to surface                             -
 Diffuses to form p-n junction                       Space-charge layers
 Apply reverse bias at high temp (room temp)
          expands intrinsic region
 Must keep cold (LN2 = 77K) or Li will diffuse
 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
                                 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”                                        2d(Å)
        LIF                Lithium flouride              4.0
        PET                Pentaery thritol              8.7
        TAP (TlAP)         Thallium acid phthalate       25.76
        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        Lα
LIF     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

      Resolution can be improved somewhat with use of collimating slits


LIF
                           PET
                                             TAP
                                                                STE


      1                    5          10                    50        100
                            Wavelength (Å)
Spectrometer                                       Accelerating
               Monochromator   Diffraction order   voltage
number         (“crystal”)
                                               Crystal Comparison

                                                                       VLPET

                     400



                                                          LPET
                     300

Intensity (cps/nA)

                     200
                                       PET

                                                                                 PbMa
                     100
                                                                                 UMb


                       0
                           0.0   0.5         1.0    1.5          2.0      2.5   3.0     3.5
                                                     length / PET
Detectors for WDS analysis
Usually gas filled counter tubes




1) X-ray enters tube and ionizes counter gas (Xe, Ar)
2) eject photoelectron
3) photoelectron ionizes other gas atoms
4) 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
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 Kα = 8.339 Å
                                                 λ         Cr KβIV = 8.34 Å
                                                 E         Al Kα = 1.487 KeV
                                                 E         Cr KβIV = 5.946 KeV
  baseline
                                                 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
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
                   Pb Ma



           Pb Mb




Pb M3-N4
S K absorption
edge                                      S Ka




                               S sKa3,4




   S Kb




     Increasing spectrometer efficiency
Th interferences on U-M region
Th absorption edges significant for high Th monazite




       ThO2




  Brabantite
                                                 Monazite GSC 8153 U-region (PET)


                                                                             UMb                           UMa
             2.30
                                                     Th-M4 edge                 Th-M5 edge        Ar-K edge
                                          Th M2-N1
             1.80
                                                                                    Th M5-P3
I (cps/nA)




                                                                                          Th M4-O2

             1.30
                                                                     ThMg
                                                                                                              ThMb
                    Ca Ka1,2
             0.80              Dy Lb 1 Tm La Gd Lb 2,15     Pm Lg Ho La          Th M3-N4 Dy La1
                                            1           Er La1               1
                                          Tm La2
                                                         Tb Lb 4
                                                                 Eu Lb 2,15    Ho La2 Sm Lb 2,15 Eu Lb 1
                                                 Tb Lb 1                             Eu Lb 3
                                                            Er La2         Gd Lb 1
             0.30
                37500    38500        39500       40500        41500        42500        43500       44500       45500   46500
                                                              Wavelength (sin-q * 105)
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
            S Ka



EDS




                   S Kb

      WDS




  EDS vs. WDS resolution

				
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posted:2/10/2012
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