BEI Detector
Annular solid-state p-n diode
creates cascade of electrons
Electron beam
Slow response time, so
slow beam scan rates
are necessary
Final probe-forming aperture
Electrical signal out
Solid-state detector
Backscattered electrons
Specimen surface
For good compositional contrast images, a polished specimen is required
MSE 421/521 Structural Characterization
Compositional Images
η0 = -0.0254 + 0.016Z – 1.86x10-4Z2 + 8.31x10-7Z3
η0 = backscattered electron coefficient, untilted
Z = atomic number
Partly because of diffraction, η also varies with θ, but in a more complicated way.
Several empirical equations exist to describe this relationship, one of which is:
η = 0.89(η0/0.89)cosθ
Contrast is typically weak and easily drowned out by topographical
effects, so best practice is to use polished specimens.
C=
η1 – η2
η1
(η1 > η2)
Weak contrast large Ic large d (poorer resolution)
Al – Pt:
C = 0.685, d = 3.6 nm
α-brass – β-brass: C = 0.003, d = 186 nm
10 µm
MSE 421/521 Structural Characterization
Channelling
Paths of low atomic density - channels
Electrons that penetrate deeply are less likely to escape from the surface once scattered
Strong when Bragg condition is satisfied for just one set of planes.
Much weaker than Z contrast: 0.0005 < C < 0.05, so requires:
1. High brightness (FEG)
2. Small semi-angle (α < 10 mrad)
3. BSE detector with large solid angle of collection
4. Clean, smooth sample surface
5. High probe current (I > 10 nA)
Channelling in
Optimum for low kV and large Z
Used in metallurgical and geological
samples to:
detect small orientational changes
defects (eg, dislocations)
Sample
Electrons out
ECCI detector
http://ssd.phys.strath.ac.uk/index.php/Electron_channeling_contrast_imaging
MSE 421/521 Structural Characterization
Channelling
Paths of low atomic density - channels
Channeling contrast
SE image of a steel
sample revealing the
different metal grains
Comparison of ECCI and SE image of a GaN thin film
MSE 421/521 Structural Characterization
EBSD
Components include:
Sample tilted 70° from horizontal
Phosphor screen (like TEM) fluoresced by electrons to form diffraction pattern
CCD camera for viewing pattern
Phenomenon
Electrons first inelastically scatter then are Bragg
diffracted to form Kikuchi bands
Peltier cooled
CCD camera
Beam
control
Phosphor
screen
Sample
Applications:
Measure crystal orientation/misorientation
Discriminate between different materials
Texture map – grain morphology, orientations, boundaries
Forward scatter
electron detector
Stage
Control
PC
Digital stage and
beam, control unit
MSE 421/521 Structural Characterization
EBSD
Width of Kikuchi bands = w = R ~ L2θ ~
Lλ
d
(Rd = Lλ)
Ni: Fm3m
Ni: Fm3m
True symmetry of crystal
4-fold symmetry of [001]
Planes with large d-spacings
produce thinner Kikuchi bands
MSE 421/521 Structural Characterization
EBIC
Electron Beam Induced Current
Incident electron knocks out hundreds of
thousands of electrons from their atoms,
leaving positively-charged holes behind
Bias voltage separates e - and h + to form
current
Image is formed with the current signal
Contrast corresponds to differences in:
Conductivity
Electron-hole pair lifetime
Electron-hole mobility
Used to study semiconductors
Imaging of perpendicular p-n junctions
in a MOSFET transistor as a function of
V (therefore depth)
R.F. Egerton
MSE 421/521 Structural Characterization
Cathodoluminescence (CL)
Electrons knock out outer electrons, emitting visible/UV photons upon relaxation
Emission is modified by impurities or defects
Resolution fairly poor (whole interaction volume), but can be improved by using low kV
or narrower beams (FEG)
Traditional polishing techniques employed for thin geological sections
create a “dead layer” that prevents work at low kV and higher spatial
resolution.
Remnants of polishing compound can be very much brighter than the
area of interest, saturating the detectors at the gain required to image
the region of interest.
Weakly luminescing
pottery imaged with
high I by Gatan
ChromaCL
Peak in photon energy corresponds to Eg, and width of peak decreases at
low T
At liquid He temperatures (~3 K) alterations in Eg due to composition
(< 10 ppb) can be detected –
CL is several orders of magnitude more sensitive than EDS or other xray techniques
MSE 421/521 Structural Characterization
Charging
Samples in the SEM must typically be conductive or be made conductive
May need to sputter coat thin (~10 nm) layer of C, Au, or other conductive material to prevent charging
More electrons
leaving the
sample surface
than are
arriving on it
Total electron yield
+ charging
Self-stabilises 1
More electrons
arriving at
sample surface
than are
leaving it
charging
charging
V1
~1 kV
Accelerating Voltage
Alternatives to coating:
Operate at low kV
(where yield ~ 1)
Leak air into vacuum
(charge is dissipated by
electron-gas interactions)
For some samples,
must also consider:
Degradation (beam damage)
Radiation damage
Volatility
V2
~5 kV
MSE 421/521 Structural Characterization
Environmental SEM
(ESEM)
ESEMs have three separate vacuum systems:
1. Gun (high vacuum)
2. Column (lower vacuum)
Cross section of coriander seed
3. Sample Chamber (1 – 10 torr)
(~102 – 103 Pa)
Can be used to image wet, volatile, or
non-conductive samples
Everhart-Thornley SEI detector cannot
be used, as high bias would cause
breakdown in poor vacuum
Gas-phase detectors are used instead
for SEI
Loss of resolution due to scattering of
electrons off gas molecules
MSE 421/521 Structural Characterization
BSE ESEM image of
live Leptospermum
flavesces stem cells
with water film