1. Optical Microscopy vs Scanning Electron Microscopy
25mm
radiolarian OM
SEM
Small depth of field
Low resolution
Large depth of field
High resolution

2. Scanning Electron Microscopy (SEM)
What is SEM?
Working principles of SEM
Major components and their functions
Electron beam - specimen interactions
Interaction volume and escape volume
Magnification, resolution, depth of field and image contrast
Energy Dispersive X-ray Spectroscopy (EDS)
Wavelength Dispersive X-ray Spectroscopy (WDS)
Orientation Imaging Microscopy (OIM)
X-ray Fluorescence (XRF)

3. What is SEM Column TV Screens Sample Chamber
The SEM is designed for direct studying of the surfaces of solid objects
Sample
Chamber
Cost: $ M
Scanning electron microscope (SEM) is a microscope that uses electrons rather than light to form an image. There are many advantages to using the SEM instead of a OM.

4. Advantages of Using SEM over OM
Magnification Depth of Field Resolution
OM 4x – 1000x mm – 0.19mm ~ 0.2mm
SEM 10x – x 4mm – 0.4mm nm
The SEM has a large depth of field, which allows a large amount of the sample to be in focus at one time and produces an image that is a good representation of the three-dimensional sample. The SEM also produces images of high resolution, which means that closely features can be examined at a high magnification.
The combination of higher magnification, larger depth of field, greater resolution and compositional and crystallographic information makes the SEM one of the most heavily used instruments in research areas and industries, especially in semiconductor industry.

5. Scanning Electron Microscope – a Totally Different Imaging Concept
Instead of using the full-field image, a point-to-point measurement strategy is used.
High energy electron beam is used to excite the specimen and the signals are collected and analyzed so that an image can be constructed.
The signals carry topological, chemical and crystallographic information, respectively, of the samples surface.

6. Main Applications Topography Morphology Composition
The surface features of an object and its texture (hardness, reflectivity… etc.)
Morphology
The shape and size of the particles making up the object (strength, defects in IC and chips...etc.)
Composition
The elements and compounds that the object is composed of and the relative amounts of them (melting point, reactivity, hardness...etc.)
Crystallographic Information
How the grains are arranged in the object (conductivity, electrical properties, strength...etc.)

7. A Look Inside the Column

8. A more detailed look inside
Electron Gun
e- beam 
Source: L. Reimer, “Scanning Electron Microscope”, 2nd Ed., Springer-Verlag, 1998, p.2

9. Image Formation in SEM M = c/x A 10cm e- beam Detector 10cm Amplifier
c-length of CRT scan
x-length of e- beam scan
To produce an image on the screen, the electron beam scans over the area to be magnified and transfers this image to the TV screen. The electron beam stops at 1,000 points as it scans horizontally across the sample and down 1,000 lines vertically. This gives 1,000,000 points of information. The signal read from the electrons coming off each point is transferred to a corresponding point on the TV screen. Since the TV screen also has 1,000 points horizontally and 1,000 lines vertically, there is a 1:1 correspondance between the scan on the specimen and the TV screen. Since the length (x) of the electron beam scan on the specimen is smaller than the length (c) of the TV screen, a magnification is produced equal to the following equation: M=c/x By changing the size of the scan on the sample, the magnification can be changed. The smaller the area of the electron beam scan, the higher the magnification. Obtaining different degrees of magnifications are important in any practical uses of the SEM.
Beam is scanned over specimen in a raster pattern in synchronization with beam in CRT. Intensity at A on CRT is proportional to signal detected from A on specimen and signal is modulated by amplifier.

10. Image Magnification
Example of a series of increasing magnification (spherical lead particles imaged in SE mode)

11. Comparison of OM,TEM and SEM
Source of
electrons
Light source
Condenser
Magnetic
lenses
Specimen
Objective
Specimen
Projector
CRT
Eyepiece
Cathode Ray Tube
detector OM
TEM
SEM
Principal features of an optical microscope, a transmission electron microscope and a scanning electron microscope, drawn to emphasize the similarities of overall design.

12. How an Electron Beam is Produced?
Electron guns are used to produce a fine, controlled beam of electrons which are then focused at the specimen surface.
The electron guns may either be thermionic gun or field-emission gun

13. Thermionic or Field Emission Gun
Electron beam Source
W or LaB6 Filament
Thermionic or Field Emission Gun

14. Thermionic Emission Gun
A tungsten filament heated by DC to approximately 2700K or LaB6 rod heated to around 2000K
A vacuum of 10-3 Pa (10-4 Pa for LaB6) is needed to prevent oxidation of the filament
Electrons “boil off” from the tip of the filament
Electrons are accelerated by an acceleration voltage of 1-50kV - +

15. Field Emission Gun
The tip of a tungsten needle is made very sharp (radius < 0.1 m)
The electric field at the tip is very strong (> 107 V/cm) due to the sharp point effect
Electrons are pulled out from the tip by the strong electric field
Ultra-high vacuum (better than 10-6 Pa) is needed to avoid ion bombardment to the tip from the residual gas.
Electron probe diameter < 1 nm is possible
1st anode to regulate the field strength at the tip by U1 and hence regulate the emission current. 2nd anode: accelerates the electrons to the final kinetic energy.

16. Source of Electrons Thermionic Gun W and LaB6 Cold- and thermal FEG
E: >10MV/cm
T: ~1500oC
                                  W
Filament
(5-50mm)
(5nm)
W and LaB Cold- and thermal FEG
Electron Gun Properties
Source Brightness Stability(%) Size Energy spread Vacuum
W 3X ~1 50mm (eV) (t )
LaB6 3x ~2 5mm
C-FEG ~5 5nm
T-FEG <1 20nm
Ideally the field emission tip is used in a vacuum of 10nPa or better. Even in that condition, however, a few gas molecules will still land on the tip from time to time and these will cause fluctuations in the emission current. Eventually the whole tip will be covered and the output will become very unstable. The cathode must then be cleaned by rapidly heating it to a high T (2000C) for a few seconds (Cold FEG). Alternatively the tip can be kept warm ( C, thermal FEG), in which case the majority of impinging molecules are immediately reevaporated. In this case acceptably stable emission is maintained even in a vacuum of 100 nPa or so.
Brightness – beam current density per unit solid angle

17. Magnetic Lenses Condenser lens – focusing
determines the beam current which impinges on the sample.
Objective lens – final probe forming
determines the final spot size of the electron beam, i.e., the resolution of a SEM.

18. Why Need a Vacuum?
When a SEM is used, the electron-optical column and sample chamber must always be at a vacuum.
If the column is in a gas filled environment, electrons will be scattered by gas molecules which would lead to reduction of the beam intensity and stability.
2. Other gas molecules, which could come from the sample or the microscope itself, could form compounds and condense on the sample. This would lower the contrast and obscure detail in the image.

19. The Condenser Lens
For a thermionic gun, the diameter of the first cross-over point ~20-50µm
If we want to focus the beam to a size < 10 nm on the specimen surface, the magnification should be ~1/5000, which is not easily attained with one lens (say, the objective lens) only.
Therefore, condenser lenses are added to demagnify the cross-over points.

20. How Is Electron Beam Focused?
A magnetic lens is a solenoid designed to produce a specific magnetic flux distribution.
Magnetic lens
(solenoid)
(Beam diameter) p
F = -e(v x B) q
Lens formula: 1/f = 1/p + 1/q
M = q/p
The lenses used in the SEM are normally weak and the lens formula can be used.
Demagnification:
f  Bo2
f can be adjusted by changing Bo, i.e., changing the current through coil.

21. The Condenser Lens
Demagnification:
M = f/L

22. The Objective Lens
The objective lens controls the final focus of the electron beam by changing the magnetic field strength
The cross-over image is finally demagnified to an ~10nm beam spot which carries a beam current of approximately A.
The objective lens is machined to very high precision and the magnetic field pattern is very carefully designed.
However, the precision attainable by machining cannot match that required for controlling a beam with Ø10 nm.
The stigmator, which consist of two pairs of pole-pieces arranged in the X and Y directions, is added to correct the minor imperfections in the objective lens.

23. The Objective Lens - Focusing
By changing the current in the objective lens, the magnetic field strength changes and therefore the focal length of the objective lens is changed.
Objective
lens
Out of focus in focus out of focus
lens current lens current lens current
too strong optimized too weak

24. The Objective Lens – The Aperture
Since the electrons coming from the electron gun have spread in kinetic energies and directions of movement, they may not be focused to the same plane to form a sharp spot.
By inserting an aperture, the stray electrons are blocked and the remaining narrow beam will come to a narrow
Electron beam
Objective
lens
Wide
aperture
Narrow
aperture
Narrow disc of least confusion
Wide disc of
least confusion
Large beam diameter
striking specimen
Small beam diameter
striking specimen
“Disc of Least Confusion”

25. The Scan Coil and Raster Pattern
Two sets of coils are used for scanning the electron beam across the specimen surface in a raster pattern similar to that on a TV screen.
This effectively samples the specimen surface point by point over the scanned area.
X-direction
scanning coil
Holizontal line scan
Blanking
y-direction
scanning
coil
Blanking means interrupting the electron beam briefly and periodically.
Objective
lens
specimen

26. Electron Detectors and Sample Stage
Objective
lens
Sample stage

27. Scanning Electron Microscopy (SEM)
What is SEM?
Working principles of SEM
Major components and their functions
Electron beam - specimen interactions
Interaction volume and escape volume
Magnification, resolution, depth of field and image contrast
Energy Dispersive X-ray Spectroscopy (EDS)
Wavelength Dispersive X-ray Spectroscopy (WDS)
Orientation Imaging Microscopy (OIM)
X-ray Fluorescence (XRF)

28. Electron Beam and Specimen Interactions
Sources of Image Information
Electron/Specimen Interactions
(1-50KeV)
Specimen interaction is what makes Electron Microscopy possible. Elastic and inelastic scattering are the elementary atomic interaction processes, though the final signal used for image formation is, with only a few exceptions, not the result of single scattering processes but of the complete electron diffusion caused by the gradual loss of the electron energy and by lateral spreading caused by multiple elastic large-angle scattering.
All signals come from different depth of sample. Right:mainly imaging and left: chemical
Cathodoluminescence (CL)-The emission of ultraviolet, visible or infrared light stimulated by electron bombardment. A great many substances, especially semiconductors and minerals show CL. CL contains much analytical information and can reveal material differences that cannot be detected by other methods, e.g., dopants distribution (information is only qualitative). However, CL is frequently used in combination with EBIC and the recombination of charge carries at lattice defects also allows lattice defects to be imaged.
EBIC-Inelastic scattering in semiconductors results in the generation of electron-hole pairs, and a few thousands of electron-hole pairs are created per incident electron. In the depletion layer of a p-n junction, the strong electric field separates the charge carries and minority carries can hence reach the junction by diffusion. This results in a charge-collection current or electron-beam-induced-current, which can be amplified and used in a quantitative manner to measure the width of the junction and its depth below the surface, the diffusion length and the surface recombination rate of minority carries.
The largest fraction of the primary electron energy that is lost during the cascade of inelastic scattering processes is converted into phonons or heat.
Electron Beam Induced Current (EBIC)

29. Secondary Electrons (SE)
Produced by inelastic interactions of high energy electrons with valence (or conduction) electrons of atoms in the specimen, causing the ejection of the electrons from the atoms. These ejected electrons with energy less than 50eV are termed "secondary electrons".
Each incident electron can produce several secondary electrons.
Primary
SE yield: d=nSE/nB independent of Z
d decreases with increasing beam energy and increases with decreasing glancing angle of incident beam
BaTiO3
The interaction of the beam electron with the solid can lead to the ejection of loosely bound electrons of the conduction band in metals or the valence band in insulators and semiconductors. This interaction results in the transfer of only a few electron volts of energy to the band electron and causes a slight energy loss and path change in the incident electron. Each incident electron can produce several secondary electrons.
Arbitrairly, such emergent electrons with energies less than 50 eV are called secondary electrons. 90% of secondary electrons have energies less than 10 eV; most from 2 to 5
There is a great difference between the amount of energy contained by beam electrons compared to the specimen electrons and because of this, only a small amount of kinetic energy can be transferred to the secondary electrons.
Production of SE is very topography related. Due to their low energy, only SE that are very near the surface (<10nm) can exit the sample and be examined (small escape depth).
Growthstep
5m
SE image

30. Topographical Contrast
Everhart-Thornley
SE Detector
Bright
lens polepiece e- SE
Scintillator
light pipe
PMT
Dark
sample
Quartz
window
Faraday
cage
+10kV
+200V
Photomultiplier
tube
Topographic contrast occurs because the efficiency of generating both SEs and BSEs depends on the angle of incidence between the scanning beam and the specimen. Thus local variations in the angle of the surface to the beam (roughness) affects the numbers of electrons leaving from point to point. The resulting “topographic contrast is a function of the physical shape of the specimen. In areas where the surface is tilted relative to the incident beam, the electrons travel greater distances in the region close to the surface of the specimen. This means more SE are generated within the escape depth in tilted areas than in areas which are normal to the beam.
In addition, SE can escape from both sides of ridges and edges.
These effects cause tilted surfaces to appear brighter than flat surfaces, and edges and ridges to be markedly highlighted, in images formed with secondary electrons
Topographic contrast is weak in BSE mode which usually use sample with flat surface.
Secondary electron attracted to collector grid (or Faraday Cage) by a positive bias ~ V. Collected electron further accelerated to scintillation disc inside the cage by kV. Photon of visible light generated when electrons strike the scintillator. Photons reach the photomultiplier, in which the light signal is amplified, through the light guide pipe. The final electrical signal from the anode is used for modulating the intensity of electron beam on the display CRT screen.
Topographic contrast arises because SE generation depend on the angle of incidence between the beam and sample.

31. Backscattered Electrons (BSE)
Primary
BSE image from flat surface of an Al (Z=13) and Cu (Z=29) alloy
BSE are produced by elastic interactions of beam electrons with nuclei of atoms in the specimen and they have high energy and large escape depth.
BSE yield: h=nBS/nB ~ function of atomic number, Z
BSE images show characteristics of atomic number contrast, i.e., high average Z appear brighter than those of low average Z. h increases with tilt.
As the name implies, elastic scattering results in little or no change in energy of the scattered electron, although there is a change in momentum. Since momentum, p=mv, and m doesn't change, the direction of the velocity vector must change. The angle of scattering can range from degrees. Elastic scattering occurs between the negative electron and the positive nucleus. Sometimes the angle is such that the electron comes back out of the sample. These are backscattered electrons.

32. Effect of Atomic Number, Z, on BSE and SE Yield

33. Interaction Volume: I The incident electrons do not go along a
Monte Carlo simulations of 100 electron trajectories
The incident electrons do not go along a
straight line in the specimen, but a zig-zag
path instead.

34. Interaction Volume: II
The penetration or,
more precisely, the
interaction volume
depends on the
acceleration voltage
(energy of electron)
and the atomic
number of the
specimen.

35. Escape Volume of Various Signals
The incident electrons interact with specimen atoms along their path in the specimen and generate various signals.
Owing to the difference in energy of these signals, their ‘penetration depths’ are different
Therefore different signal observable on the specimen surface comes from different parts of the interaction volume
The volume responsible for the respective signal is called the escape volume of that signal.

36. Escape Volumes of Various Signals
If the diameter of primary electron beam is ~5nm
- Dimensions of escape zone of
Secondary electron:
diameter~10nm; depth~10nm
Backscattered electron:
diameter~1m; depth~1m
X-ray: from the whole
interaction volume, i.e., ~5m
in diameter and depth
This escape volume limits the resolution in images produced with BSE and x-ray (element mapping) to a value that is of the order of size of the escape volume, regardless of how small the actual diameter of the incident electron beam may be.

37. Electron Interaction Volume
Pear shape
5mm a b
Interaction volume will determine resolution of EDS in SEM and will be discussed more
Later. Keep in mind interaction volume inversely proportional to atomic number, Z.
Electrons can penetrate into sample and the higher the energy, the deeper range the electrons can penetrate. Beam-sample interaction volume is proportional to the electron energy. On the other hand, the x-ray generation range depends on critical ionization energy of elements, Ec. Ec-K line >> Ec-L line. Electrons lose their energy due to multiple inelastic scattering while they are penetrating the sample. That is why the x-ray generation range of K line is smaller than that of L line, since the former needs higher energy to be excited, whereas the energy of electrons become smaller and smaller as they penetrate deeper and deeper into the sample.
a.Schematic illustration of electron beam interaction in Ni
b.Electron interaction volume in polymethylmethacrylate
(plastic-a low Z matrix) is indirectly revealed by etching

38. Image Formation in SEM M= C/x A A 10cm e- beam Detector 10cm Amplifier
The electron beam scans across a rectangular area on the specimen surface. The electron beam in the cathode ray tube (CRT) scans across the screen for viewing the image and the scanning is synchronized with that of the electron beam in the microscope. The intensity of a certain point on the screen is modulated by the intensity of the signal from the detectors (e.g. BSE or SE detectors)
Beam is scanned over specimen in a raster pattern in synchronization with beam in CRT.
Intensity at A on CRT is proportional to signal detected from A on specimen and signal is modulated by amplifier.

39. Magnification e- Increasing M is achieved by decreasing x. x Low M
Large x
40mm
High M
small x
7mm
1.2mm
2500x
15000x
The magnification is simply the ratio of the length of the scan C on the Cathode Ray Tube (CRT) to the length of the scan x on the specimen. For a CRT screen that is 10 cm square:
M= C/x = 10cm/x
Increasing M is achieved by decreasing x.
M x M x
mm mm
mm mm
Need small electron beam probe to achieve high magnification.
Changing magnification does not involve changing any lens current, only changing the current in the scan coils, and so:
focus does not change as magnification is changed
the image does not rotate with magnification change (as in TEM)

40. Resolution Limitations
Ultimate resolution obtainable in an SEM image can be limited by:
Electron Optical limitations
Diffraction: dd=1.22/
for a 20-keV beam,  =0.0087nm and =5x10-3 dd=2.1nm
Chromatic and spherical aberrations: dmin=1.29l3/4 Cs1/4
A SEM fitted with an FEG has an achievable resolution of ~1.0nm at 30 kV due to smaller Cs (~20mm) and l.
Specimen Contrast Limitations
Contrast dmin nm nm nm nm
3. Sampling Volume Limitations (Escape volume)
–fruit fly
Ultimate resolution depends on the electron-optical specification and achievable resolution is subject to operational factors (e.g., correction of astigmatism which arises from the illumination system.

41. How Fine Can We See with SEM?
If we can scan an area with width 10 nm (10,000,000×) we may actually see atoms!! But, can we?
Image on the CRT consists of spots called pixels (e.g. your PC screen displays 1024×768 pixels of ~0.25mm pitch) which are the basic units in the image.
You cannot have details finer than one pixel!
effect of pixel size on image resolution.

42. Resolution of Images: I
Assume that there the screen can display 1000 pixels/(raster line), then you can imagine that there are 1000 pixels on each raster line on the specimen.
The resolution is the pixel diameter on specimen surface.
The unaided human cannot reliably resolve features smaller than about 0.1 mm (100 µm), and so the diameter of the beam in the CRT need not be made smaller than this. Thus, the diameter of an image point on the CRT is D = 100 µm. The conjugate point on the specimen from which the image signal is produced, which is called the 'pixel' P, will have a smaller diameter, depending on the magnification, of:
P=D/Mag = 100um/Mag
At low magnifications, the resolution of the image is determined by the pixel size. At high magnifications the beam diameter limits resolution.
P=D/Mag = 100um/Mag
Mag P(m) Mag P(nm)
10x kx 10
1kx kx 1
P-pixel diameter on specimen surface
D-pixel diameter on CRT, Mag-magnification

43. Resolution of Images: II
The optimum condition for imaging is when the escape volume of the signal concerned equals to the pixel size.

44. Resolution of Images: III
Signal will be weak if escape volume, which depends on beam size, is smaller than pixel size, but the resolution is still achieved. (Image is ‘noisy’)
Small beam size is needed for high resolution. Decrease the beam size by:
1.Increasing the current on condenser lens 2.Decreasing the working distance
Decreasing the beam size also decreases the beam current and therefore the signal to noise ratio gets worse.
- single/noise ratio effect

45. Resolution of Images: IV
Signal from different pixel will overlap if escape volume is larger than the pixel size. The image will appeared out of focus (Resolution decreased)
Take EDS as an example. Beam size~5nm, but escape volume~5m, so spatial resolution at best is ~ 5m.

46. Resolution of Images: V
In extremely good SEM, resolution can be a few nm. The limit is set by the electron probe size, which in turn depends on the quality of the objective lens and electron gun.
Pixel diameter on Specimen
Magnification µm nm 10
10000
100 1
1000
0.1
0.01
100000
0.001
Pixel diameter 0.001m, screen can display 1000 pixels/raster lines,
scan length=1000x0.001 m=1 m. Magnification=10cm/1m=100,000
Here, we talking about SE image resolution.

47. Depth of Field Depth of Field 4x105W D = (m) AM To increase D
Decrease aperture size, A
Decrease magnification, M
Increase working distance, W (mm)
- One of the great advantages of SEM images is the unusually great depth of field they exhibit. This makes it possible to examine surfaces much rougher, and at much higher magnifications, than is possible with light microscopes. The reason for this great depth of field arises from the geometry of the beam optics. The final lens of the SEM focuses the electron beam to a 'crossover' at the plane of best focus. The beam diameter increases as the beam converges and diverges above and below this plane. At some distance D/2 above and below the focus plane the diameter of the beam becomes twice the pixel diameter for the mag being used, whereupon the signals from adjacent pixels overlap enough to cause the image to appear blurred.
Over the distance D between these limits, however, the image will appear to be in acceptably sharp focus, and so this distance is called 'the depth of field' or 'the depth of focus'.

48. Image Contrast SE-topographic and BSE-atomic number contrast SE Images
Image contrast, C
is defined by
SA-SB S
C= ________ = ____
SA SA
SA, SB Represent signals generated from two points, e.g., A and B, in the scanned area.
For two small objects to be detected against a background of random noise, studies show that their signal difference must be at least five times the noise level.
In order to detect objects of small size and low contrast in an SEM it is necessary to use a high beam current and a slow scan speed (i.e., improve signal to noise ratio).
SE-topographic and BSE-atomic number contrast

49. SE Images - Topographic Contrast
1mm
Defect in a semiconductor device
Molybdenum trioxide crystals
The debris shown here is an oxide fiber got stuck at a semiconductor device detected by SEM

50. BSE Image – Atomic Number Contrast
BSE atomic number contrast image showing a niobium-rich intermetallic phase (bright contrast) dispersed in an alumina matrix (dark contrast).
Z (Nb) = 41, Z (Al) = 13 and Z(O) = 8
Alumina-Al2O3

51. Field Contrast
Electron trajectories are affected by both electric and magnetic fields
Electric field – the local electric potential at the surface of a ferroelectric material or a semiconductor p-n junction produce a special form of contrast (Voltage contrast)
Magnetic field – imaging magnetic domains
Just show one example

52. Voltage contrast +U -U 500m
Voltage contrast from integrated circuit recorded at 5kV. The technique gives a qualitative view of static (DC) potential distributions but, by improvements in instrumentation, it is possible to study potentials which may be varying at frequencies up to 100MHz or more, and to measure the potentials with a voltage resolution of 10mV and a spatial resolution of 0.1m.
The image shows a heart pacemaker chip, powered up and running but removed from its usual protective casing, which is being examined in the SEM. In addition to the normal topographic contrast, large scale regions of uniform bright and dark contrast are visible. The bright areas are those which have a potential which is negative with respect to ground, while the dark areas are positive with respect to ground. The origin of such contrast is straightforward. When an area is negative then the collection field from the SE detector is increased and a higher fraction of the SE are collected. An area that is positive experiences a lower collection field from the detector, and also has a tendency to recollect its own secondary electrons so the SE signal from such areas is lower. This unique ability of the SEM to measure voltages in real-time, from small areas, and without requiring any mechanical contact has been of considerable value in the development of semiconductor technology and represents a significant fraction of the usage of this instrumentation. “Handbook of Microscopy” v.2,p.547 (1997).

53. Magnetic Field Contrast
(monolayer) t + - tc
SE electrons emitted from a clean surface ferromagnet are
spin-polarized, the sign of the polarization being opposite
to the magnetization vector in the surface of the material.
High resolution SEM image of a magnetic microstructure in
an untrathin ‘wedge-shaped’ cobalt film.
The observed pattern indicates a transition from a preference to the vertical direction for films that are thinner than about 0.6nm (bottom) to a planar structure for thicker films (top). The two different magnetization components [(a) vertical and (b) parallel to the thickness gradient] are made visible by electron polarization analysis in different detectors. For the image showing domains obtained from the perpendicular component of magnetization (bottom of (a) and top of (b)), the light areas indicate domains with magnetization pointing out of the image, black areas are domains with magnetization pointing into the plane. Grey tones represent areas with vertical polarization components in between.

54. Other Imaging Modes Cathodoluminescence (CL)
Nondestructive analysis of impurities and defects, and their distributions in semiconductors and luminescence materials
Lateral resolution (~0.5mm)
Phase identification and rough assessment of defect concentration
Electron Beam Induced Current (EBIC)
Only applicable to semiconductors
Electron-hole pairs generated in the sample
External voltage applied, the pairs are then a current – amplified to give a signal
Image defects and dislocations
Luminescence materials have wide band gaps.

55. CL micrographs of Te-doped GaAsb.
Te-tellurium
Te=1017cm-3, dark-dot dislocation contrast
Te=1018cm-3, dot-and-halo dislocation contrast which shows variations in the doping concen-tration around dislocations

56. EBIC Image of Doping Variations in GaAs Wafer
The circular area is the outline of the Schottky barrier region, and the shadow of the electrical contact on the barrier is also visible. The Schottky barrier is a metal film, usually Ti or Cr, evaporated on to the atomically clean surface of the material.
Although the impurities are only present at a concentration of 1016cm-3 (i.e., 1 part in 107) they vary the resistivity of the material and the hence the depletion depth beneath the barrier. If the range of the incident electron beam is greater than the maximum depletion depth then an increase in the depletion depth will increase the signal collected, and vice versa.
The variations in brightness across the material are due to impurities in the wafer. The extreme sensitivity (1016cm-3, i.e., 1 part in 107) and speed of this technique makes it ideal fro the characterization of as-grown semiconductor crystals.