1. Scanning Electron Microscopy (SEM)
Short description
Beam parameters
influence on image
e- gun components
filaments
lenses
Beam-sample interaction
electron scattering
Image formation
Goldstein, Scanning Electron Microscopy and X-ray Microanalysis

2. Scanning Electron Microscope (SEM)
Field of view:
5x 5 mm2 – 500 x 500 nm2
V-shaped Filament
Resolution: down to 1 nm
Extractor
Beam accelerator
Deflecting Plates
Electron
Column
Image Display
Primary
e- Beam
Scan quadrupole
e- Detector
Backscattered
Electrons
Sample

3. How to sweep an electron beam
Optical axis
First coil deviate beam from optical axis
Second coil brings beam back
at optical axis on the pivot point
Image formation point by point
collecting signal at each raster point S L
L = raster length on sample
W = working distance
S = raster length on screen
Magnification = S/L
L = 10 m, S = 10 cm
M depends on working distance

4. Effect of beam parameters on image
V0 = beam voltage
ip = beam current
p = beam convergence angle
dp = beam diameter at sample

5. Effect of beam parameters on image
Resolution
High resolution mode
Noise on signal
ip = beam current
dp = beam diameter
Good compromise
ip = 1 pA, dp = 15 nm
ip = 320 pA, dp = 130 nm
ip = 5 pA, dp = 20 nm
High current mode
Resolution too low

6. Effect of beam parameters on image
Depth of focus
If p is small, dp changes little with depth, so features
at different heights can be in focus
p = 15 mrad
p = 1 mrad

7. Effect of beam parameters on image
Electron energy
V0 < 5 kV, beam interaction limited to region close to surface, info on surface details
V kV, beam penetrates into sample, info on interior of sample
V0 = beam voltage

8. Electron column e- are produced and accelerated
Beam is reduced to increase resolution
Beam is focused on sample

9. Filament e- are accelerated to anode and the hole allows
Wehnelt: focuses e- inside the gun
Controls intensity of emitted e-
Grid connected to filament
with variable resistor
e- exit filament following + lines
The equipontential line shape
has focussing effect and
determines 0 and d0
e- are accelerated to anode and the hole allows
a fraction of this e- to reach the lenses

10. Filament Electron column Equipotential lines Filament head
Electron beam
The equipontential line shape
has focussing effect and
determines 0 and d0
Electron column

11. Operating principle: thermionic electron emission
Filament types
Lanthanum
hexaboride (LaB6)
Tungsten hairpin
(most common)
0.120 mm Tungsten wire
Operating principle: thermionic electron emission
LaB6 crystal 0.20 mm

12. Filament types Tungsten hairpin Lanthanum hexaboride (LaB6)
thermionic electron emission
Ac = 120 A/cm2K2
Ew = work function
To reduce filament evaporation  operate the electron gun
at the lowest possible temperature
Materials of low work function are desired.
Tungsten hairpin
Lanthanum hexaboride (LaB6)
Ew = 4.5 eV
Jc = 3.4 A/cm2 at 2700 K
Lifetime hours
Energy width  0.7 eV
Operating pressure 10-5 mbar
Ew = 2.5 eV
Jc = 40 A/cm2 at 1800 °K
Lifetime hours
Energy width  0.3 eV
Operating pressure 10-6 mbar

13. Thermal Field Emission
Filament types
Thermal Field
Emission
Operating principle:
thermionic electron emission +
Tunnelling
W-Zr crystal 0.20 mm
I = A/cm2 at 1800 °C
Lifetime > 1000 hours
Energy width  0.1 eV
Small source dimension (few nm)
Operating pressure 10-9 mbar

14. Lanthanum hexaboride (LaB6) Thermal Field Emission
E gun brightness
Beam current changes throughout the column
Brightness is conserved throughout the column R p dp
Tungsten hairpin
Lanthanum hexaboride (LaB6)
Thermal Field Emission
dp: 30 – 100 m
dp: 5 – 50 m
dp: 5 nm
 = 106 A/sr cm2
 = 108 A/sr cm2
 = 105 A/sr cm2

15. Electromagnetic Lenses
Demagnification of beam crossover image (d0)
to get high resolution (small dp)
d0: 5 – 100 m
for filaments
d0: 5 nm
for TFE
Beam focussing
High demag
needed
Low demag
needed
Fringe field
coils
radial
parallel

16. Electromagnetic Lenses
Focusing process
e- interacts with Br and Bz separately
-e (vz x Br) produces a force into screen
Fqin giving e- rotational velocity vqin
vqin interacts with Bz
produces a force toward optical axis
Fr = -e (vqin x Bz)
f = focal length
the distance from the point
where an electron first begins to
change direction to the point
where it crosses the axis.
The actual trajectory of the electron will be a spiral
The final image shows this spiraling action as a rotation of the image as the objective lens strength is changed.

17. Electromagnetic Lenses
Lens coil current and focal length
I = lens coil current
N = number of coils
V0 = accelerating voltage
Increasing the strength (current) of the lens reduces the focal distance
the focal length will become longer at higher accelerating voltages for the same lens current

18. Demagnification of beam crossover image (d0) = object
Comparison to optical lenses
Demagnification of beam crossover image (d0) = object
Beam crossover
d0 = tungsten diameter = 50 m
Scaling from the figure, the demag factor is 3.4 so d1 = d0/m = 14.7 m
CONDENSER LENSES: the aim is to reduce the beam diameter

19. Objective Lenses Scope: focus beam on sample They should contain:
Scanning coil
Stigmator
Beam limiting aperture
Scope: focus beam on sample
They also provide
further demagnification
Pinhole
No B outside
Large samples
Long working distances (40 mm)
High aberrations
Snorkel
B outside lens
Large samples
Separation of secondary from backscattered e-
Long working distances
Low aberrations
Immersion
Sample in B field
Small samples
Short working distances (3 mm)
Highest resolution
Low aberrations
Separation of secondary from backscattered e-

20. Effect of aperture size
Aperture size: 50 – 500 m
Decrease 1 for e- entering OL to a
a determines the depth of focus
Determines the beam current
Reduces aberrations

21. Effect of working distance
Increase in WD  increase in q 
m smaller  larger d  lower resolution but longer depth of focus

22. Effect of condenser lens strenght
Strong
Weak
Decrease q1 and
increase p2
 larger m
Higher Ibeam
Lower Ibeam
Lower dp
Higher dp
Increase in condenser strenght (current)  shorter q  larger m and smaller d
Also it brings a beam current reduction, so a compromise between current and resolution is needed

23. Gaussian probe diameter
Understand how probe size varies with probe current
Calculate the minimum probe size and the maximum probe current
Distribution of emission intensity from filament = gaussian with size dG
dG = FWHM
Knowing emitter source size, dG may be calculated from the total demagnification
With no aberrations, keeping dG constant would allow to increase ip by only increasing p

24. Spherical aberrations
Origin: e- far from optical axis are deflected more strongly
e- along PA gives rise to gaussian image plane
No aberration
e- along PB cross the optical axis in ds 
So at the focal plane there
is a disk and not a point
Spherical aberration disk of least confusion
Cs = Spherical aberration coefficient  f
Immersion and snorkel Cs ~ 5 mm
Pinholes Cs ~ mm
So one need to put a physical aperture to limit aberrations

25. Aperture diffraction To estimate the contribution to beam
diameter one takes half the diameter
of the diffraction disk nm
rad eV

26. Chromatic aberrations
Origin: initial energy difference of accelerated electrons
Chromatic aberration disk of least confusion
For tungsten filament E = 3 eV
At 30 KeV E/E0 = 10-4
At 3 KeV E/E0 = 10-3
Cs = Chromatic aberration coefficient  f

27. Astigmatism Origin: machining errors, asymmetry in coils, dirt
Result: formation ow two differecnt
focal points
Effect on image:
Stretching of points into lines
Can be compensated with octupole stigmator

28. Astigmatism

29. Beam-sample interaction
Simulation of e- trajectories
Backscattered e-
Silicon
V0 = 20 KV
TFE,  = A/sr cm2
dp = 1 nm
Ib = 60 pA
Main reason of large interaction volume:
Elastic Scattering
Inelastic scattering

30. Beam-sample interaction
Elastic Scattering 0
Elastic scattering cross section
Z = atomic number;
E = e- energy (keV);
A = atomic number
N0 = Avogadro’s number;
 = atomic density
Elastic mean free path =
distance between scattering events
Silicon
 = 2.33 g/cm3
Z = 14
A = 28
N0 =

31. Beam-sample interaction
Inelastic Scattering
Inelastic scattering energy loss rate
Z = atomic number
A= atomic number
N0 = Avogadro’s number
 = atomic density
Ei = e- energy in any point inside sample
J = average energy loss per event
Eb = 20 KeV
The path of a 20 KeV e- is of the
order of microns, so the interaction volume
is about few microns cube

32. Beam-sample interaction
20 KeV beam incident on PMMA
with different time periods
Interaction volume
Simulation
Energy
transferred
to sample

33. Influence of beam parameters on beam-sample interaction
Beam energy
10 KeV
20 KeV Fe
30 KeV
Elastic scattering
cross section
Inelastic scattering
energy loss rate
Longer 
Lower loss rate

34. Influence of beam parameters on beam-sample interaction
Smaller and asymmetric interaction volume
Incidence angle
45° Fe
60°
surface
surface
Scattering of e- out of the sample
Reduced depth
Same lateral dimensions

35. Influence of sample on beam-sample interaction
Atomic number
V0 = 20 keV
Fe (Z=26)
C (Z=6)
Fe, k shell
C, k shell
Elastic scattering
cross section
10% to 50% of the beam electrons are backscattered
They retain 60% to 80% of the initial energy of the beam
Reduced linear dimensions of interaction volume
Inelastic scattering
energy loss rate

36. Influence of sample on beam-sample interaction
Atomic number
V0 = 20 keV
Ag (Z=47)
U (Z=92)
U, k shell
Ag, k shell
More spherical shape of interaction volume

37. Signal from interaction volume (what do we see?)
Backscattered electrons
Secondary electrons
Backscattered e-

38. BSE dependence Backscattered electron coefficient 60°
Monotonic increase
Relationship between
 and a sample property (Z)
This gives atomic number contrast
60°
If different atomic species are present in the sample
Ci = weight concentration

39. BSE dependence Incidence angle 60° n = intensity at normal
Line length: relative intensity of BSE
Strong influence on BSE detector position
60°

40. BSE dependence Energy distribution Lateral spatial distribution
The energy of each BSE depends on the
trajectory inside sample, hence different
energy losses
Region good for
high resolution
Region I: E up to 50 %
Becomes peaked with increasing Z
Gives rise to loss in lateral
resolution
At low Z the external region increases

41. BSE dependence Sampling depth Sampling depth is typically 100 -300 nm
Percent of 
Fraction of maximum
e- penetration
(microns)
RKO defines a circle on the surface (center in the beam) spanning the interaction volume
Sampling depth is typically nm
for beam energies above 10 keV

42. Signal from interaction volume (what do we see?)
Energy distribution of electrons emitted by a solid
Secondary electrons
Energy: 5 – 50 eV
Probability of e- escape from solid
 = e- mean free path

43. Secondary electrons Origin: electron elastic and inelastic scattering
SURFACE
SENSITIVE
SE1 = secondary due directly to incident beam
Beam resolution
SE2 = secondary generated by backscattered electrons
BSE resolution
Carbon: SE2 /SE1 = 0.18
Low backscattering cross section
Aluminum: SE2 /SE1 = 0.48
Copper: SE2 /SE1 = 0.9
High backscattering cross section
Gold: SE2 /SE1 = 1.5
SE Intensity angular distribution: cos

44. Image formation Backscattered e- Secondary e- Volume sensitive
Surface sensitive
Sampling depth
~ nm

45. Image formation
Many different signals can be extracted from beam-sample interaction
So the information depends on the signal acquired, is not only topography

46. Image formation
The beam is scanned along a single vector (line) and the same scan generator
is used to drive the horizontal scan on a screen
Signals to be recorded
For each point the detector collects a current
and the intensity is plotted
or the intensity is associated with
a grey scale at a single point
A one to one correspondence is established between
a single beam location and a single point of the display
Magnification M = LCRT/Lsample
But the best way is to calibrate the instrument

47. Image formation Digital image: numerical array (x,y,Signal)
8 bits = 28 = 256 gray levels
Signal: output of ADC
Resolution = 2n
16 bits = 216 = gray levels
Pixel = picture element
Pixel is the size of the area on the sample from which information is collected
Actually is a circle
Considering the matrix defining the Dimension of Pixel Element
Length of the scan on sample
number of steps along the scan line
The image is focused when
the signal come only from a
the location where the beam
is addressed
At high magnification there
will be overlap between two pixel

48. Image formation There is overlapping of pixel signal intensity
For a given experiment (sample type) and experimental conditions (beam size, energy)
the limiting magnification should obtained by calculating the area generating
signal taking into account beam-sample interactions and compare to pixel size
beam
Area producing BSe-
V0 = 10 keV, dB = 50 nm
on Al, dBSE = 1.3 m  deff = 1.3 m
on Au dBSE = 0.13 m  deff = 0.14 m
10x 10 cm display
There is overlapping
of pixel signal intensity
Different operation settings
for low and high magnification

49. Depth of field
Depth of field D = distance along the lens axis (z) in the object plane
in which an image can be focused without a loss of clarity.
To calculate D, we need to know where from the focal plane the beam is broadened
Broadening means adjacent pixel overlapping
The vertical distance required to broaden a beam r0 to a radius r (causing defocusing) is
For small angles

50. Depth of field How much is r?
On a CRT defocusing is visible when two pixels are overlapped 
r = 1 pixel (on screen 0.1 mm)
But 1 pixel size referred to sample depends on magnification
To increase D, we can either reduce M or reduce beam divergence
Beam divergence is defined by the beam defining aperture

51. Depth of field
Optical
SEM

52. Detector Everhart-Thornley Secondary + BSE Grid Positive: BSE+SE
Grid negative: only BSE
The bias attracts most of SE
solid angle acceptance: 0.05 sr
Geometric efficiency: 0.8 %

53. Topographic contrast Intensity of SE and BSE depends on
beam/sample incidence angle ()
and on detector/sample angle ()
BSE coefficient increase with 
BSE emission distribution ~ cos 
SE emission distribution ~ sec 
Detector position and electron energy window are important

54. Topographic contrast Negative bias cage to exclude secondary e-
- Detector is on one side of sample  anysotropic view
- Small solid angle of acceptance  small signal
- High tilt angle
Analogy to eye view
Dierctional view
High contrast due to orientation of
sample surfaces

55. Topographic contrast Positive bias cage to accept secondary e-
Contributions:
Direct BSE+SE
SE distribution intensity I ~ sec 
Variation in SE signal between two surfaces with different 
dI = sec  tan  d
So the contrast is given by dI/I = tan  d
The SE are collected from most emitting surfaces since
the positive bias allows SE to reach the detector
Analogy to eye view

57. High resolution imaging
High resolution signal if selected in energy
SE1 : e- directly generated by beam
BSE1 : low energy loss (<2%) e- from beam
SE2 : e- generated by BSE into sample
BSE2 : higher energy loss e- from beam
High resolution signal
generated by BSE1, SE1
Separation of signal is necessary to obtain high resolution

58. Low mag High mag Silicon V0 = 30 KV TFE,  = 1 108 A/sr cm2 dp = 1 nm
Ib = 60 pA
Low mag
Scan width at X = 10x10 m2
image 1024x1024, pixel width 10 nm
SE1 - BSE1 width = about 2 nm
Scanning at low M means field of view
larger than SE2 emission area
So there is large overlap between pixel
And the changes are due only to SE2 variations
Beam penetration depth = 9.5 m
Emission area = 9.5 m
FWHM = 2 nm
High mag
Scan width at X = 1x1 m2
image 1024x1024, pixel width 1 nm
Scanning at high M means field of view
smaller than SE2 emission area
So as the beam is scanned, no changes in
SE2 but changes are due to SE1
SE2 gives large random noise

59. Ag NP on glass
Carbon nanotubes
TiO2 on silicon

60. SEM in FOOD Schematic representation of gaseous SED
the role of imaging gas in VP-SEM
B. James / Trends in Food Science & Technology 20 (2009) 114

61. SEM in FOOD ‘‘bloomed’’ chocolate. 50 μm
Blades of cocoa butter present on the surface
Image taken with sample at 5 °C
using nitrous oxide at ~ 100 Pa (0.8 torr)
as imaging gas

62. SEM in FOOD 20 μm VP-SEM image of commercially produced mayonnaise.
Image taken with sample at 5.0 °C
using water vapor at around 670 Pa (5.0 torr) as imaging gas.
Light continuous phase is water
mid grey discrete phase is oil.
Darkest grey areas are air bubbles
Disadvantages of conventional SEM techniques
insulating specimens
impossibility of examining hydrated samples without altering their state (drying or freezing)
Sample preparation treatments introduce artifacts
No studies of dynamic processes for such samples

63. Scanning Auger Microscopy (SAM) e- Detector V-shaped Filament
Extractor
Deflecting Plates
Backscattered
Electrons
e- Detector
Primary
e- Beam
Sample
Chemical Map
Electron Energy Analyzer
Scanning Auger
Microscopy (SAM)
Auger Spectrum

64. Auger Spectroscopy Ekin Evac EF e- e- EK(XYZ)= EB(X)-EB(Y)-EB(Z)- VB
M2,3 3p
M1 3s e- e-
L2,3 2p
L1 2s
K 1s
One-Hole
Initial State
De-Excitation
Auger Process
Two-Hole
Final State
Ground State
XYZ Auger Process
One-Particle Scheme
Energy Conservation
EK(XYZ)= EB(X)-EB(Y)-EB(Z)-
EK(XYZ) = KE of Auger electron
EB(X) = BE of X level
EB(Y) = BE of Y level
EB(Z) = BE of Z level

65. EK(XYZ)= EB(X)-EB(Y)-EB(Z)-F+R-
Usually additional terms must be included
accounting for the two-hole final state
correlation interaction and the relaxation effects
EK(XYZ)= EB(X)-EB(Y)-EB(Z)-F+R-
F Two-Hole Final State Correlation Energy
R Two-Hole Relaxation Energy
Eb One electron binding energy K L1 M1 VB
Evac EF
Ekin
L2,3
M2,3

66. Auger Process Nomenclature KL1M2 Auger Process
L1L2M1 Coster-Kronig Process
(the initial hole is filled by an electron
of the same shell)
CCC Core-Core-Core Transition
CCV Core-Core-Valence Transition
CVV Core-Valence-Valence Transition
KL1M2
L1L2M1
Ekin
Ekin
Evac
Evac EF EF VB VB
M2,3
M2,3 M1 M1
L2,3
L2,3 L1 L1 K K

67. Competitive processes
Auger
X-Ray
Fluorescence
Electron
Relative Probabilities of Relaxation
by Auger Emission and
by X-Ray Fluorescence Emission EF
3d M4,5
3p M2,3
3s M1
Photon
2p L3
2p L2
2s L1
1s K
For lines originating from shell L and M the Auger yield remains much higher than X-ray emission

68. Principal Auger Lines while Spanning the Periodic Table of the Elements
CHEMICAL SENSITIVITY

69. Electron distribution spectrum
Pulse Counting Mode
Derivative Mode
Since Auger emission lines are often very broad and weak, their detectability is enhanced by differentiating of the spectrum

70. Chemical environment sensitivity
Gas
Solid

71. Auger Electron Spectroscopy Quantitative Analysis
In analogy to what developed for XPS,
one can determine the atomic concentration (Ci)
of the atomic species present
in the near-surface region of a solid sample
Ci Atomic Concentration of the i-th species
si Orbital Sensitivity Factor of the i-th species
Ii Spectral Intensity Related to the i-th species

72. Au N6,7VV
Si L2,3VV
Auger Spectra as Measured at Selected Points of the Self-organized Agglomerated Au/Si(111) Interface
Island
Flat region

73. Si L2,3VV Auger Line Shape as Measured at Selected Points of the Self-organized Agglomerated Au/Si(111) Interface
Island
Flat region