1. More review problems for OM
13. What are differences between the common transmitted
(or reflected) OM and polarized OM?
14. Can a polarized OM be used to determine the orientation
(or optical axis) of an anisotropic single crystal? State your
reasoning.
15. Can you see something from eyepiece lens when an
isotropic crystal is on the sample stage of a polarized OM?
Why or why not?
2. Lecture-4 Scanning Electron Microscopy
• 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)
http://www.mse.iastate.edu/microscopy
http://www.youtube.com/watch?v=KYNknR-e5IU to 2:40 Introduction
http://virtual.itg.uiuc.edu/training/EM_tutorial
http://science.howstuffworks.com/scanning-electron-microscope.htm/printable
(SEM)
3. Principal features of an optical microscope, a transmission
electron microscope and a scanning electron microscope,
drawn to emphasize the similarities of overall design.
Comparison of OM,TEM and SEM
OM TEM SEM
Magnetic
lenses
detector
CRT
Cathode
Ray Tube
Light source
Source of
electrons
Condenser
Specimen
Objective
Eyepiece
Projector Specimen
https://www.youtube.com/watch?v=b4WOsYktdn4 comparing microscopes to 0:12
Probe
4. Optical Microscopy (OM ) vs
Scanning Electron Microscopy (SEM)
25mm
OM SEM
Small depth of field
Low resolution
Large depth of field
High resolution
radiolarian
5. What is SEM
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.
https://www.youtube.com/watch?v=sFSFpXdAiAM
http://www.youtube.com/watch?v=lrXMIghANbg How a SEM works ~1:20-2:10
http://www.youtube.com/watch?v=SaaVaILUObg Operation of SEM
Sample
Chamber
Column
http://virtual.itg.uiuc.edu/training/EM_tutorial
SEM is designed for
direct studying of the
surfaces of solid objects
6. Advantages of Using SEM over OM
Magnification Depth of Field Resolution
OM 4x – 1000x 15.5mm – 0.19mm ~ 0.2mm
SEM 10x – 3000000x 4mm – 0.4mm 1-10nm
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.
7. 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.
http://www.youtube.com/watch?v=lrXMIghANbg at~4:16-4:42
https://www.youtube.com/watch?v=VWxYsZPtTsI at~4:18-4:38
https://www.youtube.com/watch?v=nPskvGJKtDI
8. Main Applications
• Topography
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.)
9. A Look Inside the Column
Column
http://www.youtube.com/watch?v=c7EVTnVHN-s at~1:10-2:10 inside the column
http://virtual.itg.uiuc.edu/training/EM_tutorial
10. Source: L. Reimer, “Scanning Electron
Microscope”, 2nd Ed., Springer-Verlag,
1998, p.2
Electron Gun
e- beam
https://www.youtube.com/watch?v=GY9lfO-tVfE
at~2:38-4:45
A more detailed look inside
- beam convergence
<72o
https://www.youtube.com/watch?v=Mr9-1Sz_CK0
at~1:06-2:40
http://www.youtube.com/watch?v=sFSFpXdAiAM
11. • What is SEM?
• Working principles of SEM
• Major components and their functions
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. Electron beam Source
W or LaB6 Filament
Thermionic or Field Emission Gun
http://www.youtube.com/watch?v=VWxYsZPtTsI at ~1:05-1:40 thermionic gun
http://www.youtube.com/watch?v=fxEVsnZT8L8 at ~2:10-2:20
w
LaB6
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
-
+
http://www.matter.org.uk/tem/electron_gun/electron_gun_simulation.htm
http://www.matter.org.uk/tem/electron_gun/electron_sources.htm
http://www.youtube.com/watch?v=ZIJ1jI1xDhY Electron gun detail
15. Field Emission Gun
• The tip of a tungsten needle is
made very sharp (radius < 0.1
mm)
• 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
http://www.matter.org.uk/tem/electron_gun/electron_sources.htm
16. Source of Electrons
T: ~1500oC
Thermionic Gun
W and LaB6 Cold- and thermal FEG
Electron Gun Properties
Source Brightness Stability(%) Size Energy spread Vacuum
W 3X105 ~1 50mm 3.0(eV) 10-5 (t )
LaB6 3x106 ~2 5mm 1.5 10-6
C-FEG 109 ~5 5nm 0.3 10-10
T-FEG 109 <1 20nm 0.7 10-9
(5-50mm)
E: >10MV/cm
(5nm)
Filament
W
Brightness – beam current density per unit solid angle
17. Why Need a Vacuum?
When a SEM is used, the electron-optical
column and sample chamber must always
be at a vacuum.
1. 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.
http://www.youtube.com/watch?v=c7EVTnVHN-s at ~4:50-5:25
http://www.mse.iastate.edu/research/laboratories/sem/microscopy/how-does-the-
sem-work/high-school/the-sem-vacuum/ SEM Vacuum
http://virtual.itg.uiuc.edu/training/EM_tutorial external
18. Magnetic Lenses
• Condenser lens – focusing
controls the spot size and
convergence () of the electron
beam 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.
http://www.youtube.com/watch?v=lrXMIghANbg at~1:30-1:53
http://www.youtube.com/watch?v=VWxYsZPtTsI at~0:32-1:02
http://www.matter.org.uk/tem/lenses/simulation_of_condenser_system.htm
Major components and their functions
19. How Is Electron Beam Focused?
A magnetic lens is a solenoid designed to produce
a specific magnetic flux distribution.
p
q
Magnetic lens
(solenoid)
Lens formula: 1/f = 1/p + 1/q
M = q/p
Demagnification:
(Beam diameter)
F = -e(v x B)
f Bo
2
f can be adjusted by changing Bo, i.e., changing the
current through coil. Bo - magnetic field
http://www.matter.org.uk/tem/lenses/electromagnetic_lenses.htm
-
http://www.youtube.com/watch?v=G9Glw3BUTAQ Magnetic field in a solenoid
http://www.youtube.com/watch?v=a2_wUDBl-g8 e- in a magnetic field
https://www.youtube.com/watch?v=fwiKRis145E to~1:15 and at~3:08-4:15 F=-e(vxB)
https://www.youtube.com/watch?v=sCY
X_XQgnSA&feature=related 6:13-6:23
Magnetic lens
20. 1 1 1
_ = _ + _
f O i
Lens Formula f-focal length (distance)
O-distance of object from
lens
i-distance of image from
lens
I1
O i
i
O
= =
mo
Magnification
by objective
Lens formula and magnification
Objective lens
-Inverted
image
f
f
ho
hi
hi
ho
http://www.youtube.com/watch?v=-k1NNIOzjFo&feature=related at~3:00-3:40
21. 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.
23. 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 10-9-
10-12 A.
24. The Objective Lens – 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
“Disc of Least Confusion”
Electron beam
Objective
lens
Wide
aperture
Narrow
aperture
Wide disc of
least confusion
Narrow disc
of least
confusion
Large beam diameter
striking specimen
Small beam diameter
striking specimen
http://www.matter.org.uk/tem/lenses/simulation_of_condenser_system.htm aperture
Better resolution
https://www.youtube.com/watch?v=E85FZ7WLvao
25. A Look Inside the Column
Column
A disc of metal
Objective aperture
26. 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.
Out of focus in focus out of focus
lens current lens current lens current
too strong optimized too weak
Objective
lens
http://www.matter.org.uk/tem/lenses/second_condenser_lens.htm
Over-focused Focused Under-focused
27. 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
y-direction
scanning
coil
specimen
Objective
lens
Holizontal line scan
Blanking
http://www.youtube.com/watch?v=lrXMIghANbg at ~4:12
28. Electron Detectors and Sample Stage
Objective
lens
Sample stage
http://www.youtube.com/watch?v=VWxYsZPtTsI at~4:45
http://virtual.itg.uiuc.edu/training/EM_tutorial internal
https://www.youtube.com/watch?v=Mr9-1Sz_CK0
at~2:20-2:30
29. 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)
30. Electron Beam and Specimen Interactions
Electron/Specimen Interactions
Sources of Image Information
(1-50KeV)
Electron Beam Induced Current (EBIC)
http://www.youtube.com/watch?v=Mr9-1Sz_CK0 at~2:30-2:42
https://www.youtube.com/watch?v=F9qwfYwwCRM at~0.58-1:38
31. 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.
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
BaTiO3
5mm
SE image
Primary
SE yield: d=nSE/nB independent of Z
d decreases with increasing beam
energy and increases with decreasing
glancing angle of incident beam
Z – atomic number
http://www.youtube.com/watch?v=Mr9-1Sz_CK0 at~2:52-3:52
https://www.youtube.com/watch?v=F9qwfYwwCRM at~0:58-1:14
32. Topographical Contrast
Bright
Dark
+200V
e-
lens polepiece
SE
sample
Everhart-Thornley
SE Detector
Scintillator
light pipe
Quartz
window
+10kV
Faraday
cage
Photomultiplier
tube
PMT
http://www.youtube.com/watch?v=lrXMIghANbg at ~2:10-3:30 (3:09~3:18)
Topographic contrast arises because SE generation depend on
the angle of incidence between the beam and sample. 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.
https://www.youtube.com/watch?v=GY9lfO-tVfE at~4:35-6:00
http://www.youtube.com/watch?v=VWxYsZPtTsI at~3:00-3:20
33. Everhart-Thornley SE Detector System
Solid angle of collection
Both SE and B electrons can be detected, but the geometric
collection efficiency for B electrons is low, about 1-10%,
while for SE electrons it is high, often 50% or more.
34. Backscattered Electrons (BSE)
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.
Primary
BSE image from flat surface of an
Al (Z=13) and Cu (Z=29) alloy
http://www.youtube.com/watch?v=VWxYsZPtTsI at ~3:20-3:35
https://www.youtube.com/watch?v=F9qwfYwwCRM at~1:14-1:34
http://www.youtube.com/watch?v=Mr9-1Sz_CK0 at~3:52-4:26
35. Semiconductor Detector
for Backscattered Electrons
High energy electrons produce electron-
hole pairs (charge carriers) in the
semiconductor, and generate a current
pulse under an applied potential.
A and B are a paired
semiconductor detectors
https://www.youtube.com/watch?v=F9qwfYwwCRM at~4:25-4:50
38. Interaction Volume: I
The incident electrons do not go along a
straight line in the specimen, but a zig-zag
path instead.
Monte Carlo simulations of 100 electron trajectories
e-
39. 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 and e- beam
size
40. 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.
41. If the diameter of primary
electron beam is ~5nm
- Dimensions of escape
zone of
Escape Volumes of Various Signals
•Secondary electron:
diameter~10nm; depth~10nm
•Backscattered electron:
diameter~1mm; depth~1mm
•X-ray: from the whole
interaction volume, i.e., ~5mm
in diameter and depth
http://www.youtube.com/watch?v=VWxYsZPtTsI at ~3:38-4:10
42. Electron Interaction Volume
5mm
a b
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
Pear shape
43. Escape Volumes of Various Signals
SE
BE
X-ray
Lost SE
Lost BE
Primary
44. Lecture-3 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)
http://www.youtube.com/watch?v=sFSFpXdAiAM
45. Image Formation & Magnification in SEM
beam
e-
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.
A
A
Detector
Amplifier
10cm
10cm
M= C/x
http://www.youtube.com/watch?v=VWxYsZPtTsI at ~4:18-4:45
http://www.youtube.com/watch?v=lrXMIghANbg at ~4:10-4:40
46. Magnification
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
100 1 mm 10000 10 mm
1000 100 mm 100000 1 mm
Low M
Large x
40mm
High M
small x
7mm
2500x 15000x
1.2mm
e-
x
https://www.youtube.com/watch?v=Mr9-1Sz_CK0 at~11:45-11:55
48. Resolution Limitations
Ultimate resolution obtainable in an SEM
image can be limited by:
1. 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=1.293/4 Cs
1/4
A SEM fitted with an FEG has an achievable resolution of
~1.0nm at 30 kV due to smaller Cs (~20mm) and .
2. Specimen Contrast Limitations
Contrast dmin
1.0 2.3nm
0.5 4.6nm
0.1 23nm
0.01 230nm
3. Sampling Volume Limitations (Escape volume)
http://www.youtube.com/watch?v=SVK4OkUK0Yw at~1:47-3:07
dmin = 0.61/NA for OM
Cs – coefficient of spherical aberration of lens (~mm)
NA=nsin
~ 500nm
49. 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!
Pixel - In digital imaging, a pixel (picture element) is a physical
point in a raster image, or the smallest addressable element in a
display device; so it is the smallest controllable element of a
picture represented on the screen.
http://en.wikipedia.org/wiki/Pixel
50. 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.
P=D/Mag = 100um/Mag
P-pixel diameter on specimen surface
D-pixel diameter on CRT, Mag-magnification
Mag P(mm) Mag P(nm)
10x 10 10kx 10
1kx 0.1 100kx 1
51. • The optimum condition for imaging is when
the escape volume of the signal concerned
equals to the pixel size.
Resolution of Images: II
52. • 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’)
Resolution of Images: III
53. 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)
54. Resolution of Images: V
Pixel diameter on Specimen
Magnification µm nm
10 10 10000
100 1 1000
1000 0.1 100
10000 0.01 10
100000 0.001 1
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.
55. Resolution of Images
The optimum condition for imaging is when the escape
volume of the signal concerned equals to the pixel size.
e-
Effect of probe size on
escape volume of SE
10nm
1mm
5mm
BSE X-ray
The resolution is the pixel diameter on specimen surface.
P=D/Mag = 100um/Mag
56. Depth of Field
D = (mm)
AM
4x105W
To increase D
Decrease aperture size, A
Decrease magnification, M
Increase working distance, W (mm)
Depth of Field
A/2 =
=5x10-3=0.005
(1o=0.0175)
72o=1.26
OM
57. Image Contrast
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.
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
SE Images
58. SE Images - Topographic Contrast
The debris shown here is an oxide
fiber got stuck at a semiconductor
device detected by SEM
1mm
Defect in a semiconductor device Molybdenum
trioxide crystals
59. 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
2mm
60. 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
61. Voltage contrast
500mm
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 0.1mm.
+U
-U
62. Magnetic Field Contrast
+ -
t
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.
(monolayer)
tc
63. 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
64. CL micrographs of Te-doped GaAs
a. b.
a. Te=1017cm-3, dark-dot dislocation contrast
b. Te=1018cm-3, dot-and-halo dislocation contrast
which shows variations in the doping concen-
tration around dislocations
65. EBIC Image of Doping Variations
in GaAs Wafer
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.
66. Do review problems on SEM
http://science.howstuffworks.com/scanning-electron-microscope.htm/printable
Study
Next Lecture
• Energy Dispersive X-ray Spectroscopy (EDS)
• Wavelength Dispersive X-ray Spectroscopy (WDS)
• Orientation Imaging Microscopy (OIM)
• X-ray Fluorescence (XRF)
• Scanning Probe Microscopy (STM and AFM)
