More%20review%20problems%20for%20OM

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
(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.mse.iastate.edu/microscopy
http//www.youtube.com/watch?vKYNknR-e5IU to
240 Introduction http//virtual.itg.uiuc.edu/tra
ining/EM_tutorial http//science.howstuffworks.com
/scanning-electron-microscope.htm/printable
3
Comparison of OM,TEM and SEM
Probe
Source of electrons
Light source
Condenser
Magnetic lenses
Specimen
Objective
Specimen
CRT
Projector
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.
https//www.youtube.com/watch?vb4WOsYktdn4
comparing microscopes to 012
4
Optical Microscopy (OM ) vs Scanning Electron
Microscopy (SEM)
25mm
radiolarian
OM
SEM
Small depth of field Low resolution
Large depth of field High resolution
5
What is SEM
http//virtual.itg.uiuc.edu/training/EM_tutorial
Column
SEM is designed for direct studying of the
surfaces of solid objects
Sample Chamber
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?vsFSFpXdAiAM http/
/www.youtube.com/watch?vlrXMIghANbg How a SEM
works 120-210 http//www.youtube.com/watch?vSa
aVaILUObg Operation of SEM
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.

https//www.youtube.com/watch?vVWxYsZPtTsI
at418-438
http//www.youtube.com/watch?vlrXMIghANbg
at416-442
https//www.youtube.com/watch?vnPskvGJKtDI
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
http//virtual.itg.uiuc.edu/training/EM_tutorial
Column
http//www.youtube.com/watch?vc7EVTnVHN-s
at110-210 inside the column
10
A more detailed look inside
http//www.youtube.com/watch?vsFSFpXdAiAM
?lt72o
Electron Gun
e- beam
?
https//www.youtube.com/watch?vMr9-1Sz_CK0
at106-240
Source L. Reimer, Scanning Electron
Microscope, 2nd Ed., Springer-Verlag, 1998, p.2
https//www.youtube.com/watch?vGY9lfO-tVfE
at238-445
? - beam convergence
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
LaB6
w
W or LaB6 Filament Thermionic or Field Emission
Gun
http//www.youtube.com/watch?vfxEVsnZT8L8 at
210-220
http//www.youtube.com/watch?vVWxYsZPtTsI at
105-140 thermionic gun
14
Thermionic Emission Gun
http//www.matter.org.uk/tem/electron_gun/electron
_sources.htm
http//www.matter.org.uk/tem/electron_gun/electron
_gun_simulation.htm

• 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.youtube.com/watch?vZIJ1jI1xDhY
Electron gun detail
15
Field Emission Gun

• The tip of a tungsten needle is made very sharp
  (radius lt 0.1 ?m)
• The electric field at the tip is very strong (gt
  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 lt 1 nm is possible

http//www.matter.org.uk/tem/electron_gun/electron
_sources.htm
16
Source of Electrons


Thermionic Gun
E gt10MV/cm
T 1500oC
W

Filament
(5-50mm)
(5nm)

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 lt1 20nm 0.7
10-9

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.
• 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//virtual.itg.uiuc.edu/training/EM_tutorial
external
http//www.youtube.com/watch?vc7EVTnVHN-s at
450-525 http//www.mse.iastate.edu/research/lab
oratories/sem/microscopy/how-does-the-sem-work/hig
h-school/the-sem-vacuum/ SEM Vacuum
18
Magnetic Lenses
Major components and their functions

• 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?vlrXMIghANbg
at130-153 http//www.youtube.com/watch?vVWxYs
ZPtTsI at032-102
http//www.matter.org.uk/tem/lenses/simulation_of_
condenser_system.htm
19
How Is Electron Beam Focused?
http//www.youtube.com/watch?vG9Glw3BUTAQ
Magnetic field in a solenoid http//www.youtube.co
m/watch?va2_wUDBl-g8 e- in a magnetic
field https//www.youtube.com/watch?vfwiKRis145E
to115 and at308-415 F-e(vxB)
http//www.matter.org.uk/tem/lenses/electromagneti
c_lenses.htm

• 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
Demagnification
https//www.youtube.com/watch?vsCYX_XQgnSAfeatur
erelated 613-623
f ? Bo2
Magnetic lens
f can be adjusted by changing Bo, i.e., changing
the current through coil. Bo - magnetic field
http//www.youtube.com/watch?v3McFA40nP0A
Magnetic deflection of e- beam at020-150
20
Lens formula and magnification
Objective lens
ho
f
f
hi
O
i
-Inverted image
I1
1 1 1
_ _ _
Lens Formula
f-focal length (distance) O-distance of object
from lens i-distance of image from lens
f O i
i
Magnification by objective
hi

mo
ho
O
http//www.youtube.com/watch?v-k1NNIOzjFofeature
related at300-340
http//micro.magnet.fsu.edu/primer/java/lenses/con
verginglenses/index.html
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 lt 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.

22
The Condenser Lens
http//www.matter.org.uk/tem/lenses/simulation_of_
condenser_system.htm change of f
Demagnification M f/L
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

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
Better resolution
Disc of Least Confusion
https//www.youtube.com/watch?vE85FZ7WLvao
http//www.matter.org.uk/tem/lenses/simulation_of_
condenser_system.htm aperture
25
A Look Inside the Column
Column
Objective aperture
A disc of metal
26
The Objective Lens - Focusing
http//www.matter.org.uk/tem/lenses/second_condens
er_lens.htm

• 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
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
Holizontal line scan
Blanking
y-direction scanning coil
Objective lens
specimen
http//www.youtube.com/watch?vlrXMIghANbg at
412
28
Electron Detectors and Sample Stage
http//www.youtube.com/watch?vVWxYsZPtTsI at445
http//virtual.itg.uiuc.edu/training/EM_tutorial
internal
Objective lens
Sample stage
https//www.youtube.com/watch?vMr9-1Sz_CK0
at220-230
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
Sources of Image Information
Electron/Specimen Interactions
(1-50KeV)
Electron Beam Induced Current (EBIC)
https//www.youtube.com/watch?vF9qwfYwwCRM
at0.58-138
http//www.youtube.com/watch?vMr9-1Sz_CK0
at230-242
31
Secondary Electrons (SE)
http//www.youtube.com/watch?vMr9-1Sz_CK0
at252-352 https//www.youtube.com/watch?vF9qwf
YwwCRM at058-114
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 dnSE/nB independent of Z d decreases
with increasing beam energy and increases with
decreasing glancing angle of incident beam
BaTiO3
Production of SE is very topography related. Due
to their low energy, only SE that are very near
the surface (lt10nm) can exit the sample and be
examined (small escape depth).
Growthstep
5?m
SE image
Z atomic number
32
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 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.
http//www.youtube.com/watch?vlrXMIghANbg at
210-330 (309318)
https//www.youtube.com/watch?vGY9lfO-tVfE
at435-600 http//www.youtube.com/watch?vVWxYsZ
PtTsI at300-320
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)
http//www.youtube.com/watch?vMr9-1Sz_CK0
at352-426
Primary
BSE image from flat surface of an Al (Z13) and
Cu (Z29) 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 hnBS/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.
http//www.youtube.com/watch?vVWxYsZPtTsI at
320-335 https//www.youtube.com/watch?vF9qwfYw
wCRM at114-134
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?vF9qwfYwwCRM
at425-450
36
Semiconductor Detector for Backscattered Electrons
37
Effect of Atomic Number, Z, on BSE and SE Yield
38
Interaction Volume I
e-
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.

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
Escape Volumes of Various Signals

• If the diameter of primary electron beam is 5nm
• - Dimensions of escape zone of

• Secondary electron
• diameter10nm depth10nm
• Backscattered electron
• diameter1?m depth1?m
• X-ray from the whole
• interaction volume, i.e., 5?m
• in diameter and depth

http//www.youtube.com/watch?vVWxYsZPtTsI
at 338-410
42
Electron Interaction Volume
Pear shape
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
43
Escape Volumes of Various Signals
Primary
SE
Lost SE
BE
Lost BE
X-ray
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?vsFSFpXdAiAM
45
Image Formation Magnification in SEM
http//www.youtube.com/watch?vVWxYsZPtTsI at
418-445
M C/x
A
10cm
e-
beam
Detector
10cm
Amplifier
A
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.
http//www.youtube.com/watch?vlrXMIghANbg at
410-440
46
Magnification
https//www.youtube.com/watch?vMr9-1Sz_CK0
at1145-1155
e-
x
Low M Large x 40mm
High M small x 7mm
1.2mm
15000x
2500x
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
47
Image Magnification
Example of a series of increasing magnification
(spherical lead particles imaged in SE mode)
48
Resolution Limitations

• Ultimate resolution obtainable in an SEM image
  can be limited by
• Electron Optical limitations
• Diffraction dd1.22?/?
• for a 20-keV beam, ? 0.0087nm and ?5x10-3
  dd2.1nm
• Chromatic and spherical aberrations
  dmin1.29l3/4 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
• 1.0 2.3nm
• 0.5 4.6nm
• 0.1 23nm
• 0.01
  230nm
• 3. Sampling Volume Limitations (Escape volume)

NAnsin?
? 500nm
dmin 0.61l/NA for OM
http//www.youtube.com/watch?vSVK4OkUK0Yw at147
-307
Cs coefficient of spherical aberration of lens
(mm)
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 1024768 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.

PD/Mag 100um/Mag
Mag P(?m) Mag P(nm) 10x 10 10kx 10
1kx 0.1 100kx 1
P-pixel diameter on specimen surface D-pixel
diameter on CRT, Mag-magnification
51
Resolution of Images II

• The optimum condition for imaging is when the
  escape volume of the signal concerned equals to
  the pixel size.

52
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)

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
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 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
55
Resolution of Images
The resolution is the pixel diameter on specimen
surface.
The optimum condition for imaging is when the
escape volume of the signal concerned equals to
the pixel size.
Effect of probe size on escape volume of SE
e-
10nm
BSE
X-ray
PD/Mag 100um/Mag
1?m
5?m
56
Depth of Field
Depth of Field
4x105W
D (?m)
AM
A/2 ?
To increase D Decrease aperture size, A Decrease
magnification, M Increase working distance, W (mm)
OM
?5x10-30.005
??72o1.26
(1o0.0175)
57
Image 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.
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
58
SE Images - Topographic Contrast
1mm
Defect in a semiconductor device
Molybdenumtrioxide crystals
The debris shown here is an oxide fiber got stuck
at a semiconductor device detected by SEM
59
BSE Image Atomic Number Contrast
2?m
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
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
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.
62
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.
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.

1. Te1017cm-3, dark-dot dislocation contrast
2. Te1018cm-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
Study
http//science.howstuffworks.com/scanning-electron
-microscope.htm/printable

• 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)