4. A Short History
• V. K. Zworykin et al. 1942, first SEM for bulk samples
• 1965 first commercial SEM by Cambridge Scientific
Instruments
• Resolution at that time ~ 50 nm <-> Today ~ 10 nm
• Morphology only at that time <-> Today analytical
instrument
• Why e microscope ?
7. Interaction Volume: I
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.
e-
8. 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.
9. 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.
10. If the diameter of primary
electron beam is ~5nm
- Dimensions of escape
zone of
• Secondary electron:
diameter~10nm; depth~10nm
• Backscattered electron:
diameter~1μm;
depth~1μm
• X-ray: from the whole
interaction volume, i.e.,
~5μm in diameter and
depth
Escape Volumes of Various Signals
11. Where does the signals come from?
• Diameter of the interaction
volume is larger than the
electron spot
resolution is poorer than
the size of the electron spot
12. Secondary Electrons (SE)
• Generated from the
collision between the incoming
electrons and the loosely bonded
outer electrons
• Low energy electrons (~10-50 eV)
• Only SE generated close to
surface escape (topographic
information is obtained)
• Number of SE is greater than
the number of incoming
electrons
• We differentiate between SE1
and SE2
13. SE1
• The secondary electrons that are
generated by the incoming electron beam
as they enter the surface
• High resolution signal with a resolution
which is only limited by the electron beam
diameter
14. SE2
• The secondary electrons that are generated by the
backscattered electrons that have returned to the surface
after several inelastic scattering events
• SE2 come from a surface area that is bigger than the spot
from the incoming electrons resolution is poorer than for
SE1 exclusively
Sample
surface
Incoming
electrons
SE
2
15. Factors that affect SE emission
1. Work function of the surface
2. Beam energy and beam current
• Electron yield goes through a maximum at low
acc.
voltage, then decreases with increasing acc.
Voltage.
Incident electron energy /
kV
Secondary
electron
yield
16. Factors that affect SE emission
3. Atomic number (Z)
• More SE2 are created
with increasing Z
• The Z-dependence is
more pronounced at
lower beam energies
4.The local curvature of the
surface (the most important
factor)
By detecting SE1 electrons; a resolution of 1-2 nm can be
19. BSE as a function of atomic number
Signal intensity of the BSE increases with Z.
For phases containing more than one element, it is the average atomic
number that determines the backscatter coefficient
20. Factors that affect BSE emission
• Direction of the surface w.r.t. electron
beam
• Average atomic number
• When you want to study differences in
atomic numbers the sample should be as
levelled as possible (sample preparation is
an issue!)
21. X-rays
• Photons not electrons
• Each element has a
fingerprint X-ray signal
• Poorer spatial resolution
than BSE and SE
• Relatively few X-ray
signals are emitted and
the detector is inefficient
relatively long signal
collecting times are
needed
22. X-rays
• Most common spectrometer: EDS
(energy- dispersive spectrometer)
• Signal overlap can be a problem
• We can analyze our sample in different
modes
– spot analysis
– line scan
– chemical concentration map (elemental
mapping)
24. 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.
25. 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.)
28. 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 50m 3.0(eV) 10-5
(Torr)
LaB6 3x106 ~2 5m 1.5
C-FEG 109 ~5 5nm 0.3
T-FEG 109 <1 20nm 0.7
10-6
10-10
10-9
Filament
(5-50μm)
E:
>10MV/cm
(5nm
)
W
Brightness – beam current density per unit solid
angle
29.
30.
31. 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.
42. 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: η=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.
