The document provides an overview of scanning electron microscopy (SEM). It discusses the history and development of SEM, describing how early SEMs from the 1940s had lower resolution (~50 nm) and only provided morphological information, while modern SEMs can achieve ~10 nm resolution and also provide analytical capabilities. The document explains how SEM works, including how the electron beam interacts with and penetrates the sample, generating various signals like secondary electrons, backscattered electrons, and X-rays from within the interaction volume. It describes the components of an SEM, such as the electron gun, magnetic lenses for beam focusing, detectors for signals, and how changing microscope parameters can affect resolution and depth of field.

1. Scanning Electron Microscopy
(SEM)

3. 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 ?

4. Electron Beam-Specimen Interaction

5. Electron Beam-Specimen Interaction
Incoming electrons
Secondary electrons
Backscattered
electrons
Auger electrons
X-rays
Cathodo-
luminescence (light)
Sample
Involves both elastic and inelastic interactions

6. 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-

7. 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.

8. 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.

9. 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~1m; depth~1m
•X-ray: from the whole
interaction volume, i.e., ~5m
in diameter and depth

10. 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

11. 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

12. 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

13. 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
SE2

14. 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

15. 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 achieved

16. Backscattered electrons (BSE)
High energy electrons (elastic scattering)
Fewer BSE than SE
We differentiate between BSE1 and BSE2

17. Sample surface
Incoming electrons
BSE2
• Most BSE are of BSE2 type

18. 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 h

19. 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!)

20. 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

21. 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)

22. Principles of SEM

23. 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.

24. 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.)

26. SEM components

27. 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 50m 3.0(eV) 10-5 (Torr)
LaB6 3x106 ~2 5m 1.5 10-6
C-FEG 109 ~5 5nm 0.3 10-10
T-FEG 109 <1 20nm 0.7 10-9
(5-50m)
E: >10MV/cm
(5nm)
Filament
W
Brightness – beam current density per unit solid angle

30. 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.

31. The objective lens

32. The objective lens aperture
Aperture in SEM: either to limit the amount of electrons or enhance contrast

33. Depth of Field

34. Resolution

35. Resolution
Can increase the resolution by:
How these changes affect
the depth of field?

36. Detector and Sample Stage
A detector placed within the column is known as an “in-lens” detector and
produces a very different image compared to a conventionally located detector

37. Secondary Electron Detector
Side Mounted In-Lens

38. Secondary Electron Detector
Side Mounted In-Lens

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

40. Topographical Contrast
Topographic contrast
arises because SE
generation depend on
the angle of incidence
between the beam and
sample.

41. 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