It is an Microscopic technique that uses an electron as a source for visualizing microscopic parts and has a more resolution than optical microscope.

1. SCANNİNG ELECTRONMİCROSCOPY
Prepared by
SHINDE GANESH SHASHIKANT
Asst. professor
Pravara Rural college of Pharmacy , Loni(maharashtra)

2. Electron Microscopy

3. Scanning Electron Microscopy (SEM)
•What is SEM
•Working principles of SEM
•Major components and their functions
•Application

4. History
Zworykin et al. 1942, first SEM for bulk samples
1965 first commercial SEM by Cambridge
Scientific Instruments

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.
The SEM is
designed for direct
studying of the
surfaces of solid
objects
Cost: $0.8-2.4M
Column
Sample
Chamber
TV Screens

6. Scanning Electron Microscope
a Totally Different Imaging Concept
• 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.

7. Advantages of Using SEM over OM
Magnification Depth of Field Resolution
OLD 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.

8. Optical Microscopy vs Scanning
Electron Microscopy
25mm
OM SEM
Small depth of field
Low resolution
Large depth of field
High resolution
radiolarian
http://www.mse.iastate.edu/microscopy/

9. Scheme of electron-
matter interactions
arising
from the impact of an
electron beam onto a
specimen.
A signal below the
specimen is observable if
the
thickness is small enough
to allow some electrons to
pass through

10. Electron Beam and
Specimen Interactions

11. Signals from the sample
Incoming electrons
Secondary electrons
Backscattered
electrons
Auger electrons
X-rays
Cathodo-
luminescence (light)
Sample

12. The electron beams
 The types of signals produced by a SEM include
- secondary electrons,
- back-scattered electrons (BSE),
- X-rays,
- light rays (cathodoluminescence),
- Elastic Electron
- Inelastic Electron
- A standard SEM uses Secondary electrons & Back
scattered electrons

13. Elastic Electron Interactions
no energy is transferred from the electron to the
sample.
These signals are mainly exploited in
- Transmission Electron Microscopy and
- Electron diffraction methods.

14. Inelastic Electron Interactions
- Energy is transferred from the electrons to the
specimen
- The energy transferred can cause different signals such
as
- X-rays,
- Auger electrons
- secondary electrons,
- UV quanta or cathodoluminescence.
Used in Analytical Electron Microscopy … SEM

15. 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“(dislodged electron).
Each incident electron can produce
several secondary electrons.
Primary
SE decreases with increasing beam energy and increases with
decreasing glancing angle of incident beam
SE increases as primary electron energy increases and vice versa
upto certain level.
 This SE are attracted by detector & then transmittted as a signal
which amplified into images

16. Backscattered Electrons (BSE)
or reflected electron
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 have more energy than SE and shows emission 50eV and has a
definate direction and used to distinguish image from each other on
basis of atomic number
Primary

17. 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

18. 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

19. Principle
Incoming electrons
Secondary electrons
Backscattered
electrons
Auger electrons
X-rays
Cathodo-
luminescence (light)
Sample

20.  The SEM uses electrons instead of light to form an
image.
 A beam of electrons is produced at the top of the
microscope by heating of a metallic filament.
 The electron beam follows a vertical path through
the column of the microscope. It makes its way through
electromagnetic lenses which focus and direct the
beam down towards the sample.
 Once it hits the sample, other electrons
( backscattered or secondary ) are ejected from the
sample. Detectors collect the secondary or
backscattered electrons, and convert them to a signal
that is sent to a viewing screen similar to the one in an
ordinary television, producing an image.

21. How do we get an image?
156 electrons!
Image
Detector
Electron gun
288 electrons!

22. 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
Image Formation in SEM
M = c/x
c-length of CRT scan
x-length of e- beam scan

23. Components of the instrument
• electron gun (filament)
• condensers lens
•Objective lens
• scan coils
• sample stage
• detectors
• vacuum system
• computer hardware and
software (not trivial!!)

25. A Look Inside the Column
Column

26. 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

27. Electron guns
We want many electrons per
time unit per area (high current
density) and as small electron
spot as possible
Traditional guns: thermionic
electron gun (electrons are
emitted when a solid is heated)
W-wire, LaB6-crystal
Modern: field emission guns
(FEG) (cold guns, a strong
electric field is used to extract
electrons)
Single crystal of W, etched to a thin
tip

28. Electron beam Source
W or LaB6 Filament
Thermionic or Field Emission Gun

29. Electron guns
With field emission guns we get a smaller spot
and higher current densities compared to
thermionic guns
Vacuum requirements are tougher for a field
emission guns
Single crystal of LaB6
Tungsten wire Field emission tip

30. Thermionic Emission Gun
A tungsten filament heated
by DC to approximately
2700K or LaB6 rod heated
to around 2000K
oxidation of the filament
Electrons “boil off” from
the tip of the filament
Electrons are accelerated
by an acceleration voltage
of 1-50kV
-
+

31. 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

32. Source of Electrons
T: ~1500oC
Thermionic Gun
W and LaB6
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
(5-50mm)
E Cold- and thermal FEG
(5nm)
Filament
W
Brightness – beam current density per unit solid angle

33. 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.

34. 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.

35. 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- 10-
12 A. By changing the
current in the objective
lens, the magnetic field
strength changes and
therefore the focal length
of the objective lens is
changed.

36. The Objective Lens – The 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
Wide disc of
least confusion
Narrow disc of
least confusion
Large beam diameter
striking specimen
Small beam diameter
striking specimen

37. 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

38. 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.

39. Detectors
Image: Anders W. B. Skilbred, UiO
Secondary electron detector:
(Everhart-Thornley)
Backscattered
electron detector:
(Solid-State
Detector)

40. OUR TRADITIONAL DETECTORS
 SECONDARY ELECTRONS: EVERHART-THORNLEY
DETECTOR
 BACKSCATTERED ELECTRONS: SOLID STATE DETECTOR
 X-RAYS: ENERGY DISPERSIVE SPECTROMETER (EDS)

41. sample preparation
 Chemical fixation with Gluteraldehyde, optionally with OsO4 – for soft
tissues
 No fixation needed for dry specimen like bones, feathers etc
 The dry specimen is mounted on a specimen stub using epoxy resin
 ultrathin coating done by low-vacuum sputter coating or by high-vacuum
evaporation.
 Conductive materials in current use for specimen coating include gold,
gold/palladium alloy, platinum, osmium,[12] iridium, tungsten,
chromium, and graphite.

42. Salient features
Electrons are used to create images of the surface of
specimen - topology
Resolution of objects of nearly 1 nm
Magnification upto 500000 x (250 times > light
microcopes)
secondary electrons (SE), backscattered electrons
(BSE) are utilized for imaging
specimens can be observed in high vacuum, low
vacuum
In Environmental SEM specimens can be observed in
wet condition.
Gives 3D views of the exteriors of the objects like
cells, microbes or surfaces