Introduction to scanning electron microscopy
Tutor: Peter Harris
The aims of this course are:
● to introduce the principles of scanning electron microscopy
● to describe the components of the microscope and explain how they work
● to highlight some of the problems which can arise during imaging
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Electron Microscopy Lab
Introduction to scanning electron microscopy
Overview
 Scanning electron microscopy is a technique for achieving high resolution
images of surfaces. It involves scanning a fine beam of electrons over a
specimen and detecting the signals which are emitted.
 The resolution of modern SEMs is of the order of 2 nm. This compares
with a resolution of about 1 m for a conventional optical microscope. The
transmission electron microscope, in which electrons pass through a very
thin sample, has a higher resolution than the SEM (~ 0.1 nm).
 Imaging in the SEM must be carried out under vacuum, as electrons
cannot travel through air. The basic components of the SEM are illustrated
on the next slide.
Units:
1 nm = 10-9 m; 1 m = 10-6 m
Electron Microscopy Lab
Introduction to scanning electron microscopy
Components of the SEM
Electrons emitted by the
gun are accelerated,
typically by 20 kV.
They pass through
condenser and objective
lenses, and then through a
set of scan coils and an
aperture. A scan is
simultaneously generated
on a computer monitor.
Electrons emitted by the
specimen are detected,
amplified and the signal is
then used to produce an
image.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Electron sources
A number of different kinds of electron source are used in SEM. The
microscopes in CfAM use the following sources:
Tungsten thermionic source
This is simply a very fine
tungsten wire, through
which a current is passed.
Characteristics
 Low brightness
 Energy spread ~ 1-2eV
Schottky Field Emission Source
This is a crystal to
which a very high
voltage is applied.
Characteristics
 High brightness
 Energy spread < 0.5 eV
Electron Microscopy Lab
Introduction to scanning electron microscopy
Electromagnetic lenses
All modern SEMs use electromagnetic lenses. These consist of a coil of
copper wires inside iron pole pieces. solenoid of wire together with a
magnetic pole piece that creates and concentrates a magnetic field.
A current through the coils creates a magnetic
field, symbolized by red lines in the diagram
on the left. Electrons close to the centre are
less strongly deflected than those passing
through the lens far from the axis.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Interaction of electrons with specimen
When high energy electrons impinge on the
specimen, a number of signals are generated:
●
Backscattered electrons – these are
high energy electrons which are
scattered out of the specimen, losing
only a small amount of energy.
●
Secondary electrons – these originate in
the specimen itself, and have a much
lower energy than the backscattered
electrons (typically < 50 V).
●
X-rays - These give information about the
elemental composition of the sample.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Information given by secondary and
backscattered electrons
Secondary electrons originate from within a
few nm from the surface. They are therefore
very sensitive to surface structure, and provide
topographic information.
Backscattered electrons originate from much
deeper within the sample (a few m below the
surface), and interact much more strongly with
the sample. They therefore provide
compositional information, but give lower
resolution images.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Detectors: secondary electrons
The detector for secondary electrons is
the Everhart Thornley Detector (ETD).
This consists of a scintillator that emits
photons when hit by high-energy
electrons. The emitted photons are
collected by a lightguide and
transported to a photomultiplier for
detection.
A metal grid known as a Faraday cage
surrounds the scintillator, and is usually
held at a positive potential to attract the
secondary electrons.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Origin of topographic contrast in secondary electron images
Many 2ndary
electrons
escape
Fewer 2ndary
electrons escape
Sample
In order to obtain an image in the SEM, we
must have some variation in the signal from
different parts of the specimen. The yield of
secondary electrons is at a minimum when
the surface of the specimen is perpendicular
to the electron beam. This is because of the
shape of the interaction volume and its
relationship to the surface of the specimen,
as shown here. At regions of the specimen
which are not exactly perpendicular to the
beam, electrons are more likely to be
scattered out of the specimen, rather than
further into the specimen. Hence, such
regions appear bright in the secondary
electron image.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Detectors: backscattered electrons
Backscattered electrons can be detected using an Everhart Thornley detector, by
applying a negative potential to repel the secondaries. However, the collection
efficiency of such an arrangement is low.
As the name suggests, backscattered
electrons are strongly scattered back in
the direction of the incident beam.
Therefore, the detector for these
electrons is generally placed around the
final lens, as shown. Backscattered
electron detectors are usually solid state
devices. The electrons which impinge on
the detector produce electron-hole pairs
which produce a current which can be
amplified.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Resolution in the SEM
 The resolution of the SEM is determined by the size of the incident beam. This can
be reduced by introducing an aperture unto the beam path and by reducing the probe
size using the condenser lens. Note that reducing the probe size using the condenser
lens also reduces the beam current (for an explanation, see Goodhew et al., p. 131).
Therefore, as you reduce the probe size you eventually reach a point where imaging
is impossible. For a typical SEM operating at 20 kV, the minimum usable probe size
is of the order of 1 – 3 nm.
 Resolution also depends on accelerating voltage. This is because higher energy
electrons experience less spherical aberration when they pass through the lenses.
Resolution is also improved by reducing the working distance, up to a certain point.
Beyond that point the lenses may not be able to focus the beam on the sample.
 As already noted, images obtained with backscattered electrons have a lower
resolution than these obtained with secondary electrons, because they originate from
deeper within the specimen.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Depth of field 1
Depth of field is the distance above and below the plane of optimum focus
within which the image is in focus.
In the diagram on the right, d
represents the diameter of the
electron beam at the specimen.
The depth of field is h, since it
makes no difference to the
sharpness of the image if the
object is anywhere within the
range h.
Reducing the angle α increases the depth of field. This can be achieved by
using a smaller aperture or by increasing the working distance.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Depth of field 2
These diagrams
illustrate the
effect of the
convergence
angle α on the
depth of field.
Because of the
geometry of the
imaging system,
scanning electron
microscopes have a
much greater depth
of field than optical
microscopes .
Electron Microscopy Lab
Introduction to scanning electron microscopy
Astigmatism 1
Astigmatism is a problem that is commonly encountered in SEM (and
TEM). It is an aberration of lenses that causes rays in a plane parallel to
the optical axis to be focused at a different focal point from rays in a
plane at 90° to it. The effect of astigmatism is that objects in the image
generally appear “stretched” in one direction, and then in the other
direction as you go through focus.
All electron microscopes are equipped with stigmators, which allow the
user to correct the astigmatism, as shown in the next slide. Properly
corrected astigmatism is essential in achieving high resolution images.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Astigmatism 2
The SEM images shown below left illustrate how astigmatism affects the
image as you go through focus. On the right is shown the image following
correction with the stigmators.
Astigmatic
Corrected
Electron Microscopy Lab
Introduction to scanning electron microscopy
Spherical aberration
Spherical aberration is less important in the actual operation of the SEM
than astigmatism, but it is important to understand what it is.
A lens suffers from
spherical aberration if it
focuses rays more
tightly if they enter it far
from the optic axis than
if they enter closer to
the axis. It therefore
does not produce a
perfect focal point. This
is illustrated in diagram
A on the right.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Chromatic aberration
Like spherical aberration, this is not usually a major problem in SEM,
except in high resolution or low voltage operation. It is caused by a lens
having a different refractive index for different electron energies.
Chromatic aberration is present in all electron lenses, but can be reduced
by minimising the energy spread of the electron source. Field emission
sources have the lowest energy spread.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Charging
When a poorly conducting specimen is imaged in a conventional SEM, the surface
rapidly accumulates charge. This can result in a severely distorted image, as the
incident electrons are repelled by the charged areas of the surface. A typical feature
of charging is that some regions appear extremely bright, as can be seen here.
In order to avoid this problem, specimens can
be covered with a conducting coating of gold or
carbon.
An alternative to coating is to use a “low
vacuum” SEM. This kind of microscope
operates with a small pressure of water vapour
in the chamber. The water molecules become
positively charged and neutralize the negatively
charged regions on the surface.
Electron Microscopy Lab
Introduction to scanning electron microscopy
Further information
 The recommended book for this course is "Electron microscopy
and analysis", by Goodhew, Humphreys and Beanland.
 Links to some useful websites can be found on the “SEM
course” page.
 Peter Harris and other members of EMLab staff will be
happy to answer your questions.
Electron Microscopy Lab