Scanning Electron Microscopy (SEM) lecture

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SEM
Saurabh Bhargava

Objectives of this lecture
• At the end of the lecture one should be able to
answer the questions related to-
– Basic principles of SEM
– Instrumentation of SEM
– Image generation/ formation by SEM
– Collection & interpretation of images of SEM
– Forensic applications of SEM

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Resolution
• Resolution is defined as the
ability to distinguish two very
small and closely-spaced objects
as separate entities.
• Resolution is best when the
distance separating the two tiny
objects is small.
• Resolution is determined by
certain physical parameters that
include the wavelength of light,
and the light-gathering power of
the objective and condenser
lenses.

Conti..
• A simple mathematical equation defines the
smallest distance (dmin) separating the two
very small objects:
dmin = 1.22 x wavelength
N.A. objective + N.A. condenser

Electron Microscopy
• An electron microscope is a microscope that uses accelerated electrons as a
source of illumination.
• Electron microscopes were developed due to the limitations of Light
Microscopes which are limited by the physics of light.
• In the early 1930's there was a scientific desire to see the fine details of the
interior structures of organic cells (nucleus, mitochondria...etc.).
• This required >10000x magnifications which could not be achieved by simple
light/optical microscopy.
• Because the wavelength of an electron can be up to 100,000 times shorter
than that of visible light photons, the electron microscope has a
higher resolving power than a light microscope and can reveal the structure of
smaller objects.

Historical Aspects of Electron
Microscopy
– The development of a SEM began a few yrs after the
invention of a TEM by Ruska in 1931, but the
commercialization of the SEM required about 30 yrs.
– In 1935, the original prototype of the SEM, which scans the
specimen with an e- beam to obtain an image, was made
by Knoll(Germany).
– In 1942, Zworykin (USA), developed a SEM for observing a
bulk specimens.
– In 1965, Cambridge Scientific Instrument (UK) & JOEL
(Japan) first commercialized SEM individually.

Fig: SEM image of pneumonia pathogen Fig: SEM image of a ‘diatom’

Fig: SEM image of pollen grains Fig: SEM image of house fly compound eye
surface @450x
Fig: SEM image of kidney stone
(Calcium Oxalate Dihydrate crystals)

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Fig: SEM image of the tip of a needle @50x & @750x magnifications

Information Retrieved
• Topography :
– The surface features of an object (hardness, reflectivity...etc.)
• Morphology:
– The shape and size of the particles (ductility, strength, shape ...etc.)
• Composition:
– The elements and compounds that the object is composed of and
the relative amounts of them.
• Crystallographic Information:
– How the atoms are arranged in the object.
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SEM
• Scanning electron microscopy (SEM) uses a
focused electron probe to extract structural
and chemical information point-by-point from
a region of interest in the sample.
• The high spatial resolution of a SEM makes it a
powerful tool to characterise a wide range of
specimens at the nanometre to micrometre
length scales.
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Scanning Electron Microscope (SEM)
• A SEM is a type of electron microscope that
images a sample by scanning it with a high-
energy beam of electrons in a raster scan pattern.
• The electrons interact with the atoms that make
up the sample producing signals that contain
information about the sample's surface
topography, composition, and other properties
such as electrical conductivity.
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What Happens When e- Beam Strikes The
Specimen..?
• When the specimen is bombarded with the electron beam, electrons
are ejected from the atoms of the specimen surface.
• Inelastic scattering, place the atom in the excited state. The excited
atom “wants ” to return to a ground or unexcited state. Hence the
atoms will relax giving off the excess energy.
• X-rays, Cathod luminescence are some of the ways of relaxation for
an excited atom.
• A resulting electron vacancy is filled by an electron from a higher
shell, and an X-ray is emitted to balance the energy difference
between the two electrons.
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Electron Beam & Sample Interactions

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Secondary Electrons Generation
Secondary Electron
Generation
-sample electrons ejected by the
primary beam [green line]
-low energy
-surface detail & topography

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Back Scatter Electron Generation
Backscattered Electron Generation
-SEM-BSE
-primary beam electrons
-high energy
-composition and topography
[specimen atomic number]

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X-Rays Generation
•X-Rays are
produced to
balance the
difference of
energy when
an electron
from outer
shell replaces
the one from
inner shell.

Instrumentation
1. Electron optical system (to
produce electrons)
– Electron gun, condenser lens,
objective lens, deflection coil.
2. Specimen stage (to place the
specimen).
3. Secondary e- detector (to
collect secondary e-).
4. Image display unit.
5. Operating system
– The electron optical system
and a space surrounding the
specimen are kept at vacuum.
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Electron Gun
• It produces an electron beam.
• The electrons are emitted from a
filament (cathode) made of a thin
tungsten wire, (about 0.1mm ) by
heating the filament at high
temperature (about 2800k).
• These e- flow as an e- beam,
towards the metal/anode (positive
voltage).
• If the hole is made at the centre of
the plate the e- beam flows through
this hole.
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Magnetic Lens System
• Condenser lens is a lens placed right below the electron gun and is used
to adjust the width of the electron beam as per requirement.
• The condenser lens controls the intensity of the electron beam reaching
the specimen.
• The objective lens brings the electron beam into focus on the specimen.
• The scanning coils deflect the electron beam horizontally and vertically
over the specimen surface. This is also called rastering.
– A fine e- beam is required for SEM, for better performance.
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Specimen Stage/Chamber
• It receives and supports the
specimen.
• Specimen stage should be able to
perform the horizontal and vertical
movements smoothly.
• The horizontal movement is used
for selection of the field while the
vertical movement is used to
change the image resolution.
8/30/2016 bhargava 22Fig: SEM sample chamber (opened)

Secondary e- Detector
• It is used for detecting the secondary e-
emitted from the specimen.
• The secondary e- are attracted to the
high voltage (10 kV) and then generate
light when they hit the scintillator
(fluorescent substance).
• This light is directed to a photon-
multiplier tube (PMT) through a light
guide.
• The light is converted to the electrons,
and these are amplified as an electric
signal.
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Image Display Unit
• The output signals from the secondary electron detector are
amplified through PMT and then transferred to the display
unit forming a SEM image.
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Vacuum System
• The electron optical system and the specimen chamber must be
kept at a high vacuum of 10-3 to 10-4 Pa.
• High vacuum that minimises scattering of the electron beam before
reaching the specimen.
• This is important as scattering of the electron beam will increase
the probe size and reduce the resolution.
• Thus , these components are evacuated by diffusion pump.
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Working of SEM
• The electron gun produces an electron beam when tungsten wire is
heated by current.
• This beam is accelerated by the anode.
• The beam travels through electromagnetic fields and lenses, which focus
the beam down toward the sample.
• A mechanism of deflection coils guide the beam so that it scans the
surface of the sample in a rectangular frame (raster pattern).
• When the beam touches the surface of the sample, it produces:
– Secondary electrons (SE)
– Back scattered electrons (BSE)
– X - Rays...
• The emitted SE is collected by SED and converted into signal that is sent
to a screen which produces final image.
• Additional detectors collect these X-rays, BSE and produce
corresponding images.
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Secondary Electron (SE) Images
– For routine scanning electron microscope images, secondary
electrons (SE) form the usual image of the surface.
– Secondary electrons are low energy electrons formed by inelastic
scattering and have energy of less than 50eV.
– The low energy of these electrons allows them to be collected easily.
– This is achieved by placing a positively biased grill on the front of the
SE detector.
– The positive grill attracts the negative electrons and they go through it
into the detector. This is the case for the Everhart-Thornley
detector which is most commonly used.
– To increase the yield of SE emitted from the specimen, heavy metals
such as gold or platinum are routinely used to coat specimens (sputter).
An extremely thin layer is applied (~10 nm).
– This coating is applied for two main reasons:
1. Non-conductive specimens are often coated to reduce surface charging that
can block the path of SE and cause distortion of signal level and image form;
and
2. Low atomic number (Z) specimens (e.g. biological samples) are coated
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Detectors
• Secondary electron detector
(SED) –
– Everhart-Thornley Detector
– Due to the low energies of
secondary electrons (SE) (~2 to
50 eV) they are ejected only
from near-surface layers.
– Therefore, secondary electron
imaging (SEI) is ideal for
recording topographical
information.
– To attract (collect) these low-
energy electrons, usually
around +200 to 300V is applied
to the cage at the front end of
the detector.
– A higher kV (7 to 12kV) is
applied inside the cage i.e. to
the scintillator, to accelerate
the electrons into the
scintillator screen.
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SE image formation

Backscattered electron (BSE) images
• Backscattered (BS) electrons are high-energy electrons (>50 eV) ejected back out from the
sample. These BSE are used to produce a different kind of image.
• Such an image uses contrast to tell us about the average atomic number of the sample.
• The higher the average atomic number, the more primary electrons are scattered
(bounced) back out of the sample. This leads to a brighter image for such materials.
• For example, a grain of sand (in a mixture of mineral sand) that is made up of a titanium
mineral looks whiter than a grain made of a silicon material (Ti versus Si). In the image
(next slide), the left picture is taken using backscattered electrons. Here there is a
difference in contrast between the grains labelled Si and Ti whereas in the right image,
taken using secondary electrons, there is no difference in contrast between these grains.
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Backscattered electron detector
(BSED)
• The BSED is mounted below the objective lens pole piece and centred
around the optic axis.
• As the specimen surface is scanned by the incident electron beam,
backscattered electrons (BSE) are generated, the yield of which is controlled
by the topographical, physical and chemical characteristics of the sample.
• Both compositional or topographical backscattered electron images (BEI)
can be recorded depending on the window of electron energies selected for
image formation.
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Light’ region is made up predominantly
of Fe. (i.e. the heaviest element)
‘Grey’ region is made up
predominantly of Ca.
‘Dark’ region is made up predominantly
of Si and Al. (i.e. the lightest elements)

Types
1. Conventional (high vacuum) scanning electron microscopy (SEM)
– This is the most common type of machine.
– It requires a dry, conductive sample (often achieved by applying a thin
layer of metal to the surface with a technique called sputtering).
– The sample must be able to withstand a high vacuum.
2. Variable Pressure or Low Vacuum scanning electron microscopy
(LVSEM)
– This type of machine is basically like a conventional SEM but has the
advantage in low vacuum (LV) mode that the pressure can be adjusted in
the sample chamber.
– This means LVSEM can be used to image the surface of non-conductive
samples (no metal needs to be added to the surface of such samples).
– It is particularly useful for viewing polymers, biological samples, and
museum samples that cannot be changed in any way, particulate samples,
and geological materials.
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3. Cryo-scanning electron microscope (Cryo-SEM)
– Cryo stands for frozen. A cryo-scanning electron microscope is a
conventional SEM that has been fitted with specific equipment that allows
samples to be viewed in the frozen state.
– This is particularly useful for directly viewing hydrated (wet) samples,
delicate biological samples, hydrogels, food, bio-films, foams, fats, and
waxes, suspensions, pharmaceuticals and nano-particles.
4. Environmental scanning electron microscope (ESEM)
– This machine is designed to view a sample in its natural state, without the
need for desiccation.
– Sample temperature and specimen chamber vapour pressure can both be
controlled, allowing samples to be heated, cooled, wetted or dried.
– Imaging of biological processes, for example pollen tube growth in real time
through wetting of pollen.

Applications
• SEM is one of the most versatile technique, and can be used to-
I. Image morphology of samples
II. Image compositional and some bonding differences
III. Examine wet and dry samples while viewing them (only in an ESEM)
IV. View frozen material (in a SEM with a cryostage)
V. Generate X-rays from samples for microanalysis (EDS)
VI. View/map grain orientation/crystallographic orientation and study
related information like heterogeneity
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X- Ray Analysis
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•An incoming high-energy electron
dislodges an inner-shell electron in the
target, leaving a vacancy in the shell
•An outer shell electron then “jumps” to fill
the vacancy
•A characteristic x-ray (equivalent to the
energy change in the “jump”) is generated
or
•The energy released from an electron
replacement event produces a photon with
an energy exactly equal to the drop in
energy.
1. Elemental identification
2. Chemical characterization
3. Quantitative analysis

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•When an electron from a K-shell is replaced by one from the next closest shell (L), it is
designated as a Kα event
•When an electron from a K-shell is replaced by one from the second closest shell (M),
it is designated as a Kβ event

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Lα - When an electron from a L-shell is replaced by one from the next closest shell
(M).
The K shell will never donate its electron as this would require an increase in
energy, not a drop.

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•Certain events such as Mα, Lβ, and Kγ are only possible in atoms of sufficient atomic weight.
•Each element has a family of characteristic X-rays associated with it

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Instrumentation

Gunshot Residue (GSR) Analysis
• GSR is often found on criminals and also on victims if shot at close
range.
• Particles are very characteristic, therefore presence of these particles
forms evidence of firing a gun.
• Particles normally consist of Pb (lead), Sb (antimony) and Ba (barium).
• The proportion of elements present in GSR differ slightly and databases
of GSR from different manufacturers can be used to identify what
ammunition was used in a crime.
• New ammunition: environmentally friendly (no Sb).
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Gunshot Residue (GSR) Analysis
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GSR Analysis on cloth fibre
•Bullet fragments
(blue) can be
identified on cloth
fibres and
distinguished from
other metal pieces
by their elemental
composition

Analysis
• Qualitative Analysis:
– Peak energy gives qualitative information about the constituent elements
• Quantitative Analysis:
– Peak intensity gives information about the element composition to find the
changes in concentration of elements.
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FORENSIC APPLICATIONS
• In forensic science, police laboratories use the SEM to
examine and compare evidence, such as
1. Metal fragments,
2. Paint,
3. Inks,
4. Hair,
5. Fibers
– To prove a person's guilt or innocence. Through careful
examination, detectives are able to determine if samples
collected from a crime scene have properties that match
the scenario developed by detectives.
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Entomological applications
• In the field of biological
science, large objects such as
insects and animal tissues and
small objects such as bacteria
and viruses are studied by
SEM.
• There are many applications
for the SEM in this field and
one of them is entomological
taxonomy.
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Soils & Rocks
• Investigation of soils and geological
samples is common place
in scanning electron microscopy.
• Analysis of morphology can inform
about weathering processes.
• Compositional differences can be
seen with the help of
backscattered electron imaging
(image).
• Microanalysis can provide details
on elemental composition.
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Sample preparation
• The correct preparation is vital to getting good quality
information from your sample. A poorly mounted or
incorrectly processed sample can lead to viewing
artefacts.
• For non-conductive samples that need coating to make
them conductive, consider whether compositional
information is required (e.g. BSE imaging, X-ray
spectroscopy), and use a carbon coating if this is the case.
• Gold or platinum metal coatings will interfere with the
compositional signal from the sample.
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Scanning Electron Microscopy (SEM) lecture

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