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International Journal of IT, Engineering and Applied Sciences Research (IJIEASR)
Volume 4, No. 7, July 2015
ISSN: 2319-4413
Scanning Electron Microscope
Ramanjeet Kaur, Assistant Professor, Physics Department, R.S.D.College, Firozpur (Punjab) India
ABSTRACT
Two types of microscopes are used for microscopic study
of materials, Light microscopes and Electron microscope.
To overcome the limitations possessed by Light
Microscopes, Electron microscopes were developed. These
microscopes use a beam of energetic electrons to view
objects on a very fine scale. In this paper Scanning
Electron Microscope has been discussed. The main
components of SEM, beam specimen interaction, various
signals produced and their detection has been studied in
detail. The effect of various operating parameters in SEM
on the quality and resolution of image has been pointed
out. The limitations in SEM Imaging, are Contamination
and Charging of the sample used.
Keywords
Microscope, transmission, scanning, electron, electrongun
1. INTRODUCTION
Electron microscopes are scientific instruments that use a
beam of energetic electrons to examine objects on a very
fine scale. Electron microscopes were developed due to
the limitations of Light Microscopes which are limited by
the physics of light. In the early 1930's this theoretical
limit had been reached and there was a scientific desire to
see the fine details of the interior structures of organic
cells (nucleus, mitochondria...etc.).This required 10,000x
plus magnification which was not possible using current
optical microscopes. Electron microscopes are capable of
much higher magnifications and have a greater resolving
power than a light microscope, allowing it to see much
smaller objects in finer detail [1].
2. COMPARISON OF OPTICAL AND
ELECTRON MICROSCOPE
A. Optical Microscope
An optical microscope is in principle nothing else than a
simple lens system for magnifying small objects. The first
lens, called the objective, has a short focal length (a few
mm), and creates an image of the object in the
intermediate image plane. This image can be looked at
with another lens, the eye-piece, which can provide further
magnification.
The resolution of the image is limited by diffraction. The
Abbe-Rayleigh criterion states that, for a wavelength λ ,
the smallest distance dmin between two point sources such
that two can be resolved:
dmin = 1.22 x λ/ 2 ΝΑ,
where NA = n X sinα is called numerical aperture of the
objective lens. Where n is the index of refraction in the
object space, and α half the maximal angle under which
the objective lens collects light from the object.
Advantages and Disadvantages of Optical Microscope
Advantages:
• Direct imaging with no need of sample pretreatment, the only microscopy for real color
imaging.
• Fast and adaptable to all kinds of sample systems,
from gas, to liquid, and to solid sample systems, in
any shapes or geometries.
• Easy to be integrated with digital camera systems
for data storage and analysis.
Disadvantages:
• Low resolution, usually up to sub-micron or a few
hundreds of nanometers, mainly due to the light
diffraction limit.
B. Transmission Electron Microscope (TEM)
The transmission electron microscope (TEM) was the first
type of Electron Microscope to be developed and is
patterned sane as the light transmission microscope except
that a focused beam of electrons is used instead of light to
"see through" the specimen. It was developed by Max
Knoll and Ernst Ruska in Germany in 1931.
In transmission electron microscopy (TEM), a beam of
highly focused electrons is directed toward a thinned
sample (<200 nm). Normally no scanning required helps
the high resolution, compared to SEM.
These highly energetic incident electrons interact with the
atoms in the sample producing characteristic radiations
and particles providing information for materials
characterization.
Information is obtained from both deflected and nondeflected transmitted electrons, backscattered and
secondary electrons, and emitted photons.
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composition, and crystalline structure and orientation of
materials making up the sample [3].
Fig. 1 Overall design of optical microscope, a
transmission electron microscope and a scanning
electron microscope [2]
Advantages and Disadvantages of TEM Advantages:
Advantages:
• High resolution, as small as 0.2 nm.
• Direct imaging of crystalline lattice.
• Delineate the defects inside the sample.
• No metallic stain-coating needed, thus convenient
for structural imaging of organic materials,
Electron diffraction technique is used for phase
identification, structure and symmetry determination,
lattice parameter measurement, disorder and defect
identification.
Disadvantages:
• To prepare an electron-transparent sample from the
bulk is difficult (due to the conductivity or electron
density, and sample thickness).
Advantages and disadvantages of SEM:
Advantages:
• Almost all kinds of samples, conducting and nonconducting (stain coating needed) can be studied
with SEM.
• It is based on surface interaction so there is no
requirement of electron-transparent sample.
• With the help of this, imaging at all directions
through x-y-z (3D) rotation of sample is possible.
• The SEM has a large depth of field than OM, 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. It
has high magnification than OM.
The combination of higher magnification, larger depth of
field,
greater
resolution,
compositional
and
crystallographic information makes the SEM one of the
most heavily used instruments in academic and national
lab research areas and industry.
Disadvantages:
• Low resolution, usually above a few tens of
nanometers.
• Usually required surface stain-coating with metals
for electron conducting.
3. DIFFERENT COMPONENTS OF SEM
The main components of a typical SEM are electron
column, scanning system, detectors, display, vacuum
system and electronics controls (fig. 2).
C. Scanning Electron Microscope
The first scanning electron microscope (SEM) was
constructed in 1938 (Von Ardenne) with the first
commercial instruments around 1965. Its late development
was due to the electronics involved in "scanning" the beam
of electrons across the sample.
The scanning electron microscope (SEM) uses a focused
beam of high-energy electrons to generate a variety of
signals at the surface of solid specimens. Several
interactions with the sample that result in the emission of
electrons or photons occur as the electrons penetrate the
surface. These emitted particles can be collected with the
appropriate detector to yield valuable information about
the material. The most immediate result of observation in
the scanning electron microscope is that it displays the
shape of the sample. The resolution is determined by the
beam diameter. The signals that derive from electronsample interactions reveal information about the sample
including external morphology (texture), chemical
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Fig.2 Overall design of SEM [5]
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Volume 4, No. 7, July 2015
A. Electron Guns:
The purpose of the electron gun is to produce a stable
beam of electrons of adjustable energy. There are three
main types of electron guns:
Tungsten hairpin, Lanthanum hexaboride (LaB6) and Field
emission.
Field emission gun:
FEG cathode consists of a sharp metal usually Tungsten
tip with a radius of less than 100 nm. A potential
difference (V1= extraction voltage) is established between
the first anode and the tip. The result is an electric field,
concentrated at the tip, which facilitates electron emission.
The potential difference between the tip and the second
grounded anode determines the accelerating voltage. The
higher the accelerating voltage the faster the electrons
travel down the column and the more penetrating power
they have.
There are two types of FEGs: Cold and thermally assisted
Field emission guns. Both types of field emission require
that the tip remain free of contaminants and oxide and thus
they require Ultra High Vacuum condition (10-10to 10-11
Torr ). In the cold FEG the electric field produce by the
extraction voltage lowers the work function barrier and
allows electrons to directly tunnel through it and thus it
facilitates emission. The cold FEGS must have their tip
flashed (briefly heated) periodically to free absorbed gas
molecules. The thermally assisted FEG (Schottky field
emitter) uses heat and nitride coating in addition to voltage
to overcome the potential barrier level.
Although the FEG has a moderate emission current, its
Brightness value is orders of magnitude greater than the
thermionic Tungsten and LaB6sources. It is found that
brightness is 105, 106, 108 and 108 respectively for
Tungsten, LaB6, thermal and cold FEG.
Brightness is the beam current per unit area per solid angle
2
[Β=4i /(π d α ) ] and, unlike current, it is conserved down
p
p p
the column. Brightness increases linearly with accelerating
voltage (V0) of the gun. The higher the accelerating
voltage the faster the electrons travel down the column and
the more penetrating power they have.
The high brightness is due to small source size as the beam
exits the gun. This Source Size for FEGs is of the order of
nanometers rather than microns for the other emission
sources. The ability to have enough probe current (and
thus potential signal) in a probe of small diameter allows
the FEGSEM to obtain the resolution it does. The ability
to achieve a small probe diameter is directly related to the
source size or the diameter of the electron beam exiting
the gun. An electron beam emanating from a small source
size is said to have high spatial coherency. Electron beams
can also be characterized in terms of temporal coherency.
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A beam with high temporal coherency will have electrons
of the same wavelength. In reality there is a certain energy
spread associated with the beam. It is found that lower
energy spreads result in better resolution and are
particularly important in low accelerating voltage imaging.
These enhanced properties of FEG come with cost. To
achieve the vacuum levels they require very expensive
vacuum systems must be attached to these microscopes.
The advantages of coherent beam source will be negated if
the beam is interacting with molecules on its path down
the column. The vacuum at the gun level of the column is
kept at 10-10 to 10-11 Torr and the vacuum in the
specimen chamber is in the 10-5 to 10-6 Torr range [4].
B. Lenses in SEM
The purpose of the electron lenses is to produce a
convergent electron beam with desired crossover diameter.
The lenses are metal cylinders with cylindrical hole, which
operate in vacuum. Inside the lenses magnetic field is
generated, which in turn is varied to focus or defocus the
electron beam passing through the hole of the lens.
Condenser lenses
SEMs employ one to three condenser lenses to de-magnify
the electron beam crossover diameter in the electron gun
to a smaller size. The first and second condenser lenses
control the amount of demagnification. After the beam
passes the anode it is influenced by two condenser lenses
that cause the beam to converge and pass through a focal
point. What occurs is that the electron beam is essentially
focused down to 1000 times its original size. In
conjunction with the selected accelerating voltage the
condenser lenses are primarily responsible for determining
the intensity of the electron beam when it strikes the
specimen [5].
The Objective Lens
The final lens in the column called objective lens focuses
the probe on the sample. The design of the lenses often
incorporates space for the scanning coils, the stigmator
and the beam limiting aperture. Since the high currents
flowing through the lenses generate excessive heat, they
are cooled usually by circulating water.
The objective lens controls the final focus of the electron
beam by changing the magnetic field strength. The crossover image is finally de-magnified to a ~ 10nm beam spot
which carries a beam current of approximately 10-9 - 10-13
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 and hence focusing
changes.
There are three basic designs of the final (objective) lens:
Pinhole or conical lens:
In this lens the specimen is outside the lens and its
magnetic field. Typical focal lengths for the pinhole final
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Volume 4, No. 7, July 2015
lens are between 5 to 40 mm. These have two main
operational advantages. First, specimen size is limited by
the size of the specimen chamber and not by the lens.
Second the variable working distance allows control over
the depth of field and size of the imaged area.
Immersion lens:
This lens is used for specimen small enough of the order
of mm to be placed inside the lens. The immersion lens
has a very short focal distance range of 2 to 5 mm. A small
specimen is placed directly inside the lens gap. Because
lens aberrations scale with focal distance, this type of lens
yields the lowest aberrations, the smallest probe size, and
the highest image resolution. Secondary electrons spiral
upward in the strong magnetic field of the lens and are
collected by the detector positioned above the lens. This
lens generally is not suitable for detecting BSE.
Snorkel lens:
In this lens the specimen is outside the lens but inside its
magnetic field. The snorkel lens combines the best
features of both the pinhole and the immersion lenses. Its
magnetic field extends outside the lens and reaches the
specimen. This design puts restrictions for imaging of
magnetic specimens, which could be drawn inside lens
with negative consequences for the samples and the SEM.
The advantage is that it has low aberrations, and the
sample size is not limited by the lens gap. The aberrations
of the final lens and consequently the resolution are
controlled by a final lens aperture, which affects the beam
convergence angle. The final lens aperture has three
important effects on the final probe. First there is an
optimum aperture angle that minimizes aberrations.
Second, the current in the final probe is controlled by the
size of the aperture. And third, the probe convergence
angle α controls the depth of field. Smaller the angle α
greater is the depth of field. Spot size in SEM is
minimized at the expense of current.
The general approach in SEM is to minimize the probe
diameter and maximize the probe current. The minimum
probe diameter depends on the spherical aberration of the
SEM electron optics, the gun source size, the electron
optical brightness and the accelerating voltage.
The probe size, which directly effects resolution, can be
decreased by increasing the brightness. The electron
optical brightness β is a parameter that is function of the
electron gun performance and design. For all types of
electron guns, brightness increases linearly with
accelerating voltage, so every electron source is 10 times
as bright at 10 kV as it is at 1 kV. Decreasing the
wavelength and the spherical aberration also decreases the
probe size. The maximum beam current depends on
several parameters. The brightness can be increased by
increasing the accelerating voltage, the limiting tendency
here is that increasing the accelerating voltage too much
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increases the volume of generation of X-rays and thus
limits resolution. As a consequence increasing the
accelerating voltage can be done successfully up to about
30 kV, above that the resolution is effected negatively.
Decreasing the spherical aberration is another way of
increasing the maximum current. This is achieved more
successfully in the immersion type lenses. The above
considerations for maximum probe current and minimum
probe diameter ignore the contribution of chromatic
aberrations. These cannot be ignored when operating at
low accelerating voltages in the range of 2 kV or less. The
dependence of the probe current becomes much more
complex due to the chromatic aberration term.
4. BEAM SPECIMEN INTERACTION AND
VARIOUS SIGNALS
A. Interaction Volume
The combined effect of the elastic and inelastic
interactions is to distribute the beam electrons over a
three-dimensional “interaction volume” [6]. The concept
of interaction volume of the primary beam electrons and
the sampling volume of the emitted secondary radiation
are important both in interpretation of SEM images and in
the proper application of quantitative X-ray microanalysis.
The image details and resolution in the SEM are
determined not by the size of the electron probe by itself
but rather by the size and characteristics of the interaction
volume. When the accelerated beam electrons strike a
specimen they penetrate inside it to depths of about 1 μm
and interact both elastically and in elastically with the
solid, forming a limiting interaction volume from which
various types of radiation emerge, including BSE, SE,
characteristic and brehmsstrahlung x-rays, and cathodeluminescence in some materials. The combined effect of
elastic and inelastic scattering controls the penetration of
the electron beam into the solid. The resulting region over
which the incident electrons interact with the sample is
known as interaction volume.
The interaction volume has several important
characteristics, which determine the nature of imaging in
the SEM. The energy deposition rate varies rapidly
throughout the interaction volume, being greatest near the
beam impact point.
The interaction volume has a distinct shape. For lowatomic-number target it has distinct pear shape. For
intermediate and high-atomic number materials the shape
is in the form of hemi-sphere.
The interaction volume increases with increasing incident
beam energy and decreases with increasing average atomic
number of the specimen. Ultimately the resolution in the
SEM is controlled by the size of the interaction volume.
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B. Basic signals of SEM
The electrons interact with the atoms at or close to sample
surface and produce signals that contain information about
the sample's surface topography, composition, and other
properties such as electrical conductivity. The types of
signals produced by an SEM include secondary electrons,
back-scattered electrons (BSE), characteristic X-rays, light
(cathodo-luminescence), specimen current and transmitted
electrons as shown in figure 2.
Secondary electrons:
SEs are generated by 3 different mechanisms [6]: • SE(I)
are produced by interactions of electrons from the incident
beam with specimen atoms. These SEs are produced in
close proximity to the incident beam and thus represent a
high lateral resolution signal. • SE(II) are produced by
interactions of high energy BSEs with specimen atoms.
Both lateral and depth distribution characteristics of BSEs
are found in the SE(II) signal and thus it is a
comparatively low resolution signal. SE(III) are produced
by high energy BSEs which strike the pole pieces and
other solid objects within the specimen chamber. Images
produced with the SE signal will reveal something termed
the “edge effect” [7] Edges and ridges of the sample emit
more SEs and thus appear brighter in the image.
Detection
Secondary electrons are having low energy. These can be
easily collected by placing a positive voltage (100 - 300V)
on the front of the detector. The type of detector used is
called photomultiplier tube.
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attract them would also attract the incident beam. The
most common detector used is called a surface barrier
detector. It sits above the sample, below the objective lens.
BSE which strike it are detected.
Surface barrier detectors are solid state devices made up of
semiconducting materials. A semiconducting material has
a filled valence band and an empty conduction band
similar to ceramic materials. When a BSE electron strikes
the detector, electrons in the material move from valence
to conduction band. The electrons are now free to move in
the conduction band or drop back into the valence band. If
a potential is applied, the e- and e+ can be separated,
collected and the current is measured. The strength of the
current is proportional to the number of BSE that hit the
detector.
Characteristic X-rays:
These are emitted when the electron beam removes an
inner shell electron from the sample, causing a higher
energy electron to fill the shell and release energy. These
characteristic X-rays are used to identify the composition
and measure the abundance of elements in the sample.
For secondary electrons the sampling depth is from 10 to
100 nm and diameter equals the diameter of the area
emitting backscattered electrons. BSE are emitted from
much larger depths compared to SE.
Secondary electrons (SE) are accelerated to the front of the
detector by a bias voltage of 100 - 500 eV. They are then
accelerated to the scintillator by a bias of 6- 12 KeV, (10
KeV is normal). Scintillator is doped plastic or glass
covered with a fluorescent material. A thin (700Å) layer of
Al covers it to prevent light from causing fluorescence.
The 10keV potential allows the SE to get through the Al
and fluoresce. The light photons travel down the tube
(guide) to a photocathode which converts them into
electrons. The electrons move through the detector,
producing more electrons as they strike dynodes. An
output electron pulse is then detected.
Back-scattered electrons (BSE):
These are beam electrons that are reflected from the
sample by elastic scattering. BSE are often used in
analytical SEM along with the spectra made from the
characteristic X-rays. Because the intensity of the BSE
signal is strongly related to the atomic number (Z) of the
specimen, BSE images can provide information about the
distribution of different elements in the sample.
Detection
Since BSE have high energies, they can’t be pulled in like
secondary electrons. Because a potential grid used to
Fig.3 Production of various signals from the sample [8]
5. IMAGE FORMATION
One of the most surprising aspects of scanning electron
microcopy is the apparent ease with which SEM images of
three-dimensional objects can be interpreted by any
observer with no prior knowledge of the instrument. This
is somewhat surprising in view of the unusual way in
which image is formed, which seems to differ greatly from
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normal human experience with images formed by light
and viewed by the eye. The SEM image is a 2D intensity
map in the analog or digital domain. Each image pixel on
the display corresponds to a point on the sample, which is
proportional to the signal intensity captured by the
detector at each specific point. Unlike optical or
transmission electron microscopes no true image exists in
the SEM. It is not possible to place a film anywhere in the
SEM and record an image. It does not exist. The image is
generated and displayed electronically. The images in the
SEM are formed by electronic synthesis, no optical
transformation takes place, and no real of virtual optical
images are produced in the SEM. In an analog scanning
system, the beam is moved continuously; with a rapid scan
along the X-axis (line scan) supplemented by a stepwise
slow scan along the Y-axis at predefined number of lines.
The time for scanning a single line multiplied by the
number of lines in a frame gives the frame time. In digital
scanning systems, only discrete beam locations are
allowed. The beam is positioned in a particular location
remains there for a fixed time, called dwell time, and then
it is moved to the next point. When the beam is focused on
the specimen, an analog signal intensity is measured by
the detector. The voltage signal produced by the detector’s
amplifier is digitized and stored as discrete numerical
value in the corresponding computer registry. Typically,
the intensity is digitized into 8 bits (256 levels), 12 bits
(4096) or 16 bits (65,536). The digital image is viewed by
converting the numerical values stored in the computer
memory into an analog signal for display on a monitor.
6. VARIOUS OPERATING PARAMETERS
IN SEM
A. Accelerating voltage:
The accelerating voltage can be varied by the operator
from < 1 kV to 30 kV. Increasing accelerating voltage
will:
• It decreases lens aberrations. The result is a smaller
probe diameter (when considering it alone) and thus
better resolution.
• It increases the probe current at the specimen. A
minimum probe current is necessary to obtain an
image with good contrast and a high signal to noise
ratio.
• It potentially increase charge-up and damage in
specimens that are non-conductive and beam
sensitive. For imaging polymers and ceramics,
voltages below 10 kV should be used [9].
B. Probe Diameter
The probe diameter or spot size can be varied on
FEGSEM by altering current to a condenser lens.
Decreasing the probe diameter will:
•
•
•
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enable greater resolution. Resolving small specimen
features require probe diameters of similar
dimensions.
decrease lens aberration due to a stronger lens
setting.
decrease probe current.
C. Emission current:
Increase of the emission current has following effects:
• It increases the probe current at the specimen.
• Potentially increase charge up and damage in
specimens that are nonconductive and beam
sensitive.
D. Depth of Field
The height over which a sample can be clearly focused is
called the Depth of Field. The SEM has a large depth of
field which produces the images that appear 3-dimensional
in nature.
Depth of field is improved by:
• Longer working distance
• Smaller objective apertures
•
Lower magnifications
E. Objective aperture size:
The objective apertures on FEGSEM have a range of sizes
that can be selected. Decreasing the diameter of the
aperture will:
• decrease lens aberrations and thus increase
resolution.
• decrease the probe current.
• decrease the convergence angle of the beam and
thus increase depth of focus.
F. Working distance: The working distance is adjustable
on FEGSEM. Increasing the working distance will:
• increase depth of focus.
• increase probe size and thus decrease resolution.
• increase the effects of stray magnetic fields and
thus decrease resolution.
• increase aberrations due to the need for a weaker
lens to focus.
7. DEFECTS IN SEM IMAGING
A. Contamination
The term contamination describes the collective
phenomena by which the surface of a specimen undergoes
deposition of a foreign substance, generally a
carbonaceous material derived from the breakdown of
hydrocarbon. The effect of this surface deposit is generally
observed as a “scan square” when the magnification is
reduced while centered on the previous field of view.
Contrast arises in such image because of the change in the
SE coefficient caused by the deposition of foreign
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material. This can be avoided by starting imaging at low
magnifications and gradually increasing it.
Contamination can affect the signal at high resolution
imaging. Hydrocarbon molecules are attracted to the beam
location and form build-up, which obscures the fine detail
features. In addition, contamination can be a serious
limitation to voltage microscopy [10].
Contamination can be reduced in first place by improving
the vacuum in the specimen chamber. Also anticontamination devices can be employed, which cool area
in the close vicinity of the specimen so that hydrocarbon
migration is reduced. This can be achieved also by cooling
directly the sample. Another solution is exposure of the
specimen to an intense ultraviolet light prior to SEM
imaging. The UV light can crosslink a surface
contamination layer and fix it at its place, so later build-up
is minimized.
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8. CONCLUSION
Electron microscopes are preferred to Light Microscopes
because of their high power of magnification and
resolution. SEM uses electrons in the place of light beam
for microscopic view. The main components of a typical
SEM are electron column, scanning system, detectors,
display, vacuum system and electronics controls. The
beam interacts with the specimen to produce various
signals which include secondary electrons, back-scattered
electrons, characteristic X-rays, specimen current and
transmitted electrons. The operating parameters like
accelerating voltage, depth of field, probe diameter,
emission current, objective aperture size and working
distance effect the image quality, resolution and focusing
of the image. The samples used in SEM Imaging get
defected due to contamination and charging.
REFERENCES
B. Charging
When the beam interacts with the sample part of the
incident electrons are emitted back, but larger fraction
remains in the specimen as the beam electrons lose their
initial energy and are captured by the specimen. This
charge flows to ground if the specimen is conductor and a
suitable connection exists to conduct away the charges. If
the ground path is broken, even conducting specimen
quickly accumulates charge and its surface potential rises.
Charging is more often observed whenever a specimen or
portion of it is an insulator. The charges injected by the
beam cannot readily flow to ground. The resulting
accumulation of charge is a complex, dynamic
phenomenon. The specimen is in a continually changing
state of surface potential due to the accumulation and
discharge of electrons.
Charging manifests itself in images in a variety of ways.
When local charging alters the surface potential, the field
lines of the detector potential, which exist around the
sample, are disrupted, and collection of SE is greatly
altered. A contrast mechanism develops known as voltage
contrast in which the potential distribution across the
surface is imaged. Some areas appear extremely bright
because they are negative relative to the E-T detector and
enhance SE collection. Other areas appear black because
they charge positively and suppress the collection of SE.
The difficulty arises because the contrast due to surface
potential becomes so large that it overwhelms the contrast
from the true features of the sample.
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[8] C.E. Lyman, D.E. Newbury, J.I. Goldstein, D.B.
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[9] http://www.jeol.com/sem/docs/sem_guide/tbcontd.
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[10] JEOL:
A Guide to Scanning Microscope
Observation
[11] http://cfamm.ucr.edu/documents/sem-intro.pdf
Effects due to charging can be minimized by coating the
sample with conductive film, by utilizing rapid scanning,
by imaging with BSE, and low accelerating voltage [11].
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