Download Scanning Electron Microscope (SEM)

Scanning Electron Microscope
(SEM)
Prof.Dr.Figen KAYA
The scanning electron microscope (SEM) is the most widely
used type of electron microscope.
It examines microscopic structure by scanning the surface of
materials, similar to scanning confocal microscopes but with
much higher resolution and much greater depth of field.
Scanning Electron Microscope (SEM)
• An SEM image is formed by a focused electron
beam that scans over the surface area of a
specimen.
Scanning Electron Microscope (SEM)
The most important feature of an SEM is the
three-dimensional appearance of its images
because of its large depth of field.
For example, the depth of field can reach the
~10-90 micrometers at 103 × magnification and
~1-9 micrometers at 104 × magnification.
Depth of Field in SEM
Scanning Electron Microscope (SEM)
An SEM is relatively easily operated and
maintained, compared with a TEM (Transmission
Electron Microscope).
In addition, an SEM system enables us to
obtain chemical information from a specimen
by using various techniques, including by
equipping the X-ray energy dispersive
spectrometer (EDS) or X-ray Wavelenght
dispersive spemctrometer (WDS).
Instrumentation
Optical Arrangement:
A scanning electron microscope consists of an
electron gun and a series of electromagnetic
lenses and apertures.
The electron beam emitted from an electron
gun is condensed to a fine probe for surface
scanning.
• Optical Arrangement of SEM
Electron Sources
In a SEM system, an electron gun generates a high
energy electron beam for illumination.
In the electron gun, the electrons emitted from a
cathode, a solid surface, are accelerated by high
voltage (Vo) to form a high energy electron beam
with energy
E = e xVo.
Because electron energy determines the wavelength
of electrons and wavelength largely determines
resolution of the microscope, the acceleration
voltage determines the resolution to a large extent.
Acceleration
Voltage (volt)
10
40
50
60
80
100
200
500
Wavelength (nm)
0,0123
0,00601
0,0055
0,00487
0,00418
0,00370
0,00251
0,00142
Electron Gun
The general structure of an electron gun is composed of three
main parts:
1) Cathode or electron source,
2) Wehnelt electrode and
3) Anode.
Electron Gun
Electrons are emitted from the surface of the
cathode and accelerated by an electric field toward
the cathode.
The Wehnelt electrode is placed between the
cathode and the anode.
It is biased a few hundred volts negative with
respect to the cathode in order to stabilize the
electron beam against voltage fluctuation by
reducing the electron beam current whenever
necessary.
Types of Electron Guns
There are two basic types of electron guns:
1) Thermionic emission gun and
2) Field emission gun (FEG).
Thermionic Emission Gun
This is the most common type of electron gun and includes the
A) Tungsten filament gun and the
B) Lanthanum hexaboride (LaB6) gun.
Tungsten filament gun
The tungsten gun uses a tungsten filament as the
cathode. During operation, the filament is heated by
electrical current (filament current) to high temperature
(∼2800 K).
The high temperature provides kinetic energy for
electrons to overcome the surface energy barrier,
enabling the conduction electrons in the filament to leave
the surface.
The electrons leaving the surface are accelerated by a
high electric voltage between the filament and anode to a
high energy level.
The intensity of the electron beam is determined by the
filament temperature and acceleration voltage.
Lanthanum hexaboride gun
Lanthanum hexaboride (LaB6) is a better cathode than
tungsten and it is more widely used in modern electron
microscopes.
Electrons require less kinetic energy to escape from the
LaB6
surface because its surface work function is about 2 eV,
smaller than the 4.5 eV of tungsten.
Thus, a lower cathode temperature is required.
A LaB6 cathode can generate a higher intensity electron
beam and has a longer life than a tungsten filament.
One disadvantage is that a higher level of vacuum is
required for a LaB6 gun than for a tungsten filament gun.
Field Emission Gun (FEG)
This type of electron gun does not need to
provide thermal energy for electrons to
overcome the surface potential barrier of
electrons.
Electrons are pulled out by applying a very high
electric field to a metal surface.
A tunneling effect, instead of a thermionic
effect, makes metal electrons in the conducting
band cross the surface barrier.
• Tunneling effect
Field Emission Gun (FEG)
Field Emission Gun (FEG)
During operation, electrons are physically drawn off
from a very sharp tip of tungsten crystal (∼100 nm
tip radius) by an applied voltage field.
The field emission gun generates the highest
intensity electron beam, 104 times greater than that
of the tungsten filament gun and 102 times greater
than that of the LaB6 gun.
There are two kinds of field emission guns:
A) Termal guns and
B) Cold guns.
Field Emission Gun (FEG)
 The thermal field emission gun works at an elevated
temperature of about 1600–1800 K, lower than that of
thermionic emission.
It provides a stable emission current with less emission noise.
 The cold field emission gun works at room temperature.
It provides a very narrow energy spread (about 0.3–0.5 eV), but
it is very sensitive to ions in air. The bombardment of the
tungsten tip by residual gaseous ions causes instability of the
emission.
Thus, regular maintenance (called flashing) of the tip is
necessary. Generally, a vacuum of 10−10 torr is required to
operate the cold field emission gun.
Optical Arrangement
 A scanning electron microscope consists of an
electron gun and a series of electromagnetic
lenses and apertures as shown in Figure 4.1.
 In an SEM, the electron beam emitted from an
electron gun is condensed to a fine probe for
surface scanning.
 Beam brightness plays an important role in
imaging quality in an SEM.
 The acceleration voltage for generating an
electron beam is in the range 1–40 kV.
Optical Arrangement
An SEM optical path goes through several
electromagnetic lenses;
 Condenser lenses and
 Objective lens.
The electromagnetic lenses in an SEM are for
electron probe formation, not for image
formation.
Electromagnetic Lenses
 Lenses in light microscopes are made of glass;
however, glass lenses cannot be used in electron
microscopes because glass does not deflect or
focus an electron beam.
 Noting that electrons have electric charges and
that electric charges interact with magnetic fields,
we can use a magnetic force to deflect or focus
an electron beam.
 All lenses in an electron microscope are
electromagnetic.
Figure 3.3 illustrates the basic structure of an electromagnetic
lens.
The lens consists of a case and two poles.
The case is made of soft-magnetic material, and encloses a
solenoid. The poles are two pieces located at the narrow
annular opening of the case; they are machined to very
precise tolerances.
Electromagnetic Lenses
Such an arrangement creates a powerful magnetic field
concentrated in the gap between the two pole pieces (N and
S).
Thus, when an electron beam passes through the lens, it is
focused and deflected by the field’s magnetic lines of force.
Since the strength of a magnetic field can be easily
manipulated by altering the electric current that generates
magnetic field, an electromagnetic lens has a special
advantage over a glass lens:
The magnification power of an electromagnetic lens can be
altered by simply changing the current passing through the
solenoid of the lens.
The Condenser and Objective Lenses
 The two condenser lenses reduce the crossover
diameter of the electron beam;
 The objective lens focuses the electron beam as a
probe with a diameter on the nanometer scale.
 The objective lens should be considered as the
third condenser lens in the SEM because it
functions more like a condenser than an objective
lens.
 The reason is that the objective lens in SEM
demagnifies (reduces) the cross-section of the
electron beam.
The Condenser and Objective Lenses
The SEM lens system demagnifies the electron
beam by;
 10,000× → for a thermionic source and
10–100× → for a field emission source.
Scanning
Probe scanning is operated by a beam
deflection system incorporated within the
objective lens in an SEM.
The deflection system moves the probe over
the specimen surface along a line and then
displaces the probe to a position on the next
line for scanning, so that a rectangular raster
is generated on the specimen surface.
Image Formation
The signal electrons emitted from the specimen are
collected by a detector, amplified, and used to
reconstruct an image, according to
one-to-one correlation between scanning points on
the specimen and picture points on a screen
of a cathode ray tube (CRT) or liquid crystal display.
Scan Coils
The deflection system of the electron probe is
controlled by two pairs of electromagnetic coils
(scan coils).
The first pair of coils bends the beam off the
optical axis of the microscope.
The second pair of coils bends the beam back
onto the axis at the pivot point of a scan.
Magnification
• The magnification of an SEM is determined by
the ratio of the linear size of the display screen
to the linear size of the specimen area being
scanned.
• The size of the scanned rectangular area
(raster) can be varied over a wide range.
Thus, an SEM is able to provide image
magnification from 20× to greater than 100,000×.
Figure 4.2 Comparison of images taken in: (a) a light microscope; and (b) an SEM. An
SEM image (b) is able to provide the 3-D appearance of an integrated circuit while
revealing the same in-plane details as the light microscopic image (a).
For low magnification imaging, an SEM is often more favorable than a
light microscope (LM) because of the large depth of field in SEM.
A comparison between LMs and SEMs is demonstrated by the images
shown in Figure 4.2. Both the LM and SEM images in Figure 4.2 reveal
the plane configuration of an integrated circuit, but the SEM image
(Figure 4.2b) also reveals the out-of-plane details because of its threedimensional appearances.
Resolution of SEM
𝒅 𝟎,𝟔𝟏𝝀
……………….1.3
𝟐 𝝁𝑺𝒊𝒏𝜶
𝑹= =
The resolution of an SEM is controlled by the
size of the electron probe scanning the
specimen.
Depending on the instrument, the resolution
can fall somewhere between less than 1 nm and
20 nm.
Resolution and Probe size
Probe Size:
• The resolution of SEM imaging is determined by
the cross-sectional diameter of the scanning probe.
• Thus, the size of the probe limits the size of
features on the specimen surface to be resolved.
• To obtain high resolution, we should know how to
minimize probe size.
𝑑𝑝 =
4𝑖𝑝
1
2
𝛽𝜋 2 𝛼𝑓2
𝑖𝑝 →probe current
β→beam brightness
𝛼𝑓 →convergence angle of probe
The probe diameter is approximately expressed as dp.
𝛼𝑓 →(It is determined by the final aperture diameter
and the distance between aperture and specimen
surfaces (called the working distance) as illustrated in
Figure 4.4.
β→It is proportional to applied voltage (β ∞ eVo)
The effective magnification in SEM
The effective magnification limit is determined
by resolution of the microscope, for a probe size
of 10 nm, SEM systems can generate effective
magnification of 20,000×.
Contrast Formation in SEM
There are two types of contrast in SEM images:
Topographic and
Compositional.
The secondary electrons are the primary source
for surface topographic images, while
backscattered electrons are mainly used for
compositional images.
Contrast Formation in SEM
Knowledge about the interaction between the
high energy electron probe and the specimen is
necessary for understanding these mechanisms
of contrast formation.
Electron–Specimen Interactions
When high-energy electrons strike a specimen,
the electrons are scattered by atoms of the
specimen.
When high energy electrons strike a specimen,
they produce either elastic or inelastic
scattering.
Elastic scattering
Elastic scattering produces the
backscattered electrons (BSEs),
which are incident electrons
scattered by atoms in the
specimen.
Electron–Specimen Interactions
• BSEs are typically deflected
from the specimen at large
angles and with little
energy loss;
• they typically retain 60–
80% of the energy of
incident electrons.
E0→Energy of incident electrons
Supplied by acceleration
voltage.(E=e.Vo)
E1→Retined Energy after impact with
electrons of specimens
Electron–Specimen Interactions
• In contrast, SEs are typically deflected at small
angles and show considerably low energy
compared with incident electrons.
Inelastic Scattering
Inelastic scattering produces secondary
electrons (SEs), which are electrons ejected
from atoms in the specimen.
During inelastic scattering, an incident
electron transfers kinetic energy to an
electron in a specimen atom.
Any electron in atoms in the specimen
with sufficient kinetic energy will leave its
orbital to become a secondary electron.
Inelastic Scattering
The SE energy is usually in the range of about
3–5 eV.
In terms of usefulness, SEs are the primary
signals for achieving topographic contrast, while
BSEs are useful for formation of elemental
composition contrast.
Inelastic Scattering
Electron scattering results in a change of
direction of travel of electrons under the
specimen surface.
Electron trajectories during scattering in a
specimen are schematically shown in Figure 4.9.
As shown in Figure 4.9, the interaction between
electrons and specimen atoms occurs within a certain volume
under the specimen surface.
Secondary Electrons and Back
Scattered Electrons
Both SEs and BSEs generated by scattering are
used as signal sources for forming SEM
images.
However, SEs and BSEs, which are collected by a
detector, escape from different locations in the
specimen. Figure 4.10 illustrates the interaction
zone where electrons scatter under the
specimen surface.
Electron–Specimen Interactions
The zone is usually described as pear-shaped, and its size
increases with the energy of incident electrons in the
probe.
Electron–Specimen Interactions
Besides SEs and BSEs, characteristic X-rays
are also produced in the interaction zone, and these are
useful for chemical analysis.
Electron–Specimen Interactions
Figure 4.10 The interaction zone of
electrons and specimen atoms below a
specimen surface.
Secondary Electrons and Back
Scattered Electrons
 In the interaction zone, SEs can escape only from a
volume near the specimen surface with a depth of 5–50
nm, even though they are generated in the whole pearshaped zone.
 In contrast, BSEs are the products of elastic scattering,
and they have an energy level close to that of incident
electrons. Their high energy enables them to escape
from a much deeper level in the interaction zone, from
depths of about 50–300 nm.
Resolution of SEM by signal source
The lateral spatial resolution of an SEM image is
affected by the size of the volume from where
the signal electrons escape.
We can expect, as shown in Figure 4.10 that an
image formed by SEs should have a better
spatial resolution than that formed by BSEs.
Effect of Atomic number (Z) and
Acceleration Voltage on Interaction
Zone
Electron Yield
The percentage of Secondary and Backscattered electrons
emitted from the specimen is called «electron yield».
Electron yield is strongly affected by atomic numbers of
elements present in the sample.
Topographic contrast in an SEM
Topographic contrast in an SEM refers to
variation in signal levels that corresponds to
variation in geometric features on the specimen
surface.
Topographic contrast is the primary source of
contrast in an SEM image.
Contrast in SEM
Topographic contrast occurs because signal
electrons arise from two effects:
1) The trajectory effect and
2) The electron number effect.
Trajectory Effect
The trajectory effect arises from variations in
how the specimen surface is oriented with
respect to the detector.
As schematically shown in Figure 4.11, the
electrons emitted from the specimen surfaces
facing the detector will be collected abundantly,
and corresponding sites in the image will appear
bright.
Trajectory Effect
• The electrons emitted from the surfaces not
facing the detector will reach the detector
with difficulty, and thus corresponding sites in
the image will appear dark.
• The contrast created by the trajectory effect is
very much similar to the contrast we see with
our eyes of a rough surface illuminated by
light.
• The surfaces facing the detector will appear bright.
• The surfaces not facing the detector will appear dark.
• The contrast created by the trajectory effect is very much similar to
the contrast we see with our eyes of a rough surface illuminated by
light.
Trajectory Effect
The contrast created by the trajectory effect is very much
similar to the contrast we see with our eyes of a rough surface
illuminated by light.
Thus, there is little problem for us to interpret topographic
contrast such as shown in Figure 4.12.
The Electron Number Effect
In an SEM image, the electron number effect will
create bright areas in the image that do not
correspond to surface contours on the specimen.
Figure 4.13 illustrates the electron number effect.
When the electron probe hits a surface at an angle,
more electrons can escape from the specimen than
when the probe hits a flat surface directly.
The Electron Number Effect
• Thus, certain areas of the specimen (such as
edges of spherical particles, raised areas and
cavities)will appear bright in an SEM image.
Figure 4.14 shows the example of topographic contrast
from the electron number effect.
Compositional Contrast
Compositional contrast refers to the variation in
gray levels in an SEM image that correspond to
variation in chemical composition in a specimen.
An image formed by BSEs exhibits very useful
compositional contrast if the specimen consists
of more than one chemical element.
Compositional Contrast
• The origin of compositional contrast arises
because the capability of BSEs to escape from
the specimen depends on the atomic numbers
of the specimen atoms.
Compositional Contrast
The backscatter coefficient (η) characterizes such
capability.
η is the ratio of the number of BSEs escaping from the
specimen (nBSE) to the number of incident electrons
(ni).
It increases monotonically with the atomic number as
shown in Figure 4.15.
• Thus, any area in a specimen containing
chemical elements with higher atomic number
will generate more BSEs.
The difference in the number of BSEs collected
by a detector will appear as differences in gray
levels in a black and white image; that is;
An area with atoms of higher atomic numbers
will appear brighter.
Thus, a BSE image shows the atomic number contrast or
compositional contrast as demonstrated in Figure 4.16, in
which there is a side-by-side comparison of an SE image
and a BSE image for the same surface area of a nickel
alloy.
• SE/BSE images
• Pt particles on Alumina
X-ray Fluorescence Spectrometry
X-ray fluorescence spectrometry (XRF) analyzes
the chemical elements of specimens by detected
the characteristic X-rays emitted from the
specimens.
The characteristic X-rays can be analyzed from
either their wavelengths or energies.
Thus, there are two types of XRF: wavelength
dispersive spectroscopy (WDS) and energy
dispersive spectroscopy (EDS).
EDS/WDS
The main difference between the WDS and EDS
instrumentation is in the X-ray detection systems.
The WDS type uses single crystal diffraction to
detect characteristic wavelengths emitted from the
specimen, because, according to Bragg’s Law, the
single crystal is able to diffract a specific wavelength
at a specific angle between the incident X-ray beam
and a crystallographic plane.
EDS
The EDS instrument uses a photon detector,
typically a Si(Li) diode, to separate the
characteristic X-ray photons according to their
energies.
EDS/WDS
Figure 6.6 schematically illustrates the structural
similarities of, and differences between, WDS
and EDS instruments.
Wavelength Dispersive Spectroscopy
(WDS)
X-ray fluorescence spectrometry was introduced as
wavelength dispersive spectrometry (WDS) in the early 1950s;
the EDS type came along years later.
WDS provides better resolution and a wider range of
elemental analysis than EDS, although its instrumentation is
more complicated.
The WDS systems can resolve relative changes in wavelength
(Δλ/λ) in the range 0.002–0.02.
This range corresponds to the energy range 0.01–0.1 keV,
which is about one order of magnitude better than that of
EDS.
Modern WDS systems can detect elements from upward of C
(Z=6).
Nλ=2dsinϴ
There is a rotating X-ray detector system (an analyzing crystal and X-ray
photon counter in Figure 6.7) to collect the diffraction beam, and
collimators to align the characteristic X-ray
WDS
Nλ=2dsinϴ (Bragg’s Law)
Wavelength-dispersive X-ray spectroscopy (WDX
or WDS) is a method used to count the number
of X-rays of a specific wavelength diffracted by a
crystal.
The wavelength of the impinging X-ray and the
crystal's lattice spacings are related by Bragg's
law and produce constructive interference if
they fit the criteria of Bragg's law.
A WDS spectrum is presented as a diagram in which the
characteristic X-ray lines are dispersed in a range of the Xray wavelengths as shown in Figure 6.8.
• A WDS spectrum is presented as a diagram in
which the characteristic X-ray lines are
dispersed in a range of the X-ray wavelengths
as shown in Figure 6.8.
• The relative intensities of the individual X-ray
lines are represented by their heights in the
spectrum; however, there is no scale to
indicate the real intensities of the X-rays.
• An individual line is marked with the
corresponding element that generates it and
its specific line codes.
Energy Dispersive Spectroscopy
Energy dispersive spectroscopy became a
commercial product in the early 1970s, and rapidly
overtook WDS in popularity.
An EDS system is structurally simple because it does
not have moving parts such as the rotation detector
with WDS.
EDS systems are relatively faster because the
detector collects the signals of characteristic X-rays
energies from a whole range of elements in a
specimen at the same time rather than collecting
signals from X-ray wavelength individually.
EDS
For EDS, the typical resolution of energy
dispersion is about 150–200 eV, worse than the
corresponding resolution of WDS, and the
lightest element that can be detected is O (Z=8),
not C (Z=6).
But these disadvantages are not as important as
the advantages of an EDS system, which are low
cost and fast analysis.
Detector
The Si(Li) is the most commonly used detector in an
EDS system.
The Si(Li) detector consists of a small cylinder of ptype silicon and lithium in the form of a Si(Li) diode,
as schematically shown in Figure 6.9.
X-ray photons collected by the detector generate a
specific number of electron–hole pairs. The average
energy of photons needed to generate as electron–
hole pair (ei) is about 3.8 eV in the Si(Li) diode.
The higher the photon energy, the more pairs are
generated. Characteristic X-ray photons can be
separated by their energy levels according to
the numbers of electron–hole pairs they generate.
Energy Dispersive Spectra
An EDS spectrum is presented as the intensity of
characteristic X-ray lines across the X-ray
energy range.
A spectrum in a range from 0.1 to about 10–20
keV can show both light and heavy elements
because both K lines of light elements and M or
L lines of heavy elements can be shown in this
range.
For example, Figure 6.10 shows the EDS spectrum of a
glass specimen containing multiple elements including Si,
O, Ca, Fe, Al and Ba in an energy range up to 10 keV.
EDS Spectra
EDS spectra are similar to WDS spectra, but
identification of individual elements from EDS
spectra is more straightforward than from WDS
spectra because each characteristic line
generated by a specific element exhibits a
unique X-ray energy.
EDS Spectra
• However, the signal-to noise ratio is lower
than that of WDS, and the resolution, in terms
of energy, is about 10 times lower than that of
WDS.
• Even so, EDS is an attractive technique for
qualitative analysis of elements.