Electron detectors
An appropriate detector must be employed to collect and convert the
radiation of interest, that leaves a specimen, into an electrical signal for
manipulation to create an SEM image.
The most used signals in a scanning electron microscope in standard
conditions are the electrons and in particular the SE.
Considering electron imaging, BSE and SE leave the specimen with
drastically different properties in term of energy, fraction relative to the
beam, and directionality of emission.
Each specific type of radiation potentially carries information on different
aspects of the specimen characteristics.
This information can be contained in the number of BSE and SE, their
energy distribution, and their emission directions, or a combination of all
three factors.
General characteristics of a detector
The main characteristics of an electron detector are:
• the detector position with respect to the beam and the specimen
• the dimensions of the detector sensitive area (described by the solid angle Ω)
• the efficiency in converting a signal incident onto the detector sensitive area
in “useful” signal. In general, the efficiency is not constant, but it depends on
the electron energy
• the amplification properties of the signal
These widely different energy characteristics of BSE and SE present a
considerable challenge to the design of a detector that can be used for both
signals.
Alternatively, the differences permit the design of detectors that are
selective for one of the signals and thus selective for specific specimen
properties.
Everhart-Thornley detector
The electron detector most commonly used in scanning electron
microscopy is the combined secondary/backscattered electron detector
named Everhart-Thornley (E-T).
It is so popular that it is extremely rare to find a conventional SEM
without one.
The rise of the SEM as a tool of broad use to the scientific community is
due in considerable measure to the performance and utility of the E-T
detector (large solid angle of collection/efficiency, high amplifier gain,
low noise, low-maintenance performance).
Because of its efficient collection of SE, the E-T detector is often
mistakenly considered only a SE detector.
.
Everhart-Thornley detector
The E-T detector operates in
the following manner:
when an energetic electron (≈ 10
keV
energy)
strikes
the
scintillator material (S), light is
emitted.
Light is conducted by total internal reflection in a light guide (LG) (a solid
plastic or glass rod) to the photocatode of a photomultiplier (PM).
At the photocatode the photons are converted back into electrons which are
accelerated onto the successive electrodes of the photomultiplier, producing
an ever-increasing cascade of electrons until the final collector is reached.
The typical gain of a
photomultiplier is 105-106
A large fraction of the BSE,
that originates from incident
beam with energies from 10 to
30 keV, carry sufficient
energy to directly excite the
scintillator,
even
in
the
absence
of
postspecimen
acceleration.
By applying a large positive potential (10-12 kV) to a thin metal coating on the
face of the scintillator (S), the low-energy SE will be accelerated to a
sufficient energy to generate light in the scintillator.
To protect the beam from unwanted deflection and distortion by this large
potential in the specimen chamber, the scintillator is surrounded by a Faraday
cage (F) which is insulated from the scintillator bias.
To collect the low-energy SE with higher energy efficiency than simply
collecting the fraction defined by the line-of-sight solid angle, a sparate bias
potential is applied to the Faraday cage, typically in range (-50 V, +250 V).
This range from negative to positive provides the possibility of completely
rejecting SE (-50 V) or efficiently collecting SE (+250 V).
When the Faraday cage bias is
positive, the BSE directed towards
the detector are detected; but they
represent only a small fraction of all
the detected electrons. So their
contribution is negligible.
Thanks to the presence of the
Faraday
cage,
the
EverhartThornley detector is a device very
efficient to detect SE and for flat
samples it is possible to detect
nearly all the secondary electrons
If the Faraday cage bias is negative, the SE are rejected and the detector
collect only the BSE.
Obviously, without an electric field that directs the electrons towards the
scintillator, only the BSE which have the right direction are revealed.
So the collecting efficiency for BSE is very low and to detect BSE other
detectors are used.
“Through the lens” TTL detector
The high-performance field-emission-gun SEM is equipped with a “snorkel” lens
which produces a strong objective lens magnetic field that is projected into
the specimen chamber to reach the specimen plane.
TTL
Magnetic field
projected out
of lens
Snorkel Lens
ET
This contrasts with the “pinhole” lens of a
conventional SEM in which the lens
magnetic field is contained within the bore
of the lens so that the specimen resides in
a field-free region.
One major consequence of the strong
magnetic field is to trap with high
efficiency those SE emitted from the
specimen.
The SE spiral along the magnetic field lines
and pass up through the lens bore.
In this configuration SE are detected by
the TTL detector.
The upper and lower detectors have a different viewpoint of the specimen and
so they ‘see’ the specimen differently
In-lens (TTL) detector gives a shadow free image with ultra-high topographical
resolution.
Upper SE Detector
Lower SE Detector
Passive scintillator BSE detectors
They operate on the principle that, with an incident beam of 10 keV or more,
the majority of the BSE carry sufficient energy to excite the scintillator
even in the absence of a postspecimen acceleration. Without such active
acceleration, the SE have no effect on the scintillator, so the signal from an
unbiased or passive scintillator detector will consist only of contributions
from BSE.
The elimination of the bias also has the benefit that the detector potential
will not disturb the beam, so the detector can be placed close to the
specimen for mor efficient collection.
By making the scintillator of the same material as the light guide, designs
that collect over much larger solid angles are possible.
The detector is placed above the
specimen and a hole drilled trough the
material permits access for the beam.
In this configuration, the detector
surrounds
the
specimen
nearly
simmetrically, so the signal is integrated
in all directions, nearly eliminating
sensitivity to trajectory effects.
Solid state diode detectors
They operate on the principle of electron-hole production induced in a
semiconductor by energetic electrons.
A solid state diode detector (p-n junction) has the form of a flat, thin wafer
(typically several millimiters thick) which can be obtained in a variety of shape
and size, from small square to large annular detectors.
The thinness of the detector
permits it to be placed in close
proximity to the specimen, which
combined with the large area
possible, provides a large solid angle
for high geometric efficiency.
In general, it is mounted under the
objective lens, without interference
with the normal operation of the
instrument.
The main drawback is the long response time, with respect to the other
detectors, so it doesn’t permit high scan rates.