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Whilst
it is understandable that the surface topography of a specimen dictates
the contrast changes that portray the specimen image, there are other
reactions that add or subtract contrast from the image.
Contrast changes will result from a change in accelerating voltage,
spot size (probe current) and tilt angle, each of which change the
secondary electron to backscattered electron ratio.
It should be noted that the emission of secondary electrons from a
given probe current will remain relatively constant as the accelerating
voltage is changed. However as
the
accelerating voltage is increased the BSE influence increases, diminishing
the contribution to the image from the SE.
Another area of influence is the design of the microscope, the
detector position, the lens shape and the chamber shape may all have a
considerable influence upon the image contrast.
The
image produced by the Everhart-Thornley detector, is made up of a number
of electron signals.
Not only do secondary electrons become drawn into the detector, but
also the high-energy backscattered electrons entering the detector, react
with the scintillator in exactly the same way as the secondary electrons.
The detector is unable to discriminate between electrons of
differing energy, as they strike the scintillator, in either case photons
are produced and processed by the photomultiplier. In
truth although secondary electrons dominate the signal entering the
detector the majority of these electrons have been generated though
converted backscatter, thus the dominating reaction in most images is that
which produces backscattered electrons.
With many instruments it is very difficult to eliminate these
signals only on the most sophisticated instruments may “pure”
secondary electron images be produced.
Whilst as
we have mentioned above the general belief is that SE are the originators
of the image quality when it is actually the influence of BSE that adds
higher contrast, greater image depth and shadows to the image, often
making the image more “exciting” and informative.
1
Signal collection
Scanning
electron microscopes have developed in many ways but by far the most
innovative progress has been made in the way that the electron signal is
detected. Whilst retaining the
basic Everhart-Thornley (E-T) detector two styles of signal collection
have been developed.
1.1
Collection from within the specimen chamber
The conventional position
for the E-T detector is within the specimen chamber.
In this case the signal variation, or contrast, may be changed not
only as described earlier, but also through the vertical movement of the
specimen within the specimen chamber.
Moving the specimen in this manner varies the signals reaching the
E-T detector adding or subtracting the BSE influence.
Not all of this change is due to line of sight backscatter, a great
deal is due to converted backscatter from components within the specimen
chamber. The high energy
backscatter spray away from the specimen surface in all directions.
The BSE striking components of the chamber with energies near to
the accelerating voltage react with these surfaces in the same way as the
electron beam with the specimen. Secondary
electrons, backscattered electrons and x-ray are all produced from the
interaction of the BSE with the chamber components.
Depending upon the ease that these electron signals “see” the
E-T detector they will add their contribution.
SE will be attracted into the E-T detector and BSE may reach the
detector through direct line of sight or through multiple scattering
reactions. It is important to
remember that SE produced by BSE striking a surface actually carry the
information that the BSE brought from the specimen.
The manufacturers detector design may also have an influence on
which electrons reach the detector. Small
scintillators reduce the BSE content simply by offering a smaller surface
area with which to react. However
as the secondary electrons are actually attracted into the detector their
number is not related to scintillator size.
Some manufactures have an open mesh collector offering no signal
discrimination, whilst others have a cone or shield on the front or around
the detector that constrains the detected signal.
The position of the detector will have an influence upon the
signals it collects and under which conditions high levels of SE or BSE
collection take place.
1.2 Double
detector systems
Since the early 1980s instruments with two E-T detectors have been
commercially available. The
detectors are mounted one within the specimen chamber, the other being
placed above the final lens. In
this configuration the final lens acts as an electron filter, the lens
field and lens geometry preventing the upper detector having line of sight
BSE influence.
High resolution imaging of areas within large specimens had been
prevented in conventional instruments as moving the large sample nearer to
the final lens prevented most of the signal from reaching the E-T
detector; the specimen was in the way!
With a detector above the final lens such problems do not exist and
the following reactions influence signal collection.
The electron beam strikes the specimen with the normal beam-specimen
reactions. The low energy SE
are unable to break away from the lens axis due to the lens field being
sufficient to control the beam and much too strong to allow the SE to
escape. These electrons are
therefore unable to move either downwards or to one side and as a result
they spiral back up the column (often helped by an extraction voltage)
until they are outside of the lens field and at this point they are
attracted into the upper E-T detector.
In this way a pure secondary electron image may be formed.
The high energy BSE are unable to contribute to the image in a
double detector instrument unless the manufacturer has a lens design that
allows their influence to be incorporated in the upper detector signal.
Converted BSE that strike the lens surfaces producing SE may be
incorporated in the final image or excluded depending upon the lens
configuration and the specimen position.
In the dual detector system images may be obtained from specimens
less than 3mm from the final lens, longer working distances (often 5 to
7mm) offer the opportunity for the BSE influence to contribute in this
way. The constructive
influence of BSE should not be discounted when attempting to obtain the
maximum information from a specimen area, very often whilst SE offer
resolution the BSE offer more information.
The high resolution performance capabilities of a dual detector
system should not inhibit the use of the lower detector or, available on
some instruments, the ability to add upper and lower detector signals
together. The lower detector
will offer all the facilities and specimen manipulation that is available
in a single detector instrument. Far
higher contributions from BSE, therefore higher image depth and image
contrast, as well as lower magnifications, will be available from this
configuration.
In short, a double detector instrument offers far wider imaging
variations and therefore the ability to extract far more information from
the specimen; the ideal system.
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