805
to assume this setting will reliably produce the displayed
value. Well-equipped SEMs have a built-in picoammeter that
can be automatically inserted into the beam path to measure
probe current. Getting a reading in these cases is as simple as
triggering the insertion of the meter’s cup and reading the
value off the screen. Alternatively, a stage-mounted Faraday
cup (either purchased commercially or homemade) attached
through an electrical feedthrough to a benchtop picoamme-
ter can be used instead.
Since the basic idea of High-Current Mode is to deliver
sufficient probe current to the sample to generate superior
signal-to-noise ratio, optimum results are obtained at
medium to low magnifications, and often a larger final aper-
ture is useful. Frame the field-of-view desired, focus the
beam, and increase probe current until high-quality images
can be obtained within a relatively short frame time, say, a
few seconds to a minute. Check that any low-contrast fea-
tures needed for analysis are sufficiently visible, and increase
probe current further if they are not. For many situations,
this high-current imaging approach will yield excellent
images quickly and with little wasted time. If you are per-
forming X-ray microanalysis, the approach to High-Current
Mode is very similar to that for imaging, but the choice of
current is dictated not by image quality but by X-ray count
rate or, more suitably, the dead time percentage of the X-ray
spectrometer’s pulse processor.5.3.3 Resolution Mode
Resolution Mode is probably the most demanding of the four
basic SEM operational modes, chiefly because the micro-
scope is driven at or near its limits of performance. It chal-
lenges the operator mentally, since choosing optimum
imaging parameters requires deeper knowledge of electron
optics and the physics of electron beams, although suitable
images can be obtained with a basic understanding of the
principles. It also demands more skill in operating the micro-
scope, since small misalignments (e.g., residual stigmatism,
imperfectly centered aperture) are more apparent. In fact,
good alignment of the entire column is necessary to get the
best resolution from the scope, while small misalignments
are often tolerated in High-Current or High Depth-of-Field
Mode. Resolution Mode also expects more from the micro-
scope’s environment. Mechanical vibrations, electronic noise,
and AC magnetic fields near the microscope are some of the
many sources of image degradation that, while generally
unnoticeable, become obvious when operating in Resolution
Mode. Poor sample preparation, such as overly thick evapo-
rated metal coatings or insufficient metallographic polishing,
for example, is also more evident at high magnifications.
Most of these challenges are greatly reduced at lower magni-
fications, but the larger pixel sizes that result from low mag-
nification obviate the need for Resolution Mode. In short, the
same imaging conditions that enable Resolution Mode also
highlight any shortcomings in the operator’s technique, the
laboratory environment, and the sample preparation.ab. Fig. 5.17 Examples of graphical user interface controls that allow
the operator to control the beam current in discrete steps expressed as
changes in Spot Size. In a, the can choose any of several Spot Size val-
ues from a pull-down menu. In b, from the same microscope, the oper-
ator can access Spot Size via a pull-down menu or buttons that
increase or decrease the value
Chapter 5 · Scanning Electron Microscope (SEM) Instrumentation