Measures electric field gradient distribution
above the sample surface; performed using LiftMode
to track topography (Extender Electronics Module
recommended). The Extender module improves results
by instead measuring changes in phase or resonant
frequency of the oscillation, providing an improved
noise floor and allowing even more sensitive measurements
than before. In addition, the phase and frequency
signals are generally immune to interference and
artifacts. Specifically, measurements of phase
or frequency can reduce or eliminate commonly
observed optical interference artifacts and can
also reduce topographic contamination of electric
or magnetic field data that result from air damping
of the cantilever amplitude. An additional benefit
is that the measurements are more quantitative
and repeatable from probe to probe.
Surface Potential
Detection. The Extender module
can also be used to measure actual surface potential,
or voltage. Here, a solid state relay in the Extender
module switches the oscillator signal to the probe
itself. The cantilever supporting the probe is
driven into oscillation electro statically and
the oscillation amplitude depends on the DC potential
difference between tip and sample. The lock-in
amplifier produces an output of zero when the
potential on the tip matches the potential on
the sample. The servo controller sets the lock-in
output to zero by varying an additional potential
applied to the tip. At this point, the potential
on the tip and sample are the same and the NanoScope
IIIA controller records the tip potential to create
an image of the sample surface potential. Surface
potential detection has applications in semiconductor
failure analysis, composite materials, superconductor
characterization and others. This example (right)
shows the surface potential on biased metal lines
on a GaAs device.
User Manual
for Electrostatic Force Microscopy
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