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Electric Techniques
Surface Potential Imaging—Procedure
302
Dimension 3100 Manual
Rev. D
Verify that all deposited metal adheres strongly to the silicon cantilever. SCM-PIT and
DSCM-PT also work well for surface potential.
4. Set up the AFM as usual for TappingMode operation.
5. Use
Cantilever Tune
:
AutoTune
to locate the cantilever’s resonant peak. Remember,
however, that the Extender box has been reconfigured so that the phase detection circuitry
now acts as a lock-in amplifier. Any procedures that are normally used to view or adjust the
phase signal will now affect the lock-in signal instead (see
In this case, two curves should appear in the
Cantilever Tune
box: the
amplitude
curve in white
and the
lock-in
curve in yellow. In the event you find more than one resonance, select a resonance
that is sharp and clearly defined, but not necessarily the largest. It is also helpful to select a resonant
peak where the lock-in signal also changes very sharply across the peak. Multiple peaks can often
be eliminated by making sure the cantilever holder is clean and the cantilever is tightly secured.
6. Engage the AFM and make the necessary adjustments for a good TappingMode image while
displaying height data.
7. Select the
Interleave Controls
command. This brings up a new set of scan parameters that
are used for the interleaved scan where surface potential is measured. Different values from
those on the main scan may be entered for any of the interleaved scan parameter. To fix any
of the parameters so they are the same on the main and interleave scans, click on the green
bullets to the left of particular parameter. The green bullet changes to “off” (gray) and the
parameter value changes to the main Feedback Controls value. Set the interleave
Drive
frequency
to the main feedback value. Enter an interleaved
Setpoint
of
0V
. Set
Interleave
scan
to
Lift
.
8. Enter an Interleave Controls
Drive amplitude
. This is the ac voltage that is applied to the
AFM tip. Higher
Drive amplitude
produces a larger electrostatic force on the cantilever and
this makes for more sensitive potential measurements. Conversely, the maximum total
voltage (ac + dc) that may be applied to the tip is ±10V. So a large
Drive amplitude
reduces
the range of the DC voltage that can be applied to the cantilever. If the sample surface
potentials to be measured are very large, it is necessary to choose a small
Drive amplitude
,
while small surface potentials can be imaged more successfully with large
Drive
amplitudes
. To start choose a
Drive amplitude
of
2V
.
9. Set the Channel 2 image
Data type
to
Potential
. Set the scan
Line direction
for the main
and interleave scans to
Retrace
. Remember to choose the
Retrace
direction because the lift
step occurs on the trace scan and data collection occurs on the retrace.
10. Choose a
Lift start height
of
0nm
and a
Lift scan height
of
50nm
. The
Lift scan height
can be readjusted later. Set the drive phase to -90°. This compensates for mechanical lay in
the cantilever as it responds to the oscillating electric field.
11. Switch
Interleave mode
to
Enable
to start LiftMode. Now, when the microscope completes
a topographic scan line (trace and retrace) the system turns off the TappingMode piezo and
switches the oscillator signal to the cantilever. The cantilever is driven electrostatically
according to the interleave
Drive amplitude
that has been selected. Also, when
Potential
is
selected as the
Data type
for the Channel 2 image, a feedback circuit is enabled in the
Extender box which adjusts the dc voltage on the tip to maintain the cantilever oscillation
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