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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 Figure 16.4a).

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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