Instruction manual

Electric Force (EFM) Imaging
Surface Potential Imaging—Procedure
268 MultiMode SPM Instruction Manual Rev. B
13. Optimize the lift heights. Set the Lift scan height at the smallest value possible that does not
make the Potential feedback loop unstable or cause the tip to crash into the sample surface.
When the tip crashes into the surface during the Potential measurement, dark or light streaks
appear in the Potential image. In this case, increase the Lift scan height until these streaks
are minimized.
14. Optimize the drive phase. When using cantilevers other than MESPs, OSCM-PTs and
SCM_PITs, the lag in the response of the cantilever to the electric force can be compensated
for by looking at the phase cantilever with the potential feedback loop enabled, but with the
Input igain and Input pgain set to zero. By looking at the phase in this state, you can see the
input to the potential feedback loop.
•To proceed with the drive phase adjustment, with the Input Feedback set to Potential,
set the Input igain and Input pgain to zero. Set one of the Data Channels to Phase,
and set Realtime Planefit to None. Select View > Scope Mode, and look at the position
of the phase data relative to zero. Begin adjusting the Drive Phase under the Interleave
Controls panel while observing the change in the phase signal. Find the point where the
phase is minimized. For MESPs, SCM-PITs and OSCM-PTs, this value should be near
0°. For TESPs, this value is normally around -45°. Once the minimum has been found,
subtract 90° from this value. This is the optimal value of drive phase shift to compensate
for lag in the cantilever to the electrical drive. Set the Input igain and Input pgain to
their previous values. Change the Data Type back to Potential, and proceed to collect
surface potential data as needed.
15. For large sample voltages or qualitative work, select Data type = Phase instead of Potential.
When the Extender box has been configured for surface potential measurements, the “phase”
signal is actually the cantilever amplitude signal, as measured by a lock-in amplifier. If the
feedback loop is not enabled by selecting the Data type = Potential, the lock-in cantilever
amplitude depends on the voltage difference between the tip and sample in a roughly linear
fashion. (The lock-in amplifier produces a voltage that is proportional to the cantilever
amplitude.) Qualitative surface potential images can be collected using this lock-in signal.
Also, if the sample has a surface potential that exceeds ±10V (greater than the range of the
“Potential” signal), it is possible to use the lock-in signal to provide qualitative images that
reflect the sample surface potential. To view the lock-in signal with the reconfigured
Extender box, select the Data type = Phase.