Instruction manual

Electric Force (EFM) Imaging
Surface Potential Imaging—Procedure
Rev. B MultiMode SPM Instruction Manual 269/(270 Blank)
14.7.1 Troubleshooting the Surface Potential Feedback Loop
The surface potential signal feedback loop can be unstable. This instability can cause the potential
signal to oscillate or become stuck at either +10V or -10V. Here are some tips to see if the feedback
loop is working properly with no oscillation:
• Go into Scope Mode and look at the Potential signal. If oscillation noise is evident in
the signal, reduce the FM gains. If oscillations persist even at very low FM gains, try
increasing the Lift scan height and/or reducing the Drive amplitude until oscillation
stops. If the tip crashes into the surface the Lock-in signal becomes unstable and can
cause the feedback loop to malfunction. Increasing the Lift height and reducing the
Drive amplitude can prevent this problem. Once oscillation stops, the FM gains may be
increased for improved performance.
In Scope Mode, if the Potential signal is perfectly flat and shows no noise even with a small Z-
range, the feedback loop is probably stuck at ±10V. (You can verify this by changing the value of
Realtime planefit to None in the Channel 1 panel.) Reduce the Scan rate and watch the display
monitor which indicates the cantilever amplitude. On the topographic trace, the voltage displayed
should be the setpoint selected for the Main scan. On the Potential trace, this voltage drops close to
zero if the cantilever oscillation is being successfully reduced. If the value on the display monitor
instead goes to a large nonzero value, the feedback loop is probably not working properly. In this
case, try reducing the Drive amplitude and increasing the Lift scan height. It may also be helpful
to momentarily turn the Interleave mode to Disabled, then back to Enabled. Also try reducing any
external voltage that is being applied to the sample to stabilize the feedback loop, then turn the
voltage back up.