User`s guide

Contact Mode Imaging 5
Agilent 5500 SPM User’s Guide 100
two components. Comparing the friction and topography images helps
to differentiate the impact of topography versus friction.
At the end of each scan line the system will “retrace” the scan line until
it once again reaches the beginning. The scanner will then advance one
line width and another line will be scanned.
Depending on the Frames setting in the Scan and Motor window, the
system will either scan once and stop, or it will scan infinitely,
overwriting the previous scan each time. You can also choose to scan
for a specific number of frames. To stop the scan, click the red STOP
circle that will replace the Up or Down arrow when you start a scan
(Figure 68 on page 97).
Constant Height Mode
In Constant Height Mode the system maintains the tip in a plane above
the surface. It is functionally the same as Constant Force Mode, except
that the feedback circuit gains are set very low so that the system does
not react to changes in tip deflection.
To image in Constant Height Mode, in the Servo window set the I and P
gains to 0.1 %. This will effectively cause the system to no longer adjust
the tip force. This lack of feedback reduces signal noise, enabling
atomic-level resolution imaging of very flat samples. The scan speed
can also be faster since the system will no longer attempt to react to
changes in deflection.
The error signal (in volts) is used to generate an image that is sensitive
to small changes in topography.
Fine-Tuning the Image
Besides the sharpness of the scanning tip, the quality of imaging in
Contact Mode is largely dependent on three factors: the Setpoint
voltage, feedback gains, and scan settings.
Setpoint
When the Setpoint is too negative, the system will continue as if contact
is established between the tip and sample even if it actually is not. In
this case, the tip will not accurately trace surface topography—in the
extreme, the topography image will appear entirely flat. Making the
Setpoint more positive increases the force applied to the sample by the