Lateral Force Mode
Advanced LFM Operation
172
MultiMode SPM Instruction Manual
Rev. B
10.2.5 Using TMR Voltage to Measure Friction
The signal called TMR (Trace minus Retrace) in the scope mode display measures the voltage
difference between the Trace and the Retrace scan directions. In the case of LFM data, this directly
corresponds to the amount of total tip twist that occurs as the tip scans back and forth across the
sample.
It is common to study the frictional differences between two different samples. Monitoring the
TMR voltage between two different samples will give you a comparative frictional measurement.
Figure 10.2d
TMR Data
Be aware that friction force is also affected by the amount of tracking force. The force needs to be
the same when comparing two samples. You should use the force calibration curve to verify the
force after substituting samples. Also note that the laser spot position and changing the probe will
also cause changes in the LFM measurement.
10.2.6 Enhancing the LFM Data by Subtracting Two Images
It is possible to enhance the magnitude of the LFM data by subtracting the trace from the retrace.
This is done by collecting three data channels with one scan.
1. Set
Channel 1
to
Height
.
2. Set
Channel 2
to
Friction – Trace – Offline plane fit
=
none
.
3. Set
Channel 3
to
Friction – Retrace – Offline plane fit
=
none
.
4. Collect a scan at 90 degree scan angle.
5. Use the Image subtract feature in Offline mode to subtract Retrace from Trace. The resulting
data will show double the LFM data and half the background noise caused by tracking error.
Note:
The X-Y motion of an open loop scanner in trace and retrace does not overlap
exactly. You can still use image subtraction, but some erroneous height data
might be added to the LFM data in the
fi
nal image. This is not an issue for
closed loop scanners.
Less Frictional Surface
More Frictional Surface
TMR
0.123V
TMR
0.456V
