TappingMode AFM
Beyond Basics with Resonating Techniques
Rev. B
MultiMode SPM Instruction Manual
119/(118 Blank)
Scan Size, Scan Rate, and Setpoint
As discussed above, the
Scan size
,
Scan rate
, and
Setpoint
values have dramatic effects on the
data. As in contact mode, the
Scan rate
must be decreased as the
Scan size
is increased.
Scan rates
of
0.5
-
1.0Hz
should be used for large scans on samples with tall features. High scan rates help
reduce drift, but they can only be used on
fl
at samples with small scan sizes.
The
Setpoint
parameter de
fi
nes the desired voltage for the feedback loop. The Setpoint voltage is
constantly compared to the present RMS amplitude voltage to calculate the desired change in the
piezo position. When the gain values are high, as they should be when the
Data type
is set to
Height
, the Z piezo position changes to keep the amplitude voltage close to the Setpoint; therefore,
the vibration amplitude remains nearly constant.
As discussed above, changing the
Setpoint
alters the response of the cantilever vibration and
changes the amount of force applied to the sample. The force applied by the tip on the sample
increases as the setpoint is decreased. To minimize the force on the sample, increase the setpoint
using the right arrow key on the computer keyboard. Increase the setpoint until the tip is no longer
interacting with the surface. This can be observed by monitoring the trace and retrace lines in
View
>
Scope Mode
, and by monitoring the Z Center position display in the Image Monitor. When the
tip is no longer interacting with the surface, the Z Center position will move to the retracted limit.
Then, slowly decrease the setpoint using the left arrow key on the keyboard until features appear in
the trace and retrace lines in
Scope Mode
. The Setpoint can also be adjusted by using
View
>
Force
Mode
>
Calibrate
.
Force Calibration Mode
compares the amplitude of the cantilever oscillation
on the surface to the free-air amplitude. Other uses of Force Calibration include characterizing the
forces on the cantilever tip, diagnosing the performance, and calibrating the RMS amplitude
voltage as a function of the oscillation amplitude of the cantilever. The use of Force Calibration is
described in
Surface Tune
It is often desirable to check the cantilever tune after engaging. You will notice that the
Cantilever
Tune
icon is unavailable when the tip is engaged. However, you can still access the
Cantilever
Tuning
panel by selecting
View
>
Sweep
>
Cantilever Tune
. A warning will appear indicating that
cantilever tuning while engaged can result in damage to the tip or sample. This warning will prompt
the user to enter a tip offset. This is the distance above the sample that the tip will be raised before
displaying the tuning spectrum. Use a minimum offset of 100-200nm. Observe the plot of
amplitude vs. frequency to see if the cantilever tuning curve shifted during the engagement. The
drive frequency can be adjusted to compensate for such a shift by using the Offset command
located above the cantilever tune plot. the phase response curve can be adjusted as well.
