3-2
A
PPROACHING
THE
S
AMPLE
CAUTION
Be careful when lowering the tip not to crash it into the sample surface.
Feedback refers to the attenuation of the tip’s oscillation amplitude. When the tip is in
feedback at the sample surface, the sample topography can be tracked and a topographic
image taken. This also means that the tip is in the near-field range, enabling NSOM
imaging. A technique known as “false feedback” will be used to bring the tip into the near
field. This allows for a gentle yet efficient approach, whereas using the Z-motor to
approach the sample runs the risk of breaking the tip and is usually very slow.
F
INDING
THE
R
ESONANT
D
RIVE
F
REQUENCY
For each probe, it is necessary to set the drive frequency at the resonant frequency of the
tuning fork. The objective is to maximize the magnitude of the tuning fork vibration by
adjusting the driving frequency control.
For the following procedure, refer to the “Non Contact Operation” section in Chapter 2 of
the SPMLab Software Reference Manual. When the SPMLab software is configured for
the Aurora-2, the Non Contact window is automatically open, with the Active mode
checked, upon entering the Data Acquisition mode.
Step 1
Select “Internal Sensor, Feedback” from the drop-down signal source list to the
right of the upper oscilloscope.
Step 2
Select Amplitude from the Mode group box in the Non Contact window.
Step 3
In the Non Contact window, enter the following starting parameters:
Amplitude = .1 V
(drive amplitude)
Input Gain = 8.
(input gain of the I/O MOD+ board)
Step 4
In the Signal window, select S
COPE
mode.
Step 5
From the Range drop-down list, select the frequency range that is closest to 90-
100 kHz, which is typical for the tuning fork.
Step 6
In the Non Contact window, select S
PECTRUM
to run a frequency sweep.
Step 7
Zoom in on the peak so that the frequency window is 2 kHz (or less) wide.
Step 8
Make sure the peak is not cut off. The maximum voltage range of this window is
10 V. If the peak is cut off, reduce the Amplitude. Ideally, the peak will sit at
around 5 V, which corresponds to about -30nA on the internal sensor signal. Fine
tuning will be done later.
Step 9
Click on the peak to select the drive frequency, which will then appear in the
D
RIVE
F
REQUENCY
field. See Figure 3-1.
Summary of Contents for Aurora-2
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