UNIMO Technology UDR-204 Manual de instrucciones Pagina 132

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TappingMode AFM
Preparation Prior to Imaging
112 MultiMode SPM Instruction Manual Rev. B
2. If a peak in the frequency response plot does not appear, perform the following steps:
Increase the Drive amplitude to 600mV.
Increase Sweep width to the maximum value.
If the peak still has not appeared, then increase the Sweep width by rst increasing the Drive
Frequency, then maximizing the Sweep width. If there is still no peak on the response plot, check
the laser alignment.
3. After identifying the maximum amplitude peak with the lowest frequency in the frequency
response plot, center the peak on the frequency sweep plot shown on the display monitor.
This is most easily accomplished with the Zoom In and Offset subcommands in the menu
bar of the display monitor. The Offset subcommand shifts the plot by setting the center
frequency equal to the cursor position. The Zoom In subcommand stretches the plot by
decreasing the sweep width and shifting the center frequency value.
If the top of the resonance peak is off the plot, increase the Amplitude Setpoint until it appears.
Continue to Zoom In and center the peak until the peak coincides with the vertical center line
within about 10Hz. The value displayed for center frequency is now used as the resonant frequency
of the cantilever.
The system will work well in TappingMode if the drive frequency is at, or below, the peak in the
resonance plot. The Drive Frequency can be decreased to the point where the vibrational
amplitude reaches 90% of the maximum value. It is often preferable to operate at a frequency lower
than the resonant frequency due to shifting of the resonant frequency upon approach of the tip to
the surface.
7.2.7 Setting the Drive Amplitude and Setpoint
After tuning the cantilever to its resonant frequency, the free-vibration amplitude must be specied.
The desired operating amplitude depends on the sample, and other scanning conditions.
Prior to engagement, the Amplitude Setpoint parameter is used to scale the cantilever response
(vertical) axis of the frequency sweep display graph. It does not actually adjust the operating
feedback setpoint, since the computer automatically determines the initial engagement setpoint
during engagement.
Only after engagement does the Setpoint parameter in the control panel dene the operating
amplitude to be maintained by the feedback loop. Perform the following operations to adjust the
oscillation drive amplitude and the frequency sweep graph vertical axis scaling.
1. Increase the Drive amplitude until the peak amplitude in the Frequency Sweep plot reaches
the desired amplitude. A good amplitude would be 1—3V. In general, the peak amplitude
will reach 3V with the drive amplitude set to under 500mV.
The amplitude value can be determined by adjusting the amplitude setpoint value until the center
frequency crosses in the middle (horizontally) of the cantilever tune graph. You can also Quit the
Cantilever Tune menu and adjust the Drive amplitude until the RMS value is appropriate. The
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