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
fi
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 speci
fi
ed.
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 de
fi
ne 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
