Instruction manual

Contact AFM Mode
Optimization of Scanning Parameters
Rev. B MultiMode SPM Instruction Manual 101/(100 Blank)
6.5.4 Setpoint
The Setpoint parameter defines the desired voltage (and, therefore, the desired deflection of the
cantilever) for the feedback loop. The setpoint voltage is constantly compared to the present
photodiode cantilever deflection 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 photodiode output signal close to the Setpoint; therefore, the cantilever
deflection remains nearly constant. When the gain values are low as they should be when the Data
type is set to Deflection, the piezo height does not change, and the photodiode signal varies around
the Setpoint value.
In Contact Mode, increased Setpoint yields higher tip-sample forces. The Setpoint can be adjusted
to increase or decrease the cantilever deflection and, therefore, the contact force of the tip on the
sample. The Force Calibration command in the View menu of Realtime allows the setpoint to be
adjusted while viewing a graph of the tip position versus the deflection voltage. Using this
procedure, which is described in detail in the next portion of this section, the contact force of the tip
on the sample can be minimized. This is especially important on soft materials such as biological
samples.
6.5.5 Lowpass filter
The Lowpass filter invokes a digital, one-pole, lowpass filter to remove high-frequency noise from
the Realtime data. The filter operates on the collected digital data regardless of the scan direction.
Settings for this item range from Off through 9. Off implies no lowpass filtering of the data, while
settings of 1 through 9, successively, lower the cut-off frequency of the filter applied to the data
stream.
6.5.6 Highpass filter
The Highpass filter parameter invokes a digital, two-pole, highpass filter which removes low
frequency effects, such as ripples caused by torsional forces on the cantilever when the scan
reverses direction. As with the Lowpass filter, it also operates on the digital data stream regardless
of scan direction. This parameter can be Off or set from 1 through 9. Settings of 1 through 9,
successively, lower the cut-off frequency of the filter applied to the data stream. It is important to
realize that in removing low frequency information from the image, the Highpass filter distorts the
height information in the image. As a result, this filter must be Off when accurate height
information is desired. The Highpass filter is usually used only for atomic images.