Contact AFM Mode
Optimization of Scanning Parameters
Rev. B
MultiMode SPM Instruction Manual
101/(100 Blank)
6.5.4 Setpoint
The
Setpoint
parameter de
fi
nes the desired voltage (and, therefore, the desired de
fl
ection of the
cantilever) for the feedback loop. The setpoint voltage is constantly compared to the present
photodiode cantilever de
fl
ection 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
de
fl
ection remains nearly constant. When the gain values are low as they should be when the
Data
type
is set to
De
fl
ection
, 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 de
fl
ection 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 de
fl
ection 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
fi
lter
The
Lowpass
fi
lter
invokes a digital, one-pole, lowpass
fi
lter to remove high-frequency noise from
the Realtime data. The
fi
lter operates on the collected digital data regardless of the scan direction.
Settings for this item range from
Off
through
9
.
Off
implies no lowpass
fi
ltering of the data, while
settings of
1
through
9
, successively, lower the cut-off frequency of the
fi
lter applied to the data
stream.
6.5.6 Highpass
fi
lter
The
Highpass
fi
lter
parameter invokes a digital, two-pole, highpass
fi
lter which removes low
frequency effects, such as ripples caused by torsional forces on the cantilever when the scan
reverses direction. As with the
Lowpass
fi
lter
, 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
fi
lter applied to the data stream. It is important to
realize that in removing low frequency information from the image, the Highpass
fi
lter distorts the
height information in the image. As a result,
this
fi
lter must be
Off
when accurate height
information is desired
. The
Highpass
fi
lter
is usually used only for atomic images.
