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Contact AFM

Optimization of Scanning Parameters

Rev. D Dimension 3100 Manual 129

8.3.3 Scan Size and Scan Rate

In general, decrease the Scan rate as the Scan size increases. Use scan rates of 1.5 - 2.5 Hz for

large scans on samples with tall features. High scan rates help reduce drift, but use high scan rates

only on ﬂat samples with small scan sizes. When ﬁrst using the system, vary the scan rate enough to

observe a change in the image quality.

8.3.4 Setpoint

The Setpoint parameter deﬁnes the desired voltage (and, therefore, the desired deﬂection or force

of the cantilever) for the feedback loop. The setpoint voltage constantly compares to the present

vertical deﬂection signal of the photodiode to calculate the desired change in the piezo position.

When the gain values are high, and 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.

Adjust the Setpoint to increase or decrease the cantilever deﬂection and, therefore, the contact

force of the tip on the sample. The Force Calibration command in the Realtime > View menu

allows you to adjust the setpoint while viewing a graph of the tip position versus the deﬂection

voltage. Using this procedure, described in detail in Chapter 13, minimizes the contact force of the

tip on the sample. This is especially important on soft materials such as biological samples.

8.3.5 Lowpass Filter

The Lowpass ﬁlter invokes a digital, one-pole, lowpass ﬁlter to remove high-frequency noise from

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. The standard setting is Off.

8.3.6 Highpass Filter

The Highpass ﬁlter parameter invokes a digital, two-pole, highpass ﬁlter to remove low frequency

effects such as ripples caused by torsional forces on the cantilever when the scan reverses direction.

As with the Lowpass ﬁlter, the Highpass ﬁlter 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. Note 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 typically used only for atomic images.

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Dell 3100 Spezifikationen Seite 149