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TappingMode AFM

Advanced TappingMode AFM Operation

Rev. D Dimension 3100 Manual 147

The Setpoint parameter deﬁnes the desired voltage for the feedback loop. The Setpoint voltage is

constantly compared to the present RMS amplitude 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 amplitude voltage close to the Setpoint; therefore,

the oscillation amplitude remains nearly constant.

As discussed above, changing the Setpoint alters the response of the cantilever oscillation and

changes the amount of force applied to the sample surface.

The force applied by the tip on the sample increases as the setpoint is decreased. To minimize the

force on the sample:

• Increase the setpoint using the right arrow key on the computer keyboard until the

tip is no longer interacting with the surface. This can be observed by monitoring the

trace and retrace lines in View > Scope Mode, and by monitoring the Z Center

Position display in the Image Monitor. When the tip is no longer interacting with

the surface, the Z Center Position will move to the retracted limit.

• Slowly decrease the setpoint using the left arrow key on the keyboard until features

appear in the trace and retrace lines in View > Scope Mode.

The setpoint can also be adjusted by using View > Force Mode > Calibrate. Force calibration

mode is used to compare the amplitude of the cantilever oscillation on the surface to the free-air

amplitude. Other uses of Force Calibration include characterizing the forces on the cantilever tip,

diagnosing the performance, and calibrating the RMS amplitude voltage as a function of the

oscillation amplitude of the cantilever. The use of Force Calibration is described in Chapter 13.

9.4.8 Surface Tune

It is often desirable to check the cantilever tune after engaging. You will notice that the Cantilever

Tune icon is grayed out when the tip is engaged. However, you can still access the

Cantilever Tune panel by selecting View > Sweep > Cantilever Tune.

A warning appears indicating that cantilever tuning while engaged can result in damage to the tip or

sample. This warning prompts you to enter a Tip offset. This is the distance above the sample that

the tip will be raised before displaying the tuning spectrum. Use a minimum Tip offset of 100-

200nm.

Observe the plot of amplitude vs. driving frequency to see if the cantilever tuning curve shifted

during the engagement. The drive frequency can be adjusted to compensate for such a shift by using

the Offset command located above the Cantilever Tune plot. The phrase response curve can be

adjusted as well.

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Dell 3100 Especificaciones Pagina 167