Tune the Cantilever (TappingMode Only)

Tuning the AFM cantilever involves finding the resonance peak of the cantilever and adjusting the oscillation voltage so the cantilever vibrates at an appropriate amplitude. A range of oscillation frequencies are applied to the cantilever to determine the frequency that will produce the largest response—the resonance frequency. In most instances, the resonance peak has a sharp Gaussian distribution but at times the peak can be ragged. The system tolerates some deviation in the shape of the peak.

Select Microscope > Cantilever Tune or click the Manual Tune button in the Tune Cantilever panel of the Setup view:

The optimal drive frequency can depend upon the sample, fluid, and fluid volume inside the fluid cell. In liquid the cantilever resonance is highly damped. Experiment to find the best drive frequency for specific imaging conditions. Often there is not a single well-defined resonance, but instead a large number of broad peaks. The peaks are parasitic resonances and do not usually depend much on the cantilever dimensions.

A typical Cantilever Tune screen is shown in figure 1, below. It is necessary to select a frequency where there is some cantilever response, (i.e., near a peak), but experience suggests that it is best to avoid the tops and sides of extremely sharp peaks. The best frequencies appear to be on the side of a peak or in a shallow valley between peaks. Figure 1 also displays a typical operating frequency that produces good fluid TappingMode images.

Figure 1: Typical Cantilever Tune Curve in Fluid

The Manual Tune procedure for determining the correct resonant frequency in fluid is described below. However, a system that removes some of the complexity of determining the correct peak is now available.

Fluid Tune Procedure

1. In the Feedback panel (in Expert Mode), set Z Modulation to Disabled.

2. Select Microscope > Cantilever Tune or click the Tune icon on the NanoScope toolbar to open the Cantilever Tune window if it is not already open. The window opens with the Frequency Sweep (a plot of cantilever response as a function of applied oscillation frequency). See Figure 1. Assuming you are in a fluid workspace and have selected the correct probe in the Setup view, the Cantilever Tune window will set up the following parameters for you:
   • Drive Frequency
   • Sweep Width
   • Drive Amplitude
   • Amplitude Setpoint
   • Amplitude Range
3. Click the Fast Thermal Tune button. The system will measure the cantilever deflection in the absence of a drive signal and output the transformed data to the sweep plot:

Figure 2: Fluid cantilever tune with Fast Thermal Tune data displayed in the frequency sweep.

4. Identify and center on a peak near the Fast Thermal Tune peak in the frequency response plot (see the green line in Figure 2). The center frequency value is the drive frequency.

5. Adjust the Drive Amplitude until you obtain an acceptable RMS Amplitude for your application.

Previous Steps:

1. Fluid Set Up
2. Align the Laser
3. Load the Sample and Adjust the SPM Scanner
4. Locate the Tip

Next Steps:

6. Focus on the Sample Surface
7. Check Initial Scan Parameters
8. Engage and Adjust Scan Parameters
9. Withdraw and Clean Up

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