Parameter Settings

The user is encouraged to review Advanced Contact Mode AFM Operation, which discusses parameter optimization for Contact AFM.

Data Type

Data Type is the first parameter to set because the values of other parameters depend on it. The Data Type parameter in the Channel 1, Channel 2, and Channel 3 panels selects the type of data collected by the system.

• Height data corresponds to the change in piezo height needed to keep the amplitude of the cantilever constant.
• Height Sensor data corresponds to the closed-loop measured change in piezo height needed to keep the cantilever deflection constant.
• Amplitude data measures the change in amplitude relative to the amplitude setpointOperator-selected threshold used as the target of the feedback control loop..

The scan parameters required to collect good Height and Height Sensor data are different than the optimal parameters for Amplitude data. To collect Height or Height Sensor data while tracking the sample surface with minimal change in the tip’s oscillation amplitude, the feedback gainsThe gains are scaling factors for the error signal to determine how quickly corrections should be applied in the feedback loop. There are two important gain factors: the proportional gain scales to the current value of the error signal, and the integral gain scales the aggregate of the past values of the error signal (the "area under the curve"). must be high.

CAUTION: Do not conduct TappingMode microscopy with low feedback gain values, as this will cause damage to both the tip and sample. The maximum amplitude of the cantilever oscillation is not sufficient to track tall features.

Amplitude data collected with high feedback gains is the derivative of the height. Amplitude mode provides a sensitive edge detection technique. It is possible to capture both Height Sensor and Amplitude data simultaneously.

Gain Settings

The integral gainThe multiplier of correction applied in response to the average error signal. and proportional gainThe multiplier of correction applied in response to the error signal in direct proportion to the error. settings in the Feedback panel must be high enough to force the feedback system to track the sample surface. When scanning in TappingMode, set the Integral and Proportional Gains to lower values than those values used in Contact mode. The Proportional Gain can usually be set 30–100% higher than the Integral Gain. To optimize the gains, increase the Integral Gain until the piezo begins to oscillate (feedback oscillation usually occurs with Integral gains of 10–20), then eliminate the oscillations by reducing the gain with two or three clicks of the left arrow key. Repeat the process for the Proportional Gain.

Scan Size, Scan Rate, and Setpoint

The Scan Size, Scan Rate, and Setpoint values effect data output differently. As in Contact mode, decrease the Scan Rate as the Scan Size is increased. Use Scan Rates of 0.5–1.0 Hz for large scans on samples with tall features. High scan rates help reduce drift, but only use them on flat samples with small scan sizes.

The Amplitude Setpoint parameter defines the desired voltage for the feedback loop. The Amplitude 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 or Height Sensor, the Z piezo position changes to keep the amplitude voltage close to the Amplitude Setpoint; therefore, the oscillation amplitude remains nearly constant.

As discussed above, changing the Amplitude 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:

1. 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 the oscilloscope display and by monitoring the Z Center Position. When the tip is no longer interacting with the surface, the Z Center Position will move to the retracted limit.
2. Slowly decrease the amplitude setpoint using the left arrow key on the keyboard until features appear in the trace and retrace lines in the oscilloscope display.

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