Force Calibration

The ability to calibrate the amount of force exerted by the cantilever probe on the sample surface is very valuable to many imaging applications. The techniques and considerations differ slightly for Contact Mode Force Calibration versus TappingMode Force Calibration but using either requires an understanding of Force Spectroscopy.

Ramp Continuous Mode

Continuous Ramp mode is frequently used to adjust and calculate contact forces between the cantilever and the sample. Other uses of Continuous ramp mode include characterization of the forces on the cantilever tip, diagnosing AFM performance, and determination of the sensitivity of the cantilever deflection voltage in terms of voltage applied to the piezo. If used correctly, Continuous Ramp mode provides a variety of useful information.

The Continuous Ramping command in the Ramp menu allows you to quickly check the interaction between the cantilever and the sample surface. In the Continuous ramping mode, the X and Y voltages applied to the piezo tube are held constant and a triangular waveform similar is applied to the Z electrodes of the piezo tube:

Figure 1: Continuous ramping Z waveform

As a result of the applied voltage, the cantilever tip moves up and down relative to the stationary sample as shown in Figure 2, below. The Z Scan Start parameter, available through the Ramp Parameter List, sets the offset of the piezo travel, while the Ramp Size parameter defines the total travel distance of the piezo. Therefore, you can obtain the maximum travel distance by setting the Z scan start to +220 V, with the Ramp size set to 440 V.

Figure 2: Piezo Travel in Continuous ramp Mode. A) Z = Z Scan Start – Ramp Size. B) Z = Z Scan Start.

As the piezo moves the tip up and down, the cantilever-deflection signal from the photodiode is monitored. The force curve, a plot of the cantilever deflection signal as a function of the voltage applied to the piezo tube, displays in the Force Plots window. The controls detailing parameters for controlling the microscope in Continuous Ramping mode are available in the Force Plots window and the Ramp Parameters List panel.

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