Amplitude calibration of quartz tuning fork (QTF) force sensor with an atomic force microscope

Amplitude calibration of the quartz tuning fork (QTF) sensor includes the measurement of the sensitivity factor (αTF). We propose, AFM based methods (cantilever tracking and z-servo tracking of the QTF's amplitude of vibration) to determine the sensitivity factor of the QTF. The QTF is mounted on a xyz-scanner of the AFM and a soft AFM probe is approached on the apex of a tine of the QTF by driving the z-servo and using the normal deflection voltage (Vtb) of position sensitive detector (PSD) as feedback signal. Once the tip contacts the tine, servo is switched off. QTF is electrically excited with a sinusoidal signal from OC4 (Nanonis) and amplitude of the QTF's output at transimpedance amplifier (Vtf) and amplitude of VTB (Vp) is measured by individual lock-in amplifiers which are internally synchronized to the phase of the excitation signal of the QTF. Before, the measurements optical lever is calibrated. By relating the both voltages (Vp & Vtf), sensitivity factor of the QTF (αTF) is determined. In the second approach, after the tip contacts the tine, the z-servo is switched off firstly, then the feedback signal is switched to Vp and frequency sweep for the QTF, Vtb as well as z-servo are started, instantaneously. To keep the Vp at set-point the feedback control moves the z-servo to track the vibration amplitude of the QTF and thus the distance traveled by the z-servo (Δζ) during sweep is equal to the fork's amplitude of vibration (ΔxTF). αtf is determined by relating Δz and VTF. Both approaches can be non-destructively applied for QTF sensor calibration. AFM imaging of the AFM calibration grating TGZ1 (from NT-MDT Russia) has been performed with a calibrated QTF sensor.