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

Abstract

Amplitude calibration of the quartz tuning fork (QTF) sensor includes the measurement of the sensitivity factor (α TF ). 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 (V tb ) 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 (V tf ) and amplitude of V TB (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 & V tf ), sensitivity factor of the QTF (α TF ) 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, V tb as well as z-servo are started, instantaneously. To keep the V p 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 (Δx TF ). α tf is determined by relating Δz and V TF . 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.