Enhancement of Q factor in quartz tuning fork force sensors for atomic force microscopy through counter piezo excitation

Abstract

Quartz tuning forks (QTFs) are highly effective force sensors in frequency modulation atomic force microscopy (FM-AFM), owing to their inherently high quality factor (Q). This attribute significantly enhances force sensitivity and stability during surface imaging. Our study introduces a counter piezo driving scheme to restore the diminished Q of QTFs affected by mass imbalances, which may arise from geometric discrepancies or the attachment of a tip to one prong. The approach involves vibrating the two prongs of the QTF using piezo elements situated at the joint, applying differing voltages. This strategy adjusts the mechanical coupling between the prongs, thereby influencing the Q. Unlike traditional methods that focus on prong-mass tuning, this technique does not require intricate modifications to the prong structure. Experimental validation was achieved with asymmetrically structured QTFs featuring prongs of varying thickness. A notable increase in the Q was observed, several times higher than that achieved with single piezo element driving, depending on the degree of asymmetry. The findings were corroborated by finite element method simulations, which not only confirmed the substantial Q enhancement in tip-attached QTFs but also elucidated the underlying mechanism. It was demonstrated that precise tuning of piezo element voltages effectively aligns stress vectors at the joint, leading to highly efficient excitation of resonance oscillation, resulting in a significantly improved Q.