A crystal oscillator based reactance sensor for Atomic Force Microscopy

Frequency modulation Atomic Force Microscopy transduces force gradient to frequency shift of an oscillating probe driven on resonance in a phase locked loop. The technique allows for enhanced sensitivity while overcoming the bandwidth limitations associated with high-Q resonators. The sensitivity of the transducer is limited by noise in the external instrumentation typically used to detect the deflection, amplify the signal, and actuate the probe. We propose an alternative transducer concept modeled on the oscillator-based reactance sensor. The reactive element in the oscillator circuit is an AlN piezoelectric length extension resonator operating at radio frequency. The tip-surface force gradient alters the motional capacitance, or effective mechanical stiffness of the piezoelectric resonator, resulting in a relative shift of the oscillation frequency. In our proposed transducer design the sensivity is limited by the phase noise of the oscillator circuit. A sensor-reference oscillator pair form a differential frequency measurement scheme to reject common mode noise and maximize responsivity to frequency shift. The compact architecture is well suited to low temperature and ultra-high vacuum.