FPGA-Based Characterization and Q-Control of an Active AFM Cantilever

The slow transient response of Si cantilevers oscillating at resonance is a significant limiting factor to achieving higher scan rates in tapping-mode atomic force microscopy (TM-AFM). During a typical tapping-mode operation in air, when the cantilever encounters a steep drop in the topography, it experiences a transient response whose time constant is a function of the resonant frequency and quality factor (Q) of the cantilever. In order to achieve accurate surface tracking at higher scan rates, it is desirable to have a high resonance frequency and a relatively low Q factor. In this work, we demonstrate an active cantilever with resonant frequency of 46 kHz, featuring a piezoelectric layer for simultaneous actuation and sensing of cantilever's vibrations. A field programmable gate array (FPGA) is employed to mitigate the electrical feedthrough from the on-chip actuation to sensing lines for achieving a high dynamic range and also controlling the quality factor of the AFM microcantilever for higher scanning rate dynamic atomic force microscopy. After the recovery of cantilever dynamics and system identification, a dynamic range of 19.8 dB is achieved. For actively controlling the Qfactor a positive position feedback controller is implemented that results in a reduction in quality factor of the cantilever from 268 to 81.7. AFM imaging results with both uncontrolled and controlled Q factor are exhibited, demonstrating a faster response time to image surface topographies.