Active Q control may be used to modify the effective quality (Q) factor of an atomic force microscope (AFM) micro-cantilever when operating in tapping mode. The control system uses velocity feedback to obtain an effective cantilever Q factor to achieve optimal scan speed and image resolution for the imaging environment and sample type. Time delay of the cantilever displacement signal is the most common method of cantilever velocity estimation. Spill-over effects from unmodeled dynamics may degrade the closed loop system performance, possibly resulting in system instability, when time delay velocity estimation is used. A resonant controller is proposed in this work as an alternate method of velocity estimation. This new controller has guaranteed closed loop stability, is easy to tune, and may be fitted into existing commercial AFMs with minimal modification. Images of a calibration grating are obtained using this controller to demonstrate its effectiveness.

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When operating the Atomic Force Microscope in tapping mode it is possible to decrease the quality factor of the microcantilever to enhance scan speed. A new field of Atomic Force Microscopy is evolving, which makes use of multiple frequency excitation and detection of the cantilever modes making it necessary to be able to control these modes and their response to excitation. This work proposes a multimode Q control approach utilizing positive position feedback, offering full control over the first two flexural modes of the cantilever. By completely damping the first mode and adjusting the quality factor of the second mode, it is possible to scan and obtain images at the second resonance frequency which improves image quality at high scan speeds due to the increased bandwidth of the z-axis feedback loop.

T he atomic force microscope (AFM) [1] is a mechanical microscope capable of producing three-dimensional images of a wide variety of sample surfaces with nanometer precision in air, vacuum, or liquid environments. This article provides an overview of the AFM and its three main modes of operation, with a focus on the tapping mode of operation. The challenges associated with obtaining high-speed images with a tapping-mode AFM while minimizing image artifacts are outlined and control techniques that have been developed to overcome these challenges are reviewed.

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