A novel phase-shift-based amplitude detector for a high-speed atomic force microscope.

In any atomic force microscope operated in amplitude modulation mode, aka "tapping mode" or "oscillating mode," the most crucial operation is the detection of the cantilever oscillation amplitude. Indeed, it is the change in the cantilever oscillation amplitude that drives the feedback loop, and thus, the accuracy and speed of amplitude detection are of utmost importance for improved atomic force microscopy operation. This becomes even more crucial for the operation of a high-speed atomic force microscope (HS-AFM), where feedback operation on a single or a low number of cantilever oscillation cycles between 500 kHz and 1000 kHz oscillation frequency is desired. So far, the amplitude detection was performed by Fourier analysis of each oscillation, resulting in a single output amplitude value at the end of each oscillation cycle, i.e., 360° phase delay. Here, we present a novel analog amplitude detection circuit with theoretic continuous amplitude detection at 90° phase delay. In factual operation, when exposed to an abrupt amplitude change, our novel amplitude detector circuit reacted with a phase delay of ∼138° compared with the phase delay of ∼682° achieved by the Fourier analysis method. Integrated to a HS-AFM, the novel amplitude detector should allow faster image acquisition with lower invasiveness due to the faster and more accurate detection of cantilever oscillation amplitude change.