Torsional Resonance Mode Imaging for High- Speed Atomic Force Microscopy

The instrumentation of high-speed imaging has been a challenge for scanning probe-based technologies. Mechanical stability of the system, surface tracking at sharp topographic transitions and prolonging tip lifetime have been the determining factors for practical applications. In this paper we report a new type of feedback control based on the torsional resonance amplitude (TRmode ) of cantilever probes. Atomic Force Microscope (AFM) dynamics are improved over TappingMode due to the much faster response of the TR amplitude signal when the tip is interacting with a surface. For a given cantilever, the amplitude error generation rate due to the topographic variation of surfaces is substantially higher in torsional resonance mode compared to that while tapping, leading to improvement in feedback loop response. We have demonstrated that the improved dynamics of the new TR imaging mode achieve much better surface tracking during high speed scanning across sharp steps. When torsional resonance amplitude is used in a nested feedback loop applying microactuated ZnO levers, the tracking error at 200 nanometer step edges can be controlled within 3 nm for a scanning speed around one millimeter per second. The tip/surface interaction of TRmode and TappingMode were studied in controlled experiments. In addition, we found that tip wear under torsional amplitude feedback control is comparable to that in low force TappingMode operation.