Contact Stiffness Measurements with an Atomic Force Microscope
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RIMENTAL INSTRUMENTS AND TECHNIQUE
Contact Stiffness Measurements with an Atomic Force Microscope A. V. Ankudinova,* and M. M. Khalisovb a Ioffe
b Pavlov
Institute, St. Petersburg, 194021 Russia Institute of Physiology, Russian Academy of Sciences, St. Petersburg, 199034 Russia *e-mail: [email protected] Received April 3, 2020; revised April 3, 2020; accepted April 3, 2020
Abstract—We propose a method for improving accuracy of nanomechanical measurements by an atomic force microscope. We describe the contact interaction of the cantilever with the sample using an analytic model taking into account different mechanisms of the cantilever probe operation (it can be clamped or can slide over the sample surface), the geometrical and mechanical characteristics of the sample and the cantilever, and their mutual arrangement. For the case of sliding, a filter is developed for correcting signals of contact stiffness and deformation measured on a sample with a developed relief. The application of the filter is illustrated by images obtained with an atomic force microscope in the visualization regime based on point-bypoint recording of the forced quasi-static interaction of the cantilever probe with the sample. DOI: 10.1134/S1063784220110031
INTRODUCTION In atomic force microscopy (AFM), the force of interaction of the probe with the sample is determined from the force curves. In new AFM regimes, e.g., HybriD (NT–MDT SI), PeakForce QNM (Bruker), and fast volume mapping (Asylum Research), the height of the sample relief and, for instance, its local mechanical properties are determined from these curves. The friction in the probe–sample contact can considerably affect the shape of the force curve. If the cantilever probe slides over the surface, it experiences the action of the normal force alone. Such a force bends the cantilever console, and the bending angle along the console increases monotonically [1]. When the probe is clamped to the sample by a large friction force acting along the surface, the console can sag (bending angle varies nonmonotonically) [1, 2]. In most AFM instruments, the cantilever deformations are recorded using the optical beam deflection (OBD) scheme. The profile of the console angle of deflection is not monitored; only its value at a single point on the console at the focus of the OBD laser is determined. For this reason, the microscope control system does not distinguish between bending and deflection [3, 4], which leads to erroneous results of measurement of the amplitude and direction of the force of interaction. In the OBD scheme, two parameters (bending angle α and torsion angle β) of the console are measured at a chosen point (Fig. 1). This information is insufficient for determining three projections of the displacement vector of the contact point or the applied force. A
commercially reasonable scheme of recording deformations of the cantilever, which combines OBD scheme with an interferometer for measuring the missing third parameter (vertical displacement of the laser focal p
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