Shear force interaction in the viscous damping regime studied at 100 pN force resolution

We report a very good force resolution for tuning fork based shear force microscopy as used for feedback regulation in scanning near-field optical microcopy (NSOM). The sensitivity and dynamics of fiber tips attached to 100 kHz tuning forks are investigated both experimentally and theoretically applying a finite element analysis. Operating the tuning fork at vibration amplitudes smaller than 10 nm allows to discriminate between viscous damping due to capillary wetting, and fiber bending upon tip-sample approach to hydrophilic sample surfaces indicating the direct transition from "noncontact operation" (pure viscous damping due to contamination layer) down to tip-sample contact. Viscous damping manifests in frequency shifts of less than 50 mHz, as deduced from resonance curves recorded under feedback control. For relative amplitude changes of less than 0.5% the viscous damping force acting lateral on the fiber tip is calculated to similar to 100 pN using the finite element method. This detection limit proves that tuning fork based shear force control is superior to other feedback mechanisms employed in NSOM. (C) 2000 American Institute of Physics. [S0021-8979(00)02106-X].