Oscillating String as Force Sensor in Scanning Force Microscopy

        We present a sensor that uses an oscillating string to detect forces. A cantilever beam serves as a sample stage.
        The string is attached to the free end of the beam along its deflection axis. A change of the force on the free end of the beam modifies the string tension and hence the resonance frequency of transverse oscillations. These oscillations can have amplitudes of the order of microns without causing noticeable wavering of the cantilever beam.
        The scheme is particularly suitable for measuring contact forces for which stiff sensors are required. A prototype was built using a carbon fiber 5 um in diameter and 4 mm in length, oscillating at 4 kHz and attached to a beam with a spring constant of 100 N/m. A force resolution of 4 nN was achieved in vacuum for a detection bandwidth of 300 Hz.
        For a conventional beam deflection sensor of equal stiffness this corresponds to a deflection sensitivity of 0.4 angstrom. To detect the fiber oscillation, a laser beam is focused onto the fiber and the bypassing light is absorbed in a photodiode. To excite the resonance, the fiber is placed in a magnetic field perpendicular to the fiber and an ac current is sent through it.

By: A. Stalder, U. Durig

Published in: Proceedings of NATO ASI on Forces in Scanning Probe Methods, edited by H.-J. Guenterodt et al., Dordrecht, Kluwer, p.69-75 in 1995

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