Nanoguitar: Oscillating string as force sensor

Abstract

A force sensor is presented that makes use of an oscillating string. In a conventional scanning force microscopy setup comprising a tip and a sample consisting of a flexible cantilever beam, a string is attached to the free end of the beam along its deflection axis. Changes of the tip-sample interaction force modify 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. This kind of sensor is particularly suitable for applications that require a stiff sensor. A prototype was built using a carbon fiber 5 μm in diameter and 4 mm in length, oscillating at 4 kHz. A force resolution of 2.5 nN was achieved in vacuum for a response time of 1 ms and a sensor stiffness of 160 N/m. For a conventional beam deflection sensor of equal stiffness this corresponds to a deflection sensitivity of 0.015 nm. © 1995 American Institute of Physics.