Mechanical oscillator tip-to-sample separation control for near-field optical microscopy

A nonoptical system for controlling the tip-to-sample separation in a near field scanning optical microscope is described. It consists of a tapered aluminum-coated fiber glued to a high-Q Si paddle mechanical oscillator. The paddle is driven at one of its resonances by a capacitively coupled electrode, and the amplitude of oscillation is detected through a second electrode. This resonant amplitude, which varies as the tip approaches the surface, is used as a feedback signal to control the separation between tip and sample. A separation stability better than 0.8 nm is achieved in the 0-200 nm range. We present a complete characterization of this system and demonstrate its capabilities with shear-force images of two samples. (C) 1997 American Institute of Physics.