CALIBRATION OF OPTICAL-LEVER SENSITIVITY FOR ATOMIC-FORCE MICROSCOPY

Accurate force determinations in atomic force microscopy require the p recise measurement of cantilever deflections. A limiting factor in mak ing these measurements is the calibration of the optical lever detecti on system, particularly when the tip cannot be pressed against a hard surface. Here we show that, for a given instrument, a fixed displaceme nt of the photodiode detector produces a change in the detector voltag e that correlates strongly with optical lever sensitivity. This provid es a simple method for optical lever sensitivity calibration not requi ring contact of the tip with any surface. The data also suggest that d ifferences in the shape of the laser spot on the photodiode are a majo r source of variability in optical lever sensitivity. (C) 1995 America n Institute of Physics.