Development of a magnetic levitation force microscope

Abstract

Classical atomic force microscopes use a soft cantilever spring to convert the interaction between a tip and a surface into a displacement. This leads to indirect force measurement and instability in the case of attractive forces. We have developed a new kind of force sensor to overcome these limitations. The principle is to place a tip glued on a small magnet in levitation in a magnetic field. A servo-loop ensures the stability of the equilibrium in the vertical direction and allows the measurement of forces. It is monitored by an optical sensor that measures continuously the actual position of the tip with a sensitivity of 10 pm. With such a device it is possible to obtain the van der Waals attraction up to the molecular contact without jump. A digital signal processing board stops the approach of the tip just before contact to prevent any deterioration of the surface during the measurement. This allows images of soft and brittle surfaces, such as a microfiltration membrane. Copyright © 1999 John Wiley & Sons, Ltd.