Development of a magnetic levitation force microscope

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 fo rces. We have developed a new kind of force sensor to overcome these limita tions. The principle is to place a tip glued on a small magnet in levitatio n in a magnetic field. A servo-loop ensures the stability of the equilibriu m in the vertical direction and allows the measurement of forces. It is mon itored 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 t o obtain the van der Waals attraction up to the molecular contact without j ump. A digital signal processing board stops the approach of the tip just b efore contact to prevent any deterioration of the surface during the measur ement. This allows images of soft and brittle surfaces, such as a microfilt ration membrane. Copyright (C) 1999 John Wiley & Sons, Ltd.