  In this paper, the dynamic behaviour of an oscillating tip-microlever system
at the proximity of a surface is discussed. The attractive tip-surface
interaction is simply described with a Van der Waals dispersive term and a
sphere-plane geometry. We show that the non linear behaviour of the oscillator
is able to explain the observed shifts of the resonance frequency as a function
of the tip-surface distance without the need of introducing a particular short
range force.
