Capacitive coupling is most commonly used in NEMS/MEMS or in Scanning Probe Techniques to either induce a displacement or to detect an external interaction applied to the micro/nanosystem. A parabolic elastic deformation of the sensor signs a capacitive interaction as the applied voltage is varied. In this paper, we present detailed force measurements performed in the submicron range with a picoNewton sensitivity, and using a UHV AFM and a silicon microlever equipped with a metallized microsphere,. A complete treatment of the sensor sphere/plane geometry enables one to detect quantitatively a V4 departure from the parabolic behaviour. This shows that using this setup allows to detect the increased deformation of a lever much before the mechanical instability. More importantly, this effect must be carefully taken into account if sphere/plane absolute separations are to be quantitatively measured on the basis of electrostatic calibration. To our knowledge, this behaviour in AFM is reported for the first time. We believe that it is relevant for the description of NEMS/MEMS behaviour. Moreover, it provides a method to accurately measure the lever spring constant without any direct contact.
