Scanning capacitance force microscopy and Kelvin probe force microscopy are used to image Sn nanometer sized structures embedded in silicon oxide thin films. The capacitance variation occurring between probe and sample in presence of a metallic cluster modifies the oscillation amplitude of the AFM probe at twice the frequency of the applied voltage. The extreme localisation of the interaction due to the small geometries involved allows a lateral resolution of few nm. Issues related to the contrast mechanism and the spatial resolution are discussed with the support 2D finite element calculation of the electrostatic field distribution between probe and sample.