Imaging contrast in SMM: the mechanical approaches

Scanning Microdeformation Microscopy (SMM) is a dynamic force microscopy technique, which uses a microcantilever vibrating at its resonance frequency (range of some kHz up to several 10 kHz) while scanning a sample surface. It uses a microcantilever associated with a tip of radius in the 1-20 microns range (contact radius is typically about 1 μm). The absolute amplitude and phase of the cantilever vibration as well as the frequency shifts of the cantilever resonance frequencies are measured. Due to the size of the tip, the SMM works on mesoscopic scale, allowing to investigate a large subsurface volume (typically about 10 times the contact radius between the tip and the sample). This paper is devoted to the contrast analysis in the SMM for the investigation of subsurface defects. More precisely we analyse different mechanical approaches to explain the contrast induced by the defects in the subsurface volume. The approach used for each case depends on the materials and geometry (size, depth, elastic properties). Experimental results performed on calibrated samples are presented, allowing us to give a clear analysis of the contrast sources