Surface and Bulk Investigation of Materials with Scanning Microdeformation Microscopy: What is Really Investigated?
Many near-fiield acoustic microscopes have been developed in order to analyze the local elastic or viscoelastic properties of solids [1–4]. Most of them derive from AFM and use a vibrating tip in contact with the sample. These microscopes enable surface investigation of solids but the volume in the vicinity of the surface also contributes to the detected signal. Thus, SNAM (Scanning Near-Field Acoustic Microscopy) could be either used to measure the topography of the surface or to estimate the local elastic properties if the local contact radius is known. Our technique, called SMM (Scanning Microdeformation Microscope)[5,6], operates at the mesoscopic scale: the contact radius is typically in the 100nm-1µm range. This scale yields a larger subsurface investigation because the investigation depth is mainly related to the contact radius (with AFM tips, the contact radius is typically ~10 nm without surface damage and the investigation depth is restricted). The investigation depth is not easily estimated because many parameters have to be taken into account: sensitivity of the vibration probe, contact section, sample geometry and inhomogeneities, acoustic properties of the sample support and working frequency. The aim of this paper is to give a global approach of this problem, based on solid mechanics, on near-field acoustics and on experimental results. The different potential interpretations of the image contrast are discussed.
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