Determination of Deformation Fields and Visualization of Buried Structures by Atomic Force Acoustic Microscopy

Abstract

Advanced scanning probe microscopy techniques combine atomic force microscopy (AFM) with ultrasound. Atomic force acoustic microscopy (AFAM) and ultrasonic force microscopy (UFM) become increasingly powerful tools for the determination of material properties on nanoscale. AFAM is mainly applied to the analysis of materials with elastic properties locally varying on micro- and nanoscale. In AFAM, flexural and torsional cantilever vibrations are excited by out-of-plane and in-plane sample surface vibrations. The ultrasound is transmitted from the sample into the cantilever while forces act between sensor tip and sample. The sample surface is scanned by the sensor, and an ultrasonic image is acquired simultaneously to the topography image. The contrast comprehended in the ultrasonic image depends on surface topography and on the local elastic and adhesive properties of the sample. Voids, inclusions, or cracks, which represent regions of different elastic constants in the interior of the material, are sensed by the local elastic response of the tip at the sample surface. As a consequence, information on hidden structures can be derived from the acoustic images. Usually, this subsurface information is overlaid by additional topographic information, also contained in the ultrasonic image. Therefore a test sample having only clearly interpretable topographic information was produced, allowing a distinct proof of the sub surface capability of AFAM. Additionally the AFAM set up is combined with tensile and bending modules. This approach allows generation of static deformation fields on surfaces and in-situ imaging and analysis of these fields in the AFM or AFAM. A software module for micro deformation analysis by means of correlation based algorithms (MicroDAC) is used to determine the local surface deformation quantitatively