Visibility of buried structures in atomic force acoustic microscopy

Advanced Scanning Probe Microscopy (SPM) modes such as Atomic Force Acoustic Microscopy (AFAM) and Ultrasonic Force Microscopy (UFM) combine Atomic Force Microscopy (AFM) with an excitation of the sample or cantilever by ultrasound. These techniques become increasingly powerful tools for the determination of material properties on nanoscale. Non-destructive evaluation of subsurface and buried structures is getting more and more important in semiconductor industries and electronics system integration technology. Existing methods that allow subsurface measurements with high local resolution are mostly based on destructive concepts as surface ablation by Focused Ion Beam (FIB) devices. It is widely discussed in literature that AFAM and UFM techniques should have the capability to detect subsurface features. But direct proofs of this capability are hard to find. The difficulty comes from the point that in UFM and AFAM images besides elastic contrast also topological contrast is mixed in. So, for a direct proof samples are needed which (a) show subsurface contrast and (b) having definitely no surface topology correlated with the subsurface feature in question. These samples are not so easy to obtain. An appropriate sample fabrication technology was developed based on the focused ion beam technique. Using the machined samples the buried structure visibility for the AFAM technique could be proved uniquely. The results are compared with conclusions from modeling.