Quantitative Subsurface Imaging by Acoustic AFM Techniques

Abstract

We review the modeling techniques developed for analyzing the effects of 2-D and 3-D subsurface structures on the stiffness measurements by acoustic AFM. Starting from the analytical Hertzian model, we describe important parameters such as penetration depth and subsurface resolution for acoustic AFM imaging. These definitions point to the need for analytical–numerical models based on mechanical surface impedance method and finite element modeling of arbitrary 2-D and 3-D structures buried under the surface. By using the 2-D and 3-D models, the dependence of penetration depth and subsurface resolution on material properties, subsurface structure geometry, and imaging parameters are investigated. It has been shown that high contrast between subsurface structure and substrate increases the detectability of the structure and the visible depth of the structure depends highly on the contact radius. Soft subsurface structures or voids can be detected with appropriate tip radius and force even if they are as deep as 450 nm. However, the sensitivity is higher while detecting stiff structures under thin soft layers. These results can be extrapolated for different applications using the presented guidelines.