Dispersion of IDT-induced high-frequency surface acoustic waves – Application to the mechanical and dimensional characterization of mesoporous silicon

A method based on the dispersion of surface acoustic waves (SAW) is proposed for the effective characterization of mesoporous silicon (MPS) layers with the possibility of estimating Young's modulus and the thickness of the porous layer, as well as the porosity. For this purpose, IDT were developed to generate SAW in the frequency range of 20 MHz to 250 MHz. Then, the displacement generated by these waves at several points on the surface of the silicon wafer was detected using a Polytec UHF-120 vibrometer. From these displacements, it was then possible to determine the experimental dispersion curves using the Slant Stack transform. Finally, an inversion method was implemented to estimate the thickness, Young's modulus, density, and porosity of the MPS. In this study, the MPS samples were produced by electrochemical anodization of 2-mm thick Si (100) wafers in an ethanoic hydrofluoric acid solution. The current densities and anodization time of the heavily doped P++ silicon samples were chosen between 20 mA/cm2 and 200 mA/cm2 and 100 s and 3600 s, respectively. Finally, the thicknesses and Young's moduli were compared to those obtained using the Scanning electron microscopy (SEM) photography and the nanoindentation technique.