Imaging buried features with terahertz s-SNOM*

Scattering-type scanning near-field optical microscopy (s-SNOM) at terahertz (THz) frequencies has emerged as a powerful tool for nanoscale imaging and spectroscopy. It is increasingly used as a non-contact probe of devices with buried structures, where resolving features deeper than the tip radius is challenging. Here we demonstrate THz near-field imaging of metal interconnects buried ∼200 nm beneath SiO₂ on a thinned Si integrated circuit. Using a 40-nm-radius PtIr tip in tapping AFM and broadband THz excitation, we raster-scan a 4×4 µm² region containing three copper lines capped by ~200 nm SiO₂, and demodulate the scattered field at the 2nd–4th harmonics. The buried lines are well resolved at all harmonics. These results demonstrate THz s-SNOM imaging on buried features at depths >5× the tip radius, enabling nondestructive mapping of embedded structures through dielectric capping layers. We also compare our experiments to finite-dipole-model simulations and use them to explore how tip size, layer thickness, and illumination direction influence image contrast and resolution.