Achieving 10 nm Spatial Resolution in Thermal Near-Field Microscopy with Sharpened Tungsten Tip

We report a passive infrared near-field microscopy method that achieves ~ 10 nm spatial resolution in the long-wavelength infrared region ( λ = 14.1 ± 0.5 μm). The technique employs a scattering-type scanning near-field optical microscope that requires no external illumination. Instead, the tungsten probe detects thermally excited evanescent waves generated by spontaneous charge carrier fluctuations at the sample surface. A two-step electrochemical etching process, combining AC etching for taper formation and DC etching for apex refinement, was developed to fabricate sharpened tungsten tips with apex radii below 10 nm. Near-field measurements on lithographically patterned Au/SiO 2 samples revealed thermal contrast with ~ 10 nm lateral resolution, surpassing the conventional 20 nm limit. Comparative analysis with a coarser tip confirmed that smaller apex radii yield higher spatial resolution, highlighting the critical role of tip sharpness in passive s-SNOM. This passive approach enables non-invasive, ultra-high-resolution thermal imaging and opens new opportunities for nanoscale infrared metrology.