Detecting structures below a cover film at the nanoscale resolution is of essential importance. In this work, we explored factors affecting subsurface material contrast and structural visibility in scattering-type scanning near-field optical microscopy (s-SNOM). A kind of multilayered reference samples containing different buried structures was fabricated and applied for s-SNOM imaging. The dependence of near-field optical contrast on structure geometry, dimension and cover thickness was investigated. Results demonstrate that distinguishing the buried slit pattern is easier than the circular hole with the same critical dimension. The s-SNOM can sense material difference under a more than 100 nm thick polymethyl methacrylate layer and it has a subsurface spatial resolution better than 100 nm.

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