Appendix: Calibration of 3D-AFM

Accurate CD-AFM measurements require calibration of both the tip width and the lateral scale. Tip width calibration uses a reference structure with a known, near-vertical sidewall — a vertical parallel structure (VPS). The tip is scanned across the VPS and the measured apparent linewidth, minus the known true linewidth, gives the effective tip width including the flare geometry. This is precisely the calibration application for which the grid resolution standard in this project is intended: the PBW-fabricated grid, with a demonstrated sidewall angle of 89.66° and sub-nanometre roughness, provides a well-characterised VPS against which CD-AFM tip width can be established traceably.