Fig. 1: Schematic of MPSN fabrication process, related control signals, and resultant MPSN.

a Pre-etching process using KOH to achieve remaining thickness H, i.e., distance between tip of micro-pyramid and silicon/etchant interface, ranging from 114 nm to 19 nm (i); local etching process using photoinhibition-assisted KOH etching to regulate etching rate and final pore size, i.e., the narrowest opening diameter, of MPSN (ii and iii). b Representative transmission spectra used to closely monitor remaining thickness during pre-etching process. Spectral peak λ_p of 520 nm corresponds to H of 1270 nm. c The spectral peak was measured using a spectrometer, while the remaining silicon thickness was measured using a contact profilometer. The black dots represent experimental data from 4 samples, and the red line represents the fitting result. Based on four wafer tests, the standard deviation of the measured remaining silicon thickness at the tested spectral peak is approximately 8 nm. d Current signal (I_P, blue line) measured during local etching process. Depending on remaining thickness H, different irradiation t_imes t_i are needed to achieve silicon perforation, leading to over-etching stage signified by current ramp with slope tanθ_I over 0.5 nA/sec. Pore size of MPSN is then precisely controlled by over-etching time t_o. Front-view SEM image (e) and back-view TEM image (f) of small MPSN S₀ with pore size of 4.4 nm fabricated using remaining thickness of 19 nm, irradiation time of 20 s, and over-etching time of 0.6 s. Pore size is further characterised using image grayscale analysis (g) of red line in f.