Solid-state nanopores offer unique possibilities for biomolecule sensing; however, scalable production of sub-5 nm pores with precise diameter control remains a manufacturing challenge. In this work, we developed a scalable method to fabricate sub-5 nm nanopores in silicon (Si) nanomembranes through metal-assisted chemical etching (MACE) using gold nanoparticles. Notably, we present a previously unreported self-limiting effect that enables sub-5 nm nanopore formation from both 10 and 40 nm nanoparticles in the 12 nm thick monocrystalline device layer of a silicon-on-insulator substrate. This effect reveals distinctive etching dynamics in ultrathin Si nanomembranes, enabling precise control over nanopore dimensions. The resulting nanopore sensor, suspended over self-aligned spheroidal oxide undercuts with diameters of just a few hundred nanometers, exhibited low electrical noise and high stability due to encapsulation within dielectric layers. In DNA translocation experiments, our nanopore platform could distinguish folded and unfolded DNA conformations and maintained stable baseline conductance for up to 6 h, demonstrating both sensitivity and robustness. Our scalable nanopore fabrication method is compatible with wafer-level and batch processing and holds promise for advancing biomolecular sensing and analysis.