Ultra-thin h-BN films with precisely controlled thickness are important for fabricating 2D-material based nanostructures and devices. We developed a layer-by-layer etching method to obtain h-BN films with single atomic level accuracy. The method involves structural deformations of the top layer of the h-BN film by oxygen-radical adsorption, followed by the removal of the weakened layer using nitrogen-ion bombardment. The mechanism is verified by ab initio simulations and experimental results. The fabricated h-BN films show low leakage current and high breakdown field even with only a few nanometers thickness.
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