The 2-step process exhibits roughness of the walls from about a third of the length until two thirds of the length, which obscures the scallops. In the 3-step nanopores, the scallops can be observed over the whole length of the nanopore, indicating that the 3-step process offers much more control thus giving a consistent etch for the whole length, as desired from the outset. At the low pressure used in the boost step, the number of ion collisions in the plasma are reduced. This narrows the ion angular distribution and decreases the loss of ion energy through collision, effectively increasing the ion energy at constant depth. Increased ion energy reduces the number of ions lost to the walls to image force and thus reduces the pore widening.