Pore widening occurs in the middle section of the length of the nanopore as shown in Figures and, and may occur in nanopores with no mask undercut or tapering. Figure shown an example of pore widening in a sample created using a 2-step process. The sample has no mask undercut and a small amount of positive tapering, only the diameter at the middle of the nanopores is larger than the mask diameter. In wider nanopores, for example 500 nm diameter, this phenomenon manifests mainly as a rough appearance in the middle section of the pore as is highlighted in Figure . In smaller diameter nanopores, for example the 280 nm nanopores in Figure , this roughness becomes a widening of the middle section of the pores to a diameter greater than that of the mask. This roughness and widening only affects the middle section of the nanopore. In all cases observed throughout this study, the scallops are visible at the bottom of the pore and often the diameter has returned to that of the mask or even smaller. In an attempt to elucidate when in the etch process this roughness begins to form, the cycle number was incrementally reduced from N = 200, where "funnel-like" profile. Red boxes provide a higher magnification image of the the tops of the pores to further highlight the difference in form at the start of the pore.