As the pore becomes longer, the influence of image force increases and the angular range θ becomes narrower reducing the effective number of reactive species resulting in the etch becoming slower. Increasing the etch step time as the nanopore length increases, should allow more reactive species to access the bottom of the pore, thus increasing the length. To this end, the t f was increased beyond 3 s. Figure shows the top of a set of nanopores with a 2 to 3 s ramped etch step time, exhibiting distinct scallops and minimal mask undercut. When t f is increased to 4 s to give a 2 to 4 s ramped etch step time, the scallops lose definition, becoming uneven, the nanopores also become wider, see Figure . When we further increase t f to 5 s, parts of the silicon walls between the nanopores to begin to break down. Longer etch step time gives more time for removal of the protecting fluorocarbon from the walls before re-protection, causing the walls to become laterally etched. This means that one must always compromise between nanopore length, as longer nanopores require longer t f , and the integrity of the walls of the nanopore.