While mask undercut and scallops may appear to be two separate issues, they are integrally linked. Mask undercut is caused by the quasi-isotropic nature of the etch step, most importantly in the first few etch cycles which form the most pronounced scallops. The initial 5 to 10 scallops are often much wider and longer than subsequent scallops. The increased width is due to the effective angular distribution of the ions which decreases with nanopore depth. As is shown in Figure , early on after a few etch steps when the nanopore is shallow, ions with a broad angular range θ, can readily access the exposed silicon wall which results in lateral etching to form the early scallops. After more and more steps, see Figure , the walls at the top of the nanopore are protected by fluorocarbon and the deeper, exposed silicon walls are only accessible by ions with an increasingly narrow angular range θ. This reduction in angles with depth also affects the F radicals, which have no directionality and are a significant part of the plasma, though at higher pressures radical-radical collisions within the pore will increase the angular range of the etching radicals. The length of the scallops is related to the number of reactive species, ion and radicals, that are available to etch during an etch step. The decrease in scallop length with increasing nanopore length is due to the decreasing number of available species for etching in deep pores. Firstly, ions and radicals traveling at oblique angles with respect to the surface, that is, more oblique than the critical angle θ for a particular depth (as shown in Fig. ), will be lost to the side walls. Secondly the influence of image force becomes more prominent as the nanopore lengthens as the ions must successfully pass a longer silicon wall before reaching the exposed silicon bottom to etch. Thirdly, only ions with sufficient energy from the whole energy distribution can reach the end of a longer pore with enough energy to remove the protective fluorocarbon layer and continue the etching process.