Widening in the middle length of a pore is caused by the balance between ion energy and image force. The positively charges ions continually lose energy as they travel along the negatively charged wall. At some point, which is defined by the energy of the ions and the diameter of the nanopore, the ions will have lost so much energy that they are pulled to the wall by the image force and hence they begin to laterally remove the protecting As the neutral etching species, such as radicals, are not affected by image force and are the majority component of the plasma, after the loss of ions which caused the pore widening the etch continues unperturbed by image force effects.