The examples of negative tapering, or widening of the bottom of the nanopores, found in the course of this work have been few, though negative taper was briefly mentioned in section III B 3. An example of negative taper is shown in Figure . In this case the effects of varying the length of the boost step was investigated. A 2 to 4 second etch step time was used and the boost step time was increased from a 1 to 2 s ramp in Figure to 1 to 3 s ramp in Figure .