Since in the 2-step process both the removal of the fluorocarbon and the etching of the silicon occur during the same etch step, only a single set of parameters is available to perform two different tasks during this step. To gain more control over the process, a 3-step Bosch process is implemented, as schematically illustrated in Figure . The 3-step process splits the etch step into two parts, removal of fluorocarbon and etching silicon. This process is also referred to as Deposition, Removal, Etch, Multistep or DREM. The removal step is performed using low pressure SF 6 . The lower pressure results in a longer mean free path for the SF 5 + ions, resulting in less collisions and thus a greater directionality. This reduces the loss of fluorocarbon on the walls that is caused by the high angular distribution of the SF 5 + ions. The low pressure also means there are fewer ions and radicals available to etch the silicon, so while some silicon will be etched during the removal step, the amount of etching should remain small due to low concentration of SF 5 + ions and F radicals. The pressure of SF 6 is then increased for the etch step, returning to the faster, quasi-isotropic etching by ions with a wide angular distribution and radicals. The software of the SPTS Pegasus calls the fluorocarbon removal step the "boost step". The boost step is part of the etch cycle, meaning a 3 s etch cycle with 1 s boost would be 1 s of low pressure fluorocarbon removal and 2 s of higher pressure etching. From this point on, the fluorocarbon removal step will be referred to as boost step in keeping with the SPTS software.