Our ability to develop and build ever smaller microelectronic devices depends strongly on the capability to generate a desired device pattern in an image layer (photoresist) by lithography and then to transfer this pattern into the layers of materials of which the device consists. In the past the pattern transfer was almost exclusively accomplished by wet etching. Chemical dissolution of a film region that had to be removed took place in a suitable solvent. Although the wet etching processes stop precisely at a chemically different underlying layer, they typically have isotropic etch characteristics, which cost the researcher control over critical lateral dimensions. Such a tradeoff is not acceptable in the manufacture of micron- and submicron-scale devices, and wet, solution-based etching techniques were replaced in the late 1970s by dry, directional etching processes using plasmas or ion beams. Figure 1 shows a plasma-based dry etching system at IBM. In dry etching processes, surface atoms are removed via momentum transfer from bombarding, directed ions (physical sputtering), thus achieving a directional etch.