The large - and constantly growing - number of available process simulation tools can be explained with the rapid evolution of semiconductor technology as well as with the lack of a unified and established basis for the development and implementation of TCAD tools. Fast technology changes call for the development of specialized simulation tools, each holding an expert's position for a particular process simulation task. For example, the definition of arbitrary diffusion models on each material segment of a wafer is an advanced feature of the PROMIS NT process simulator [PS95] [Puc96], which other diffusion simulators do not offer. Of all available tools, to optimally model a process technology, it is of paramount importance to use the tool best suited for each process step instead of confining oneself to a particular product's or vendor's solutions. (Cf. Table 4.1 for some examples for existing process simulators; this list shows only a negligible fraction of all process simulation tools in use at universities and in industry).