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1.3 Topography Simulation

The considerations from above underline, why simulation is rapidly gaining importance. Simulation and modeling is not restricted to special fields but has already been implemented successfully for all unit steps itemized in the previous section. Besides the development of device simulators, important work has been done in process simulation, where a considerable number of commercial and academic software packages for the simulation of lithography, development, ion implantation, diffusion, oxidation, metalization, etching, and deposition steps is available. Even so, and especially when talking about three dimensions, generation of virtual devices, suitable as input for device simulators relies to a major part on interactive editing of structures and contacts, applying analytical doping profiles. Three-dimensional simulation is a challenging task in all applications, since it requires concepts for efficient use of memory and CPU time resources. Nevertheless, shrinking feature sizes bringing along three-dimensional phenomena not longer screened by two-dimensional effects predominant in larger structures not only justify but demand the development of three-dimensional models and simulators. This is especially pronounced for topography simulation, the only tool available for the physically based, step by step generation of device geometries from the layout.

Topography simulation in general comprises all process steps changing the shape of the wafer, thus including also oxidation. Oxidation is a very complex process and a field of scientific interest of its own. Careful modeling has to deal with a broad variety of effects ranging from quantum chemistry of oxide formation, diffusion, defect generation, and recombination up to surface advancement techniques and interface tracking with moving, expanding and colliding grids. For these reasons the definition of topography simulation within the scope of this thesis is restricted to etching and deposition, still giving space for an abundant number of resist development, sputtering, plasma, and metalization processes.

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