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3.1.6 Oxidation

The last process step to be introduced within the scope of solid modeling is oxidation. Similar to CMP oxidation is a process step composing a very complex scenery of diffusion, chemical reactions, volumetric expansion, and mechanical stress. It is far beyond the solid modeling approach to cover all these aspects. Just for tracking the evolution of the resulting geometry, it is enough to apply a ``mask'' with a variable thickness which follows some fitting function. Obviously, this simple, geometric method does not allow any statement about resulting mechanical stresses or doping distributions. Nevertheless it expands the possibilities for geometry generation and improves the results of resistance and capacitance extraction for metal lines applied on top of the highly non-planar oxides.

Figure 3.7: Bipolar transistor with field oxide.

Fig. 3.7 shows the topography of a bipolar transistor where the oxidation module was used to generate the field oxide. Beside the circuit behavior which is determined by the doping profile inside the silicon, the current densities and the resulting electro-thermal effects in the poly-silicon deposited onto the oxide determine the operational characteristics of the device. Accurate simulation of the effects in the poly-silicon lines are supported and improved by the rigorous, non-planar treatment of the geometry.

The simplified geometric model of oxidation concludes the section about solid modeling. All steps introduced above have in common, that they only consider the change in the shape of the structures accompanying the single process steps. The change in the topography is emulated with geometric models not necessarily based on the physical principles underlying to the formation of the considered processes.

Similar to these solid modeling approaches, the process steps introduced in the next section also use geometric models based on morphological operations. In contrast to the purely geometric emulation from above, the morphological operations used next are a representation of the physical principles building the base for the etching and deposition steps under discussion. Therefore the models for isotropic, anisotropic and unidirectional processes form a link between geometrically motivated solid modeling and physically based topography simulation.

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W. Pyka: Feature Scale Modeling for Etching and Deposition Processes in Semiconductor Manufacturing