7.1.6 Process Control

The initial step is the generation of the input geometry for the simulation [Fig. 7.5(a)]. This step is carried out interactively with the solid modeling tool introduced in Chapter 3 and performed on the cellular geometry representation. Once the input geometry is defined, the following surface extraction supplies a triangular surface description [Fig. 7.5(b)] which serves as input for the meshing procedure. Meshing results in a three-dimensional unstructured tetrahedral mesh depicted in Fig. 7.5(c). The following FEM calculation delivers the steady state distribution of the reactants and the deposition rate [Fig. 7.5(f)] on the tetrahedral mesh, which has then to be transferred back to the surface cells of the cellular structure. The surface propagation is again carried out on the cellular geometry representation [Fig. 7.5(e)]. Unless the total deposition time or the required film thickness is reached, the circular flow has to be restarted from the surface extraction. Otherwise, the final geometry is obtained in the cellular format and the simulation is finished [Fig. 7.5(d)].

**Figure 7.5:** Results of the single modules composing the overall high-pressure CVD model and information transfered between the single modules: Initial structure consisting of a circular via with a physical vapor deposition (PVD) barrier layer, extracted surface at an intermediate time-step, three-dimensional mesh, final geometry, propagated surface, and distribution of the reduced gas species resulting from the continuum transport and reaction simulation.
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Hence, the control system has to handle four different tools (surface extraction, meshing, FEM solver, and surface propagation) and has to make sure that the four different file formats (cellular geometry, triangulated surface, tetrahedral mesh, and resulting concentration distribution on the triangulated surface) are correctly transferred between the tools. Furthermore it has to trace the actual film thickness and the elapsed time in order to decide for the correct termination of the simulation.

With respect to the complex combination of the specialized tools, the most critical issue for the control system is that all modules are executed without necessitating any further user interaction. The crucial point hereby is the automation of the meshing tool, which is everything else than a trivial task in three dimensions.

Fortunately, the automatic mesh generation was assured by the advancing front algorithm of the very stable and mature meshing tool being at disposal [16]. When the surface has propagated after each time-step of the deposition procedure it is possible that some of the initially defined points of the tensor product point clouds are ``below'' the new surface. Since the advancing front used for meshing propagates from the initial surface in a well defined direction, those points are never reached and implicitly discarded. Therefore it is possible to use one and the same control file and point clouds for the meshing of the structures of different time-steps. Thus, the very important requirement for the automation of the simulation sequence is fulfilled.