Segment-based etch algorithm and modeling

A novel approach to modeling the etch process is presented. A structure is represented as a string of nodes connected by line segments. By focusing on the segments one can deduce the behavior of the nodes and derive accurate etch models. These etch models handle naturally the rounding of a corner, handle precisely discontinuities due to material boundaries or shadow edges, and handle fully the complexities of angle-dependent etching. An efficient and accurate node movement algorithm has been developed. Nodes are allocated solely on the basis of curvature and adjust automatically as the structure evolves. The normal tradeoff between time step and accuracy is avoided by moving each node independently in time. This also allows nodes to be stopped at collisions, eliminating the need to detect and deal with loops