1. A non-transitory computer-readable medium holding computer-executable instructions for modeling plasma etches on a 3D structural model of a semiconductor device, the instructions when executed causing the computing device to: receive, for a semiconductor structure to be virtually fabricated in a computing-device generated virtual fabrication environment, a selection of 2D design data and a process sequence that includes a plurality of processes; receive a set of material-specific behavioral parameters for a plurality of types of etch behavior to be respectively applied to a plurality of etchable materials in at least one etch process in the process sequence, each material-specific behavioral parameter being a behavioral parameter for a type of etch behavior for an indicated one of the plurality of ecthable materials, at least one of the types of etch behavior being taper behavior, the taper behavior caused by a combination of directional etching and polymer deposition; perform with the computing device a virtual fabrication run for the structure using the process sequence and 2D design data, the virtual fabrication run building a 3D structural model containing the plurality of etchable materials and an etchable surface including one or more etchable materials, the virtual fabrication run using the material-specific behavioral parameters to simulate the etch process by evolving the etchable surface during the virtual fabrication run.