The process design Aid (PDA), a design environment which bridges the gap between design and specification of semiconductor fabrication processes, is presented. The PDA, for the first time, uses a hierarchical process model supporting reuse of well-characterized process fragments, integrates design goals with process and simulation data in a single knowledge base, and provides direct support for the numerical optimization of process parameters and simulator calibration parameters. Details of a hierarchical, object-oriented process representation are discussed. Process objects provide high-level building blocks for process design, focusing attention on critical design parameters and supplying default values for other parameters. Spectral editors for process objects, simulation objects, mask data, and process flows; a menu-driven interface; and online documentation make the PDA easy to use. Results of modeling a 0.8-μm BiCMOS process with the PDA are presented, demonstrating automatic generation of consistent simulation and manufacturing specifications, efficient incremental simulation, and fast, interactive, closed-loop optimization of process parameters. 89% of the process parameters required for the BiCMOS process were supplied as defaults by the PDA process library. Incremental simulation greatly accelerated optimization of the width of a transistor base.