Direct transistor-level layout for digital blocks

We present a complete transistor-level layout flow, from logic netlist to final shapes, for blocks of combinational logic up to a few thousand transistors in size. The direct transistor-level attack easily accommodates the demands for careful custom sizing necessary in high-speed design, and is also significantly denser than a comparable cell-based layout. The key algorithmic innovations are (a) early identification of essential diffusion-merged MOS device groups called clusters, but (b) deferred binding of clusters to a specific shape-level layout until the very end of a multi-phase placement strategy. A global placer arranges uncommitted clusters; a detailed placer optimizes clusters at shape level for density and for overall routability. A commercial router completes the flow. Experiments comparing to a commercial standard cell-level layout flow show that, when flattened to transistors, our tool consistently achieves 100% routed layouts that average 23% less area.