Design Implementation With Noninteger Multiple-Height Cells for Improved Design Quality in Advanced Nodes

Abstract

Standard-cell libraries can be developed with different cell heights (e.g., in FinFET technology, corresponding to different numbers of fins). Larger cell heights provide higher drive strengths, but at the cost of larger area and power consumption as well as pin capacitance. Cells with smaller heights are relatively smaller in area, but have weaker drive strengths and are more likely to suffer from routing congestion and pin accessibility issues. Existing design methodologies and tool flows are able to mix cells with different heights at the block level (i.e., each block contains cells with heights being integer multiples of a particular “single row” cell height). To our knowledge, no design methodology in the literature or in production mixes cells with different, noninteger multiple heights in a fine-grained manner. In this paper, we propose a novel physical design optimization flow to implement design blocks with mixed cell heights in a fine-grained manner. Our optimization resolves the “chicken-and-egg” loop between floorplan site definition and the optimized choices of cell heights after placement with full comprehension of the constraints and costs of mixing cells of different heights (e.g., the “breaker cell” area overheads of row alignment between sub-blocks of 8 T and 12 T cell rows), our optimization achieves up to over 30% area and power reductions versus 12 T-only implementation while maintaining the same performance, and up to over 10% performance improvement along with power and area reductions versus 8 T-only implementation.