Mixed-cell-height placement considering drain-to-drain abutment

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Along with device scaling, the drain-to-drain abutment (DDA) constraint arises as an emerging challenge in modern circuit designs, which incurs additional difficulties especially for designs with mixed-cell-height standard cells which have prevailed in advanced technology. This paper presents the first work to address the mixed-cell-height placement problem considering the DDA constraint from post global placement throughout detailed placement. Our algorithms consists of three major stages: (1) DDA-aware preprocessing, (2) legalization, and (3) detailed placement. In the DDA-aware preprocessing stage, we first align cells to desired rows, considering the distribution ratio of source nodes to drain nodes. After deciding the cell ordering of every row, we adopt the modulus-based matrix splitting iteration method to remove all cell overlaps with minimum total displacement in the legalization stage. For detailed placement, we propose a satisfiability-based approach which considers the whole layout to flip a subset of cells and swap pairs of adjacent cells simultaneously. Compared with a shortest-path method, experimental results show that our proposed algorithm can significantly reduce cell violations and displacements with reasonable runtime.