Identifying redundant inter-cell margins and its application to reducing routing congestion

A modern standard cell is embedded with extra space, called inter-cell margin, on its left and right ends. Margins are sometimes redundant, and so margins between some cell pairs can be removed for the benefit of area. Lithography simulations on whole layout to identify redundant margins take excessive amount of time, and thus are impractical. We propose to determine in advance the redundancy of margins between each cell pair; a few methods of approximation are introduced to accelerate the process, e.g. grouping cell pairs of similar boundary patterns, refining each group with geometry parameters, etc. Experiments indicate that the redundancy of margin is accurately determined in 93.7% of cell pairs; the remaining 6.3%, which are actually redundant, are declared irredundant by our method, so our method is inaccurate for those cell pairs yet is still safe. We take advantage of redundant margins and address the problem of routing congestion reduction. Placement is locally perturbed to identify more redundant margins; the cells in high congestion region are spread out after the margins in low congestion area are removed. The proposed method was evaluated on a few test circuits using 28-nm technology. The number of routing grids with congestion overflow was reduced by 43% with no impact on total wirelength.