Pin Accessibility-Driven Detailed Placement Refinement

The significantly increased number of routing design rules at sub-20nm nodes has made pin access one of the most critical challenges in detailed routing. Resolving pin access issues in detailed routing stage may be too late due to the fixed pin locations, especially in the area with high pin density. In placement stage when cell movement is allowed, the consideration of pin access has more flexibility. We propose a refinement stage after detailed placement to improve pin access. To respect the given placement solution, the refinement techniques are restricted to cell flipping, same-row adjacent cell swap, and cell shifting. A cost function is presented to model pin access for each pin-to-pin connection. Based on the cost function, two phases are proposed to improve pin access for all the connections simultaneously. In the first phase, we refine the placement by cell flipping and same-row adjacent cell swap. The problem is solved by dynamic programming row by row. In the second phase, only cell shifting is used, and a linear program is formulated to further refine the placement. Experimental results demonstrate that the proposed detailed placement refinement can improve pin access and reduce unroutable nets by about 33% in the detailed routing stage.