Cell height reduction by routing over-the-cells

The authors apply the concept of over-the-cell routing to the global routing problem in leaf cell synthesis. A cell layout is divided into 5 routing regions in the model. The global routing problem can be formulated to route those critical net segments over the diffusions or the middle channel such that the overall cell height is minimum. The routing of transmission gates has been a problem because it produces a cyclic constraint in the middle channel. A new routing pattern is proposed which is cyclic free for transmission gate connections. In this approach, tree patterns are generated for each net. Then, the global routing is modeled and solved as an integer linear programming problem. Finally, the detailed routing of the cell layout is completed. All the cell benchmarks were solved in very short time. In comparison with previous results, the approach produced denser layouts