Integrated Approach of Airgap Insertion for Circuit Timing Optimization

Airgap technology enables air to be introduced in inter-metal dielectric (IMD). Airgap between certain wires reduces coupling capacitance due to the reduced permittivity; this can be utilized to decrease circuit delay. We propose an integrated approach of airgap insertion with the goal of circuit timing optimization. It consists of three sub-problems. We first select the layers that employ airgap, called airgap layers, that maximize total negative slack (TNS) improvement; this yields TNS improvement of 7% to 15% and worst negative slack (WNS) improvement of 2% to 8%, compared to a simple assumption of airgap layers. Second, we reassign the layers of wires such that more wires on critical paths can be placed in airgap layers. This is formulated as integer linear programming (ILP), and a more practical heuristic algorithm is also proposed. It provides an additional 17% TNS improvement and 6% WNS improvement. Finally, we perform airgap insertion through ILP formulation, where a number of design rules are modeled with linear constraints. To reduce the heavy runtime of ILP, a layout partitioning technique is also applied. It implements a feasible airgap mask in a manageable time where the amount of inserted airgap is close to the optimal solution.