Metal Stack and Partitioning Exploration for Monolithic 3D ICs

Abstract

In this work, we investigate the effect of metal stack and tier 3D IC partitioning methodologies on the Quality of Results (QoR) of monolithic 3D circuits compared to their 2D counterparts. Two interconnect options are considered. For the interconnect option, termed Single, a single metal stack is used where cell pins lie on two lower metal layers. A Face to Face (F2F) interconnect option is also considered where cell pins lie on symmetrical lower and upper metal layers in two different tiers. In addition, two 3D circuit partitioning methodologies are investigated including the Greedy Bin-Based Fidducia-Mattheyses (GBBFM) and a Displacement-based 3D Legaliser (3DLG). For both the 2D and 3D circuits, a multi-pass timing-driven In-Place Optimisation (IPO) is performed with an industrial P&R tool to extract the best QoR. For the 3D circuits, the IPO is applied after tier partitioning. The 45 nm nanoCAS library, a 3D library based on NANGATE 45 nm, is utilized and three typical benchmark circuits are analysed. The performance of 3D circuits is improved between 1% to 3%, total wirelength is significantly reduced, via usage is increased, yet the estimated power and cell area do not necessarily decrease. Single metal stack overall demonstrates better QoR than F2F integration.