How to Cope with Slow Transistors in the Top-tier of Monolithic 3D ICs: Design Studies and CAD Solutions

In this paper we study the impact of low thermal budget process on design quality in monolithic 3D ICs (M3D). Specifically, we quantify how much the tier-to-tier transistor performance difference affects full-chip power and performance metrics in a foundry 14nm FinFET technology. Our study first shows that 5%, 10%, and 15% top-tier device degradation in a wire-dominated, timing-closed monolithic 3D IC design leads to 7%, 12%, and 18% full-chip timing violation, respectively. Next, we address this impact with our CAD solution named Tier-Aware M3D (TA-M3D) flow that identifies potential timing-critical paths and partitions them into the faster (bottom) tier to minimize the top-tier degradation impact. One unique challenge in timing closure in this case, is how to conduct buffering and sizing on the paths that lie entirely in the top or bottom-tier as well as those that span both tiers. Our approach handles all 3 types of paths carefully and closes timing under the given top-tier degradation assumption, while minimizing the total power consumption. Our enhanced monolithic 3D IC designs, even with 5%, 10%, and 15% slower transistors in the top-tier, still offers 26%, 24%, and 5% power savings over 2D IC, respectively. Our study also covers other types of circuits.