Three-tier 3D ICs for more power reduction: Strategies in CAD, design, and bonding selection

Low-power is one of the key driving forces in modern VLSI systems. Several recent studies show that 3D ICs offer significant power savings over 2D ICs, primarily due to wirelength and buffer saving. However, these existing studies are mainly limited to 2-tier designs. In this paper, our target is extended to 3-tier 3D ICs. Our study first shows that the one additional tier available in 3-tier 3D ICs does offer more power saving compared with their 2-tier 3D IC counterparts, but more careful floorplanning, through-silicon via (TSV) management, and block folding considerations are required. Second, we find that the three tiers can be bonded in different ways: (1) face-to-back only and (2) face-to-face and face-to-back combined. Our study shows that these choices pose additional challenges in design optimizations for more power saving. Lastly, we develop effective CAD solutions that are seamlessly integrated into commercial 2D IC tools to handle 3-tier 3D IC power optimization under various bonding style options. With our low-power design methods combined, our 3-tier 3D ICs provide -14.8% more power reduction over 2-tier 3D ICs and -36.0% over 2D ICs under the same performance.