Wire Length Characteristics of Multi-Tier Gate-Level Monolithic 3D ICs

Monolithic inter-layer vias (MIVs) used in 3D integration technology are much smaller than through-silicon vias (TSVs) used in TSV-based 3D integration. Thus, monolithic 3D integration provides shorter wire length, better performance, and lower power consumption than TSV-based 3D integration. Multi-tier monolithic 3D integration stacks more than two silicon layers, thereby increasing the device density, which reduces the wire length and power consumption and improves performance further than two-tier monolithic 3D integration. Since the size of an MIV is comparable to that of a local via, the wire lengths of multi-tier gate-level monolithic 3D IC layouts are expected to be close to the ideal wire length reduction ratio, for example, 29, 42, and 50 percent for two-, three-, and four-tier designs, respectively. However, it is still unknown whether it would be possible to achieve the ideal wire length reduction ratios by multi-tier gate-level monolithic 3D integration. In this paper, we propose an efficient routing methodology to design multi-tier monolithic 3D ICs. Using the design methodology, we design two- to eight-tier gate-level monolithic 3D IC layouts and thoroughly investigate the wire length characteristics of the layouts. The simulation results show that the proposed routing methodology reduces the wire length by 12 to 53 percent compared to the 2D designs. In addition, the proposed routing methodology builds multi-tier monolithic 3D IC layouts more efficiently than a state-of-the-art methodology with comparable routed wire length.