We examine the performance of custom circuits in an emerging technology known as three-dimensional integration. By combining multiple device layers with a high-density inter-layer interconnect, 3D integration of a given circuit is expected to provide better timing and energy performance relative to a single-wafer implementation of the same circuit. In this paper, we show that by using our performance-driven design tool for 3D ICs, the interconnect energy dissipation of standard-cell circuits can be reduced by 24% to 42% using two to five device layers respectively. Similarly, the interconnect energy-delay product can be reduced by 30% to 50%.At the same time, thermal performance in 3D ICs is expected to be a critical issue. By incorporating thermal management and analysis into our placement tool, we may investigate the thermal scalability of 3D integration. We find that the thermal performance actually can be improved with the use of a modest number of additional device layers. Also, we show that the absolute die temperature can be controlled through the use of extra silicon.

We present an overview of a new monolithic fabrication technology known as three-dimensional integration. 3D integration refers to any process by which multiple conventional device layers may be stacked and electrically interconnected. By combining state-of-the-art single-wafer integration with a high-density inter-wafer interconnect, our 3D integration process is capable of providing improved circuit performance in terms of metrics such as wire length, area, timing, and energy consumption. In this paper, we will discuss the overall 3D integration process flow, as well as specific technological challenges and the issues they present to circuit designers. We will also describe how these issues may be tackled during the placement, routing, and layout stages of physical design. Finally, we will present some performance results that may be obtained by integrating circuits in three dimensions.

In this paper, we determine the accuracy of Rahman's interconnect prediction model for three-dimensional (3-D) integrated circuits. Utilizing this model, we calculate the wiring requirement for a set of benchmark standard-cell circuits. We then obtain placed and routed wirelength figures for these circuits using 3-D standard-cell placement and global-routing tools we have developed. We find that the Rahman model predicts wirelengths accurately (to within 20% of placement and of routing, on average), and suggest some areas for minor improvement to the model.

We present a set of design tools for 3-D Integration. Using these tools - a 3-D standard-cell placement tool, global routing tool, and layout editor - we have targeted existing standard-cell circuit netlists for fabrication using wafer bonding. We have analyzed the performance of several circuits using these tools and find that 3-D integration provides significant benefits. For example, relative to single-die placement, we observe on average 28% to 51% reduction in total wire length.

In this paper, analytical models for predicting interconnect requirements in field-programmable gate arrays (FPGAs) are presented, and opportunities for three-dimensional (3-D) implementation of FPGAs are examined. The analytical models for two-dimensional FPGAs are calibrated by routing and placement experiments with benchmark circuits and extended to 3-D FPGAs. Based on system-level modeling, we find that in FPGAs with more than 20 K four-input look-up tables, the reduction in channel width, interconnect delay and power dissipation can be over 50% by 3-D implementation.

Twenty-first century opportunities for GSI will be governed in part by a hierarchy of physical limits on interconnects whose levels are codified as fundamental, material, device, circuit, and system. Fundamental limits are derived from the basic axioms of electromagnetic, communication, and thermodynamic theories, which immutably restrict interconnect performance, energy dissipation, and noise reduction. At the material level, the conductor resistivity increases substantially in sub-50-nm technology due to scattering mechanisms that are controlled by quantum mechanical phenomena and structural/morphological effects. At the device and circuit level, interconnect scaling significantly increases interconnect crosstalk and latency. Reverse scaling of global interconnects causes inductance to influence on-chip interconnect transients such that even with ideal return paths, mutual inductance increases crosstalk by up to 60% over that predicted by conventional RC models. At the system level, the number of metal levels explodes for highly connected 2-D logic megacells that double in size every two years such that by 2014 the number is significantly larger than ITRS projections. This result emphasizes that changes in design, technology, and architecture are needed to cope with the onslaught of wiring demands. One potential solution is 3-D integration of transistors, which is expected to significantly improve interconnect performance. Increasing the number of active layers, including the use of separate layers for repeaters, and optimizing the wiring network, yields an improvement in interconnect performance of up to 145% at the 50-nm node.

In this paper, the wire (interconnect)-length distribution of three-dimensional (3-D) integrated circuits (ICs) is derived using Rent's rule and following the methodology used to estimate two-dimensional (2-D) (wire-length distribution). Two limiting cases of connectivity between logic gates on different device layers are examined by comparing the wire-length distribution and average and total wire-length. System performance metrics such as clock frequency, chip area, etc., are estimated using wire-length distribution, interconnect delay criteria, and simple models representing the cost or complexity for manufacturing 3-D ICs. The technology requirement for interconnects in 3-D integration is also discussed.