Three-dimensional chip (3-D) stacking technology provides a new approach to address the so-called memory wall problem. Memory processor chip stacking reduces this memory wall problem, permitting faster clock rates (with suitable processor logic) or permitting multicore access to shared memory using a large number of vertical vias between tiers in the stack, for ultrawide bit path transfer of data and address information to and from various levels of cache. Although a limited amount of parallel access is possible using conventional two-dimensional (2-D) chip memory-processor approaches, 3-D memory-processor stacking greatly extends this to much larger capacity memories. We evaluate high-clock-rate processors as well as shared memory processors with a large number of cores. Various architectural design options to reduce the impact of the memory wall on the processor performance are explored and validated through simulations. Certain architectural features can be implemented in a 3-D chip, such as an ultrawide, ultrashort vertical bus with low parasitic resistance and the elimination of conventional electrostatic discharge, and packaging parasitics required in multiple package 2-D solutions. The objective is to reduce the clocks per instruction figure of merit for high clock speeds in order to deliver significant performance levels. High-clock-rate processors can be designed with SiGe heterostructure bipolar transistors to obtain processors operating on the order of 16 or 32 GHz.

Forty years ago Gene Amdahl published a figure of merit for parallel computation, which proved extremely controversial. The controversy still rages today, although those that have looked closely at this figure of merit conclude that it is correct, but perhaps misinterpreted. In this paper we will look at a small variation on that law that suggests computer designers should take a closer look at two emerging technologies, SiGe HBT BiCMOS and 3D chip stacking. We may be overlooking a way to continue the clock race, and in so doing accomplish better parallelism.

A major impediment to the continuation of Moore's Law in the years to come is the performance of interconnections in ICs at high frequencies. Microprocessors are using a greater portion of their clock cycle charging and discharging interconnections. Silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs) provide a fast track technology for the exploration of the effect of interconnections on high-speed computer design. Industry has pursued low-k dielectrics to decrease wire capacitance. Cu metallization has been used to reduce wire resistance which becomes important as the wire dimensions are scaled down. These are not the only issues for high-frequency interconnections. Some other high-frequency issues include coupling, transmission line propagation, skin effects, and dielectric and substrate loss. These phenomena cause signal attenuation, noise, and dispersion in addition to delay. In the limit of zero device delay, interconnection delay will remain in addition to these problems. Wire shortening has been possible using more layers of interconnections, but this approach may be reaching its limit. An unconventional approach, three-dimensional (3-D) integration, attempts to shorten wiring through increased circuit component placement flexibility. The approach considered here for 3-D integration uses wafer-to-wafer aligning and bonding, wafer thinning and deep, high-aspect-ratio Cu via formation. This provides an intimate interconnection between CPU components and an extremely wide path to memory that would be infeasible in conventional or multichip module packaging. This combination of SiGe HBT BiCMOS and 3-D chip stack technologies enables small computing engines in the 16-32-GHz range.

This paper describes the implementation of a large scale SiGe FPGA that serves as a high speed FPGA test platform. In the FPGA core, 20 x 20 building blocks (Basic Cells) are used to implement logic applications. This chip contains 106 devices including SiGe NPNs and MOSFETs. This chip is fabricating with the IBM SiGe 7HP process with cut off frequency of 120GHz. The target running frequency of this FPGA is 10GHz. Clock repeaters are added for improved clock distribution. A test circuit whose building block cell runs up to 10GHz is fabricated and measured by the same process. Future work and some potential applications of the SiGe FPGA are also described.

The current trends in high performance integrated circuits are towards faster and more powerful circuits in the giga-hertz range and even further. As the more complex Integrated Circuits (IC) such as microprocessors have been entering the giga-hertz operating frequency range, various speed related roadblocks have become increasingly difficult to overcome. The migration to smaller devices has raised serious challenges. The major impediment to fulfill Moore's Law effectively in the years to come is increasingly becoming the interconnect. ICs are using a greater fraction of their clock cycles charging interconnect wires. IC interconnect related speed degradation has stimulated much research effort in the area of low dielectric constant materials. A relatively novel approach, wafer scale 3-dimensional (3D) integration attempts to by-pass the large wire parasitics by shortening wires. This paper is going to elaborate on a 3D microprocessor test vehicle. We intend to demonstrate the speed advantages, which may be derived from 3D integration, through a combination of fabrication, testing and simulation.