4.3.1 3D Integration
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4.3.1 3D Integration

3-dimensional integration (3DI) is the practice of bonding very thin silicon dies to each other in a vertical stack. This practice provides potential benefits, but also poses significant fabrication challenges [Kni08].

Figure: Latency Benefit of 3D Integration

Perhaps the most important benefit of 3DI is decreased path length through the system, as shown in Figure . A 3-centimeter silicon die is replaced with a stack of four 1.5-centimeter dies, in theory decreasing the maximum path through the system by a factor of two, keeping in mind that each layer is quite thin. In addition, given proper attention to design and placement, long horizontal electrical connections (which are both slow and power hungry) can be replaced by short vertical electrical connections, which are both faster and more power efficient.

However, delays due to levels of clocked logic will not be decreased by 3D integration, and significant manufacturing, testing, power-supply, and heat-dissipation problems must be solved for 3D integration to reach production while still delivering on its promise. The heat-dissipation problems might be solved using semiconductors based on diamond, which is a good conductor for heat, but an electrical insulator. That said, it remains difficult to grow large single diamond crystals, to say nothing of slicing them into wafers. In addition, it seems unlikely that any of these technologies will be able to deliver the exponential increases to which some people have become accustomed. That said, they may be necessary steps on the path to the late Jim Gray's ``smoking hairy golf balls'' [Gra02].