Three-dimensional processors took a baby step towards commercial reality today, thanks to IBM’s water-cooling research. Big Blue and the Fraunhofer Institute have successfully tested a multistack CPU prototype that’s cooled by pumping water directly through the separate layers of the processor. If you aren’t used to thinking of processors in terms of layers, you may need to check Jon Stokes’ “Dagwood Sandwich” analogy before continuing on.
3-D chip stacking uses a technology referred to as “through silicon via” (TSV) to build processors vertically, rather than just horizontally. By using both dimensions, CPU engineers can reduce wire delay, improve CPU efficiency, and significantly reduce total power consumption. We’ve previously covered both Intel and IBM’s efforts in this area; readers should consult those articles for a more comprehensive treatment of the subject.
Thermal dissipation, however, is the Achilles’ heel of any three dimensional processor. The more layers in a processor, the more difficult it is to effectively remove heat emanating from the lower levels. CPU architects can compensate for this by placing the hotter parts of a core on upper layers and by avoiding designs that stack core hotspots vertically, but the complexity of the problem increases with every additional layer. Simply leaving more space between the individual layers is not a solution, as this would quickly recreate the wire delay problems three-dimensional processors are meant to alleviate.