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.



IBM's water-cooled solution. Water radiates through the core in an enclosed structure.

According to the IBM project lead, conventional processor cooling systems simply aren't up to the task. "As we package chips on top of each other to significantly speed a processor’s capability to process data, we have found that conventional coolers attached to the back of a chip don’t scale. In order to exploit the potential of high-performance 3-D chip stacking, we need interlayer cooling."

IBM's water-cooled prototype has 100 micron high cooling layers between the die layers of the hermetically sealed processor. These layers aren't just filled with water, they're also packed with the vertical interconnects between the two dies, at a density of 10,000 interconnects per cm2. According to IBM, standard fabrication technology was used to build the chip, although specialized equipment was needed to connect the wires between the two die packages.

The final results of IBM's test were impressive; the die-stacked processor's interlayer cooling solution was capable of removing 180W/cm2 for a stack with a footprint of 4cm2. We're still a long way off from three-dimensional processors (three-dimensional RAM is imminent) but IBM's prototyped water-cooling solution proves that designs could one day see the light of day.

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