LAKE WALES, Fla. — The Defense Advanced Research Projects Agency’s Intrachip/Interchip Enhanced Cooling (ICECool) program, which teamed IBM and the Georgia Institute of Technology to solve the liquid cooling problem for 3-D chip stacks, has yielded an approach that uses an insulating dielectric refrigerant instead of water. Researchers who worked on the prototype say the approach will lower the cost of cooling supercomputer CPUs by pumping refrigerants through microfluidic on-chip channels and will cool the interior of even the thickest 3-D chip stacks by safely running refrigerants between each die.
DARPA�s Intrachip/Interchip Enhanced Cooling (ICECool) program sought to overcome the limitations of remote cooling with �embedded� thermal management using microfluidic cooling inside the substrate, chip, or package.
(Source: DARPA)
“Our prototype was an eight-core based Power7 supercomputer with microfluidic channels etched in its backside to remove heat, sitting alongside an air-cooled Power7 supercomputer for comparison,” said Tim Chainer, principal researcher at IBM’s Thomas J. Watson Research Center (Yorktown Heights, N.Y.), told EE Times in an interview. “The resultant improvements lowered junction temperatures by 25°C [44°F] while using 7 percent less power and a much simpler cooling infrastructure. We also aim to overcome the scaling limits of Moore's Law by enabling 3-D chips to be stacked to any height,” said Chainer, whose team worked with IBM Research colleagues in Zurich, with support from researchers at Georgia Tech.
Traditional air-conditioned cooling, using cold air and tall heat sinks (top), had already been proved inferior to warm-water cooling (center), but technology developed under DARPA�s ICECool program promises to drive size and cost even lower through the use of dielectric vapors (bottom).
(Source: IBM)
Chainer recalled how the shift to multicore architectures had overcome the 5-GHz speed limit for processors several years ago. Now that junction temperatures can be dropped by 44°F, engineers can start cranking up the clocks again. Insulating-dielectric-cooled 3-D chip stacks likewise will overcome the scaling limits of Moore’s Law, Chainer said.
Cooling 3-D chip stacks with an insulating dielectric fluid, which boils into a vapor to withdraw heat from within 50-micron stacks spaced 100 microns apart, enables bare-metal through-silicon vias to be run between chips to interconnect them without shorting them out, as water coolants would.
(Source: IBM)
"We are living in one of the most exciting times of computer innovation, driven by the ingenuity of engineers overcoming the limits of Moore’s Law that were once thought to be insurmountable,” he said.
IBM has already eliminated the need for data-center air conditioning with its water cooling systems, illustrated here, but the insulating dielectric vapor system developed for the ICECool program also eliminates the need for chillers (top right) and cooling towers (top left).
(Source: IBM)
IBM evaluated more than a dozen refrigerants before settling on Honeywell International Inc.’s Solstice ze R-1234ze because that refrigerant is a liquid at room temperature but vaporizes at the temperature of typical chips (as high as 85°C, or 185°F), drawing out their heat in the vaporization process. Because the refrigerant returns to liquid form at room temperature, there is no need for a compressor as is used in conventional refrigerators. Instead, Solstice ze R-1234ze merely needs to be channeled through a coil of copper piping (resembling liquor distiller or car radiator pipes), and thereby returned to liquid form before being pumped back through or between chips.
The Honeywell refrigerant is also a dielectric, so it can be pumped between chips without the need for insulation from metal parts, including through-silicon vias, which water would short out. Microfluidic channels can be run through single chips; 3-D-stacked chips can be flooded en masse. The optimal use of the noncorrosive refrigerant in 3-D chip stacks is to slim down the CMOS chips to 50 microns thick and leave gaps of 100 microns between them. Hollow rectangular spacers around the edges contain the refrigerant within the stack, with nipples on each side to pump in the liquid on one side and remove its vapor on the other side. The vapor is then run through the distiller, which returns the refrigerant to liquid form for pumping back into the chip stack.
“We fully expect that Honeywell, 3M, and other green-refrigerant makers will research and produce custom formulas just for the semiconductor industry, but for now Solstice R-1234ze is the best we’ve found,” Chainer said. While IBM and Georgia Tech concentrated on commercial high-performance computers under the ICECool program, Raytheon and Boeing produced solutions to cool radar installations and other very high-frequency equipment for defense applications. The DARPA program achieved its goals in about four years. Now IBM, Raytheon, and Boeing are passing the technology from their respective research labs to manufacturing. The technologies are expected to appear in commercial products and military gear as soon as 2018.
Chainer and his colleagues describe their work in the pay-to-access article “Improving Data Center Energy Efficiency with Advanced Thermal Management.” IBM will also detail the technology at the IEEC 29th Annual Electronics Packaging Symposium (EPS; Niskayuna, N.Y.) in September and at SuperComputing 2017 (Denver) in November.
— R. Colin Johnson, Advanced Technology Editor, EE Times 
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