PORTLAND, Ore. IBM's Zurich Research Laboratory on Thursday (June 5) demonstrated three-dimensional chip stacks that are cooled with water. The company expects to commercialize such stacks for its multicore servers as early as 2013.
IBM plans to stack memory chips between processor cores to multiply interconnections by 100 times while reducing their feature size tenfold. To cool the stack at a rate of 180 watts per layer, water flows down 50-micron channels between the stacked chips.
"Electrical interconnects are in a wiring crisis; the wiring does not scale the way transistors scale, because the width of wires is shrinking but their length is not," said IBM Zurich researcher Thomas Brunschwiler. "Our solution is to go to the third dimension to stack multicore dice and have the interconnections go in between them vertically, which can decrease their length by up to 1,000 times."
IBM's paper on the approach, "Forced convective interlayer cooling in vertically integrated packages," received a Best Paper award at the IEEE ITherm conference, held in late May in Orlando, Fla. This marks the third consecutive year the IBM Zurich Lab's Advanced Thermal Packaging team has won such awards. The Zurich group claims to be fixated on water cooling because water is up to 4,000 times more effective than air at removing heat from electronics.
Earlier this year, the same group described the water cooling method for IBM's Hydro-Cluster supercomputer. For the Hydro-Cluster Power 575, the Zurich group replaced heat sinks with water-filled copper plates above each core. The Zurich team predicts high-end IBM multicore computers will migrate from the copper-plate water-cooling-method to the 3-D chip-stack in five to 10 years.
Three-dimensional water-cooled chip stacks will interleave processor cores and memory chips so that the interconnects run vertically chip to chip through copper vias that are surrounded by silicon oxide. Thin-film soldering (using electroplating) enables the separate dice to be electrically bonded to the layers above and below them, with the insulating layers of silicon oxide separating the flowing water from the copper vias.
The power density dramatically increases for such 3-D chip stacks, since enough heat gets trapped between layers to melt the cores. To solve the problem, IBM etched a liquid aqueduct into the silicon oxide on the back each die. That creates a water-filled separating cavity with 10,000 pillars, each housing a copper via surrounded by silicon oxide. The cooling technique runs water through the aqueduct between each layer in the chip stack, enabling IBM to channel heat away from 3-D multichip stacks of nearly any scale.
The technology "forces water between the layers in the chip stack, picking up the heat right at its source," said Brunschwiler. "We found that to [create] an efficient heat remover, [we] had to use a structure with very little resistance to the fluid flow. . . . we found that round pillars aligned in the flow direction and put under pressure gave the best convective heat transfer."
IBM packages the chip stacks in a sealed pressurized silicon housing with an inlet reservoir on one side and an outlet reservoir on the other. The only way water can get from the inlet side of the silicon box to its outlet side is by going through the silicon oxide layers separating the layers of the 3-D stack. Cool water enters a 3-D chip stack and exits heated. The protected copper vias connect the chips vertically. After being forced through the layers between the chips in a stack, the heated water could be fed to the hot tap of the customer's plumbing, turning a data center's wasted heat into a means for reducing the data center's carbon footprint, according to IBM.
Next, the team plans to optimize the cooling structures for smaller chip dimensions, more interconnects and more-sophisticated heat transfer structures. In particular, the lab is experimenting with ways of adding extra cooling to designated hot spots on cores.
Eventually, IBM envisions a hierarchy of cooling structures similar to those in the brain, which branch out to cover a large surface area combined with many interconnections between layers.