Power/Performance Bits: Oct. 13

Cooling down FPGAs

Georgia Institute of Technology researchers found a way to put liquid cooling a few hundred microns away from where the transistors are operating by cutting microfluidic passages directly into the backsides of production FPGAs.

The research, backed by DARPA, is believed to be the first example of liquid cooling directly on an operating high-performance CMOS chip.

To make their liquid cooling system, the team removed the heat sink and heat-spreading materials from the backs of stock Altera 28-nanometer FPGA chips. They then etched cooling passages into the silicon, incorporating silicon cylinders approximately 100 microns in diameter to improve heat transmission into the liquid. A silicon layer was then placed over the flow passages, and ports were attached for the connection of water tubes.

Liquid ports carry cooling water to specially designed passages etched into the backs of FPGA devices to provide more effective cooling. The liquid cooling provides a significant advantage in computing throughput. (Source: Rob Felt, Georgia Tech)

In multiple tests a liquid-cooled FPGA was operated using a custom processor architecture provided by Altera. With a water inlet temperature of approximately 20 degrees Celsius and an inlet flow rate of 147 milliliters per minute, the liquid-cooled FPGA operated at a temperature of less than 24 degrees Celsius, compared to an air-cooled device that operated at 60 degrees Celsius.

“We have created a real electronic platform to evaluate the benefits of liquid cooling versus air cooling,” said Muhannad Bakir, associate professor at Georgia Tech. “This may open the door to stacking multiple chips, potentially multiple FPGA chips or FPGA chips with other chips that are high in power consumption. We are seeing a significant reduction in the temperature of these liquid-cooled chips.”

In a separate research project, Bakir’s group demonstrated the fabrication of copper vias that would run through the silicon columns that are part of the cooling structure fabricated on the FPGAs.

“The moment you start thinking about stacking the chips, you need to have copper vias to connect them,” Bakir said. “By bringing system components closer together, we can reduce interconnect length and that will lead to improvements in bandwidth density and reductions in energy use.”

Room temperature skyrmions

Physicists at UC Davis and the National Institute of Standards and Technology (NIST) have now succeeded in making magnetic skyrmions, formerly found at temperatures close to absolute zero, at room temperature.

“This is a potentially new way to store information, and the energy costs are expected to be extremely low,” said Kai Liu, professor of physics at UC Davis.

Skyrmions were originally described over 50 years ago as a type of hypothetical particle in nuclear physics. Actual magnetic skyrmions were discovered only in 2009, as chiral patterns of magnetic moments in materials close to absolute zero temperature, in the presence of a strong magnetic field.

The researchers designed a nanosynthesis approach to achieve artificial “Bloch” magnetic skyrmions at room temperature. They created a pattern of magnetic nanodots, each about half a micron across, on a multilayered film where the magnetic moments are aligned normal to the plane. They used ion beam irradiation to modify the interface between the dots and the film to allow “imprinting” of the magnetic moments of the dots into the film.

Nanodots induce magnetic skyrmions (arrows) in the film below. Skyrmions are stable magnetic structures and could be a new way to store data at low energy cost. (Source: Dustin Gilbert and Kai Liu/UC Davis)

Using neutron-scattering at NIST Center for Neutron Research, they were able to resolve the magnetic profiles along the depth of the hybrid structure. Combined with magnetic imaging studies at NIST and Lawrence Berkeley Laboratory, they were able to find the first direct evidence of arrays of stable spiral magnetic skyrmions beneath the nanodots at room temperature, even without an external magnetic field.

The availability of stable magnetic skyrmions at room temperature opens up new studies on their properties and potential development in electronic devices, such as nonvolatile magnetic memory storage.