Paul van Gerven
30 September

Integrating a microfluidic cooling system in the heart of a power chip improves cooling performance by a factor of 50, Swiss researchers found.

As sophisticated as modern semiconductor devices have become over the years, their thermal management hasn’t changed much. Cooling is a matter of getting the heat away from the belly of the chip and directing it towards a device that disperses it into the environment. In a PC or notebook processor, for example, the heat produced by the billions of transistors is led to an air-cooled heat sink.

This way of heat extraction is fundamentally limited by the thermal resistance between the semiconductor die and its packaging: there’s only so much heat you can rid get off. It’s also inefficient, as cooling requires relatively large amounts of energy. And, of course, bulky heat sinks take up a lot of space, which in many electronic products is a scarce resource.

Researchers of the École Polytechnique Fédérale de Lausanne (EPFL) decided they can do better, for power electronics at least. By designing extensive plumbing into the chip, thus putting the coolant mere micrometers away from where heat is produced, the Swiss managed to increase cooling performance by a factor of 50.

Credit: EPFL

Cold plate

Elison Matioli and colleagues at EPFL’s Insitute of Electrical Engineering demonstrated microfluidic cooling systems integrated into gallium nitride-on-silicon (GaN-on-Si) chips, which are anticipated as the next-generation power semiconductors. Wide-bandgap semiconductor GaN has excellent qualities for power applications, but it’s difficult and expensive to make (large) wafers out of the material. Hence the alternative approach to apply it as a thin layer on a silicon substrate. The silicon typically lacks functionality other than ‘carrying’ the GaN, but the Swiss researchers turned it into an active cooling layer by incorporating microfluidic channels into it – directly underneath the active transistor areas.

Using microfluidics to cool chips is, in itself, not a new idea. In one general approach, a microfluidically cooled ‘cold plate’ is stacked on top of the chip. The drawback of this approach is restricted heat flow. In another general approach, coolant is brought directly into contact with the die’s surface, which is much more efficient. Pumping coolant through parallel microchannels etched directly in the die offers great performance, for example, but requires high-powered pumping and generates thermo-mechanical stress because of the high temperature gradient that arises.

Big step

Embedding the microchannels into the chip itself is an attractive solution from a thermal management perspective, but such an approach would increase the complexity and cost of constructing the device. The EPFL researchers managed to simplify the process by combining two design steps into one: in what they call a monolithically integrated manifold microchannel (MMMC) system, the microchannels right underneath the heat sources are integrated and co-fabricated in a single die.

The MMMCs are fabricated in three steps. First, slits are etched into the GaN-on-Si substrate. Next, an etching procedure is employed to widen the slits in the silicon, as well as to form sections of channels that connect to produce an interconnected channel system through which the coolant can flow. Finally, the channel openings at the surface are plugged with copper. The power integrated circuit can then be fabricated in the GaN top layer.

Working at Imec, Tiwei Wei, who wasn’t involved in the research, describes the results as “impressive.” Experiments in an AC-DC current converter fitted with MMMC cooling “show that heat fluxes exceeding 1.7 kilowatts per square centimeter can be cooled using only 0.57 watts per square centimeter of pumping power. Moreover, the liquid-cooled device exhibits significantly higher conversion efficiency than does an analogous uncooled device, because degradation caused by self-heating is eliminated.”

Wei does point out that some aspects of the invention require further investigation, such as the stability of the modified GaN layer over time. Still, the Swiss research “is a big step towards low-cost, ultra-compact and energy-efficient cooling systems for power electronics,” he concludes.