Today, heat from dense electronics is an expensive resource. To keep the system at the right temperature for optimal computational performance, the U.S. data center cooling system consumes as much energy and water as all the residents of Philadelphia. Now, by integrating liquid cooling channels directly into semiconductor chips, researchers hope to reduce this loss at least in power electronics, making them smaller, cheaper and less energy-intensive.
Traditionally, electronic devices and thermal management systems have been designed and manufactured separately, says Elison Matioli, a professor of electrical engineering at ecole Polytechnique in Lausanne, Switzerland. This presents a fundamental barrier to improved cooling efficiency, as heat must travel relatively long distances across multiple materials before it can be removed. In today's processors, for example, siphons of hot material transfer heat from the chip to bulky, air-cooled copper fins.
For a more energy efficient solution, Matioli and his colleagues developed a low-cost process that puts a 3D network of microfluidic cooling channels directly into the semiconductor chip. Liquid removes heat better than air, and the idea is to keep the coolant micrometer away from the hot spot on the chip.
But unlike previously reported microfluidic cooling, he said, "We design the electronics and the cooling system from the beginning." Thus, the microchannel is located just below the active region of each transistor device, where it is at its highest temperature, which increases cooling performance by a factor of 50. They reported their joint design concept in a recent issue of nature.
Researchers proposed microchannel cooling technology as early as 1981, and startups like Cooligy have been pursuing the idea of processors for years. But the semiconductor industry is moving from planar devices to three-dimensional devices and toward future chips with multilayer structures, making cooling channels impractical. "This embedded cooling solution is not suitable for modern processors and chips such as cpus," said TiweiWei, who researches electronic cooling solutions at Interuniversity Microelectronics Center and KU Luuven in Belgium. "Instead, this cooling technology makes the most sense for power electronics," he said.
Power electronic circuits manage and convert electrical energy and are widely used in computers, data centers, solar panels and electric vehicles. They used large-area discrete devices made of wide-bandgap semiconductors such as gallium nitride. The power density of these devices has skyrocketed in the last few years, meaning they have to be "hooked up to a giant radiator," Matoli said.
More recently, power electronics modules have shifted to liquid cooling, either through cold plate or microchannel cooling systems. But, until now, all microchannel cooling systems have been manufactured separately and then combined with the chip. Bonding layer adds heat resistance, channels and circuit equipment are not tightly aligned.
"We took it to the next level," Says Matoli, by making devices and cooling channels in the same chip. They etched micron-wide cracks in a layer of gallium nitride coated on a silicon substrate. Slit length 30μm, deep 115μm. Using special gas etching techniques, they widen gaps in the silicon substrate to form channels through which the liquid coolant passes.
The researchers then used copper to seal tiny openings in the layers of gallium nitride and build devices on them. "We only have microchannels in tiny areas of the wafer that are in contact with each transistor," he said. This makes the technology much more effective because we can extract a lot of heat from nearby, but the pumping power we use is very small."
As a demonstration, the researchers built an AC-DC rectifier circuit consisting of four Schottky diodes, each capable of handling 1.2kV of voltage. Circuits like these typically require a fist-sized radiator. But the circuit chip with the liquid cooling system is mounted on a printed circuit board the size of a USB flash drive. The circuit board consists of three layers with channels carved into it to deliver the coolant to the chip.
The display shows that hot spots with a power density of more than 1,700 watts per square centimeter can be cooled using just 0.57 watts per square centimeter of pumping power. This is a 50-fold improvement in performance compared to the previously reported microfluidic channel cooling.
"The reliability of gallium nitride films and copper seals should be studied over time. But this innovative cooling solution is a big step toward a low-cost, ultra-compact and energy-efficient power electronics cooling system.