Today’s densely-packed electronics generate a whole lot of heat. The problem is that heat is an expensive resource to manage and drain, as is keeping systems cool. Data centers in particular are feeling the pain, some of which consume as much energy and water as entire cities. In fact, Microsoft, in an effort to combat data center heat, put one on the ocean floor to keep cool.

Now, researchers at École Polytechnique Fédérale de Lausanne (EPFL) in Switzerland hope to reduce heat and its subsequent resource drain in power electronics devices by integrating liquid cooling channels directly into a semiconductor chip. This will make them smaller, cheaper, and more efficient. 

Their research, which has been published in the journal Nature, describes how the EPFL team developed its integrated microfluidic technology together with electronics that can efficiently manage the large fluxes of heat generated by transistors.

Integrating Cooling Directly Into a Chip

Traditionally, electronics and heat management systems are designed and developed separately from one another. However, according to Elison Matioli, a professor of electrical engineering at EPFL, this creates inefficiencies because heat must then propagate long distances and through multiple materials for it to be removed. 

As a more efficient alternative, Matioli and his team at EPFL have developed a low-cost process that integrates a three-dimensional (3D) network of microfluidic cooling channels directly into a silicon chip.

The idea behind this is that fluids remove heat much better than air does, and by placing these channels a mere few micrometers away from the chip’s hottest areas, they’ll efficiently dispel the heat and do away with the additional cooling methods.

Microfluidic channels are placed very close to the transistor’s hot spots, enabling heat to be extracted in exactly the right place for maximum efficiency.

Microfluidic channels are placed very close to the transistor’s hot spots, enabling heat to be extracted in exactly the right place for maximum efficiency. Image used courtesy of EPFL

Unlike microfluidic cooling techniques that have previously been reported, however, the EPFL team has designed the electronics and the cooling system “together from the beginning.” This means that the microchannels are directly below the active region of each transistor device—where the most heat is generated—increasing cooling efficiency by a factor of 50.

In contrast, previous forays into microchannel cooling systems have been made by building the two parts separately and then bonding them to one another, which adds heat resistance. 

The Process: a Gas-Etching Technique

In this research, the EPFL researchers etch micrometer-wide slits 30 µm-long and 115 µm-deep in a gallium nitride (GaN) layer coated on a silicon substrate. Using a gas-etching technique, these slits are widened in the silicon substrate to form the channels through which the liquid coolant is pumped. 

Researchers used the following setup to evaluate thermo-hydraulic performance

Researchers used the following setup to evaluate thermo-hydraulic performance. Image used courtesy of Nature
 

These slits are then sealed with copper and the chip itself is built on top. “We only have microchannels on the tiny region of wafer that’s in contact with each transistor,” says Matioli. He adds that this makes the technique particularly efficient because lots of heat can be extracted with very little pumping power required. 

A 50-Fold Performance Improvement

To demonstrate their chip’s viability, the researchers built an AC-DC rectifier circuit made from four Schottky diodes. This type of circuit would typically require a large heat sink, but with the integrated cooling system, the chip sits on a small PCB which consists of three layers with channels carved into it for coolant delivery. 

The results of this test show that hot spots on the device with densities of over 1,700 cm2 can be cooled with only 0.57 W/cm2 of pumping power—a 50-fold increase in performance in contrast to previously reported microfluidic cooling techniques.

Source: All About Circuits