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The same overheating problem that happens to our laptops also plagues computer servers and data centers around the world — and heat management is only getting harder as computer chips get more compact and powerful.
Understanding how heat moves through chips at the micro scale is essential for continuing to improve their performance. Unfortunately, most methods for measuring heat flow struggle with multilayered devices like the electronics that power our modern world.
Now MIT researchers have demonstrated a new way to study how heat moves through multilayered materials, combining X-rays that penetrate multiple layers with laser pulses for delivering heat. The researchers used the technique to measure how heat moves inside a promising device for transistors and flexible electronics.
The method was so precise it allowed the researchers to quantify the effect of a single micron-scale defect in the device, revealing a surprising fourfold reduction in the material’s ability to transfer heat at that spot. They also found that the defect caused heat to spread unevenly, moving more easily in one direction than in the other.
The team believes the approach could help researchers understand overheating in devices and help companies develop more power-dense electronics for everything from AI applications to wearables and clean energy systems.
“Chip developers need devices that can handle heat,” says Mingda Li, an associate professor of nuclear science and engineering at MIT and co-corresponding author on an open-access paper about the work in Nature Communications. “I think overheating has become the real bottleneck in device performance. When doing these diagnoses using traditional techniques, they couldn’t get down to the micro- or nanometer scale. But eventually they’d like to go beyond that to study the heat carriers and understand exactly what causes failure, in order to avoid local hotspots and design better devices. This approach is a step in that direction.”
Joining Li on the paper are co-lead authors Thanh Nguyen PhD ’24 and MIT postdoc Chuliang Fu; PhD candidate Mouyang Cheng; Abhijatmedhi Chotrattanapituk ’21, SM ’26; Denisse Córdova Carrizales SM ’26; Eunbi Rha SM ’26; Tyra Espedal ’26; Buxuan Li PhD ’24; Shivam Kajale SM ’23, PhD ’26; Tongtong Liu PhD ’23; Kuan Qiao PhD ’22; University of Texas at Austin Assistant Professor Zhantao Chen SM ’18, PhD ’22; Argonne National Laboratory researchers Kumar Neeraj, Donald Walko, and Haidan Wen; MIT Principal Research Scientist Svetlana Boriskina; MIT Associate Professor Deblina Sarkar; and co-corresponding author and MIT Associate Professor Jeehwan Kim.
Tracking heat
Making more powerful computers and electronics often comes down to cramming more transistors into a smaller area. But the closer those transistors get to each other, the hotter the device gets as it operates, and the more heat needs to be moved.
Most people learn about the problem from their laptops overheating on their lap. At the data center scale, it means an enormous amount of energy must be devoted to cooling the servers.
The quest to design more power-dense computers and electronics is thus a quest to find materials that can transport heat most efficiently.
Researchers have used a number of methods to measure and model heat flow across materials, but they all have limitations when it comes to studying realistic device architectures. One common optical method to study heat at the microscopic level, for instance, is called time domain thermal reflectance.
“Because that technique uses optics, it doesn’t allow you to study different layers,” Kim explains. “Real devices have five or more layers. It also only provides an overall signal, and that makes it hard to see thermal transport happening in layers buried under the surface.”
Other techniques, like infrared cameras, don’t capture tiny changes at a fast enough frame rate to be useful at small scales.
To address those limitations, the researchers wanted to create something that could measure heat transfer at the nanoscale in multilayer systems. To do that, they used an emerging analysis technique that sends electron pulses and ultrafast X-rays at a material and measures changes in energy.
“Over the last few years, researchers have developed what is basically the brightest X-ray source in the world,” Nguyen says. “That allows you to focus an X-ray beam and get incredibly fine spatial resolution. You can also use a laser to heat the sample while the X-ray scans and shows how the heat dissipates across space in real-time.”
The technique offered a better view of heat transfer because the laser-powered electron pulse can capture changes in material strain at the atomic level while the X-rays can penetrate into multiple layers of the material, and the measurements can be combined to provide a clearer view of how a material moves heat.
“Using previous measurement techniques, in real devices, you couldn’t resolve what happens on one layer versus another, so you’d just measure the average,” Fu says. “X-rays can clearly show how heat propagates across the interface through their diffraction.”
The researchers applied their technique to a test device made of a layer of gallium nitride, which has shown promise for conducting heat efficiently, on top of silicon. The material combination has been studied for years, but its thermal performance has been shown to deteriorate because of tiny defects created during processing.
The researchers measured a fourfold reduction in heat dissipation across a wrinkle defect on the device and a 25 percent drop in heat dissipation across materials, showing more disruption to heat flow than they had expected.
“When people model heat dissipation, they model perfect crystals without defects,” Li says. “But these types of large wrinkle defects are very common in 2D materials. People never even knew how much heat is blocked by these wrinkles. Those are things we can now directly observe with this technique.”
Designing better chips
Li says a leading semiconductor industry consortium has already reached out to collaborate on applying the measurement technique to study different types of chips. He believes the technique will work to study a wide array of materials and devices.
“We can now pass a current and shine an X-ray on a device and see how the heat dissipates at a very small scale,” Kim says. “That’s something the industry has been longing for.”
Li says the approach will provide researchers with new information to improve the design of electronic systems.
“This will enable better thermal design of electronic systems,” Kim says. “Even with the same type of materials, the geometry and how the materials are laid out is quite complicated, so it will show us how those differences impact thermal flow by providing direct experimental measurements.”
The work was supported, in part, by the U.S. Department of Energy, the U.S. National Science Foundation, and the MIT School of Engineering Distinguished Energy Efficiency Fellowship.
