A study recently published in the Proceedings of the National Academy of Sciences (PNAS), headed by scientists at Rensselaer Polytechnic Institute (RPI), has demonstrated how subjecting metals to intense pressures could lower their electrical resistance, which could potentially lead to increased speed and performance in many technologies.
Strain engineering, or the manipulation of materials using pressure to enhance device performance, has primarily been used to influence materials like silicon in semiconductor manufacturing. However, this recent research in strain engineering in metals, specifically, points to exciting new applications that metal interconnects with lower electrical resistance could have, particularly in the field of electrical insulation or conductivity.
In the paper, “Pressure Enabled Phonon Engineering in Metals,” researchers at RPI, consisting of scientists and RPI professors from diverse backgrounds – Saroj Nayak, specializing in physics, applied physics, and astronomy; Morris Washington, associate director of the Center for Materials, Devices, and Integrated Systems, specializing in physics, applied physics, and astronomy; and E. Bruce Watson, Institute Professor of Science, specializing in earth and environmental sciences, as well as materials science and engineering – combined the use of RPI’s supercomputer at the Center for Computational Innovations, theoretical predictions, and heavy-duty equipment capable of exerting crushing pressures of 40,000 atms to conduct the study.
“Understanding the pressure response of the electrical properties of metals provides a fundamental way of manipulating material properties for potential device applications,” the study states, addressing its own significance. “This study suggests innovative ways of controlling transport phenomena in metals.”
Though the speed of transistors has vastly improved over the years, scientists and engineers have been stymied by the limited speed of the metal wirings between transistors, known as interconnects. When electricity is conducted through a metal, electrons travel through a lattice structure consisting of metal atoms, carrying the current. However, this process is accompanied by a vibration, known in physics as a phonon, of metal atoms in the lattice. When a phonon combines with electrons, a quantum mechanical phenomenon known as electron-phonon coupling occurs, which amplifies greatly at the atomic level and produces a great deal of electrical resistance that slows down the speed of the interconnect.
However, previous research completed using RPI’s supercomputer shows that electron-phonon coupling varies depending on the size of the wiring in question; namely, nanoscale wiring has a higher resistance than normal “bulk” wiring.
Nicholas Lanzillo, Ph.D candidate at Rensselaer Polytechnic Institute and lead author on the study, along with his graduate advisor Nayak, were intrigued by this information, and decided to research it further.
“Our goal was to understand what limits the resistivity, what accounts for the different resistance at the atomic scale,” Nayak said in a recent press release. “Our earlier findings showed that sometimes the resistance of the same metal in bulk and at the atomic scale could change by a factor of 10. That’s a big number in terms of resistivity.”
“We looked at a fundamental physical property, the resistivity of a metal, and show that if you pressurize these metals, resistivity decreases, “ Lanzillo said. “And not only that, we show that the decrease is specific to different materials – aluminum will show one decrease, but copper shows another decrease. This paper explains why different materials see different decreases in these fundamental properties under pressure.”
Limited by the difficulty of creating atomic-scale wires from scratch and measuring electron-phonon coupling experimentally, Lanzillo and Nayak turned to an alternate method of research after noting that atoms were more tightly-packed in the atomic-scale wiring than the bulk wiring.
“We theorized that if we compress the bulk wire, we might be able to create a condition where the atoms are closer to each other, to mimic the conditions at the atomic scale,” Nayak said.
Lanzillo and Nayak worked with Watson and Washington, who had previously collaborated with Nayak at New York State Interconnect Focus Center at Rensselaer, to craft a suitable experiment to test out their theories. Washington was vital in his knowledge of experimenting with materials for metal interconnects of less than 20 nanometers, while Watson, with his experience in simulating extreme geochemical pressures like those in the center of the Earth, was integral in designing the experiment.
In this study, aluminum and copper were subjected to high pressures of 20,000 atms, yielding important information about their electrical resistivity. The team thus concluded experimentally that Lanzillo’s and Nayak’s initial theoretical predictions were correct.
“We can make this prediction with a computer simulation but it’s much more salient if we can get experimental confirmation,” Lanzillo said. “If we can go to a lab and actually take a block of aluminum and a block of copper and pressurize them and measure the resistivity. And that’s what we did. We made the theoretical prediction, and then our friends and colleagues in experiment are able to verify it in the lab and get quantitatively accurate results in both.”
“The experimental results were vital to the study because they confirmed that Saroj and Nick’s quantum mechanical calculations are accurate – their theory of electron-phonon coupling was validated,” Watson said. “And I think we would all argue that theory backed up by experimental confirmation makes the best science.”
This study signals an important new path in scientific research, as it shows that the resistivity of metals can be determined through computer simulations; specifically, by combining strain with semiconductor wafer fabrication techniques to chart changes in resistivity. This find could not only yield exciting information about semiconductors, interconnects and more, but could potentially save researchers a great deal of money and time.
