The force that pushes atoms from the hot side to the cold side of a material is demonstrated by the continuous dragging of microscale patterns on a surface. When there is a significant temperature difference across a microcircuit in a device such as a mobile phone, atoms can migrate and eventually lead to poor electrical connections. Currently, this microscale tracking, called thermomigration, has shown a diffusion-induced force as the driving factor of the movement. The scientists looked at shallow depressions, or “basins,” that formed on the surface of a square silicon wafer that was heated on one side and cooled on the other. They watched the atoms move as a result of the temperature difference as they moved across the basins.
Other researchers may develop new strategies to regulate the growth of nanostructures with the help of the extracted force and overall characterization of the process.
The gradient (temperature difference) between two locations affects how quickly thermomigration moves from one area to another. According to Aix-Marseille University's Leroy, engineers spend a lot of time designing to avoid thermal gradients. But Leroy argues that the fundamental ideas underpinning this movement are not better understood. “We propose a method to quantify migration to have a very accurate value of the force driving the movement.”
To record atomic motion, Leroy and his colleagues started with a silicon wafer that was 9 mm wide and had a very flat, uniform surface.
A temperature difference of approximately 100° C was produced by applying a heat source to one side and a heat sink to the other. The team predicted that silicon atoms would move from hotter to cooler parts of the surface under this gradient. However, it would be difficult to actually see this movement. Because atoms move so fast, Leroy explains, "We cannot measure atomic motion directly."
Instead, the scientists examined wafer surface depressions one atom deep using an electron microscope. The scientists used successive photographs to determine the migration of these structures, several micrometers wide, toward the hot edge of the wafer at a speed of about 0,2 nanometers per second (nm/s).
The movement of the basin is explained by the researchers as being caused by silicon atom diffusion. As in a 2-dimensional gas, atoms break away from the hot wall of the basin and begin to move chaotically around the bottom of the basin. When one of these dispersed atoms reaches the colder wall of the basin, it can recombine. The basin walls move in the direction of the heat source as a result of the overall process.
Using this diffusion model, scientists calculated a thermomigration force of approximately 108 eV/nm, which is millions of times less than the forces that cause chemical bonding. Leroy suggests that due to the higher temperature gradients present in microcircuits, the thermomigration force should be stronger here. However, further research with different types of materials will be required to determine how strong the force will be. These experiments could reveal whether the diffusion mechanism the team identified is a feature of thermomigration in general.
Japanese Surface Scientist Hibino was surprised that the movement of the basins appeared so pronounced in the data, given that atomic motion is quite complex and the temperature difference between the basins is minimal (about 0,04 °C). According to Hibino, well-designed experiments are what allowed the authors to successfully extract the thermomigration effect from challenging processes.
The work demonstrates “beautiful technical prowess” and “the experimental measurements are impressive,” according to French condensed matter theorist Olivier Pierre-Louis. Still, he thinks more research is needed to improve the theoretical model. According to him, a better understanding of thermomigration could result in entirely new methods for producing nanostructures that use heat gradients to move atoms across a surface. “Thanks to their paper, we now have the numbers to tell us what is possible and what is not,” says Pierre-Louis.
📩 17/09/2023 19:03