Thermal considerations are rapidly becoming one of the most serious design constraints in microelectronics, especially on submicron scale lengths. A study by researchers from the University of Illinois at Urbana-Champaign has shown that standard thermal models will lead to the wrong answer in a three-dimensional heat-transfer problem if the dimensions of the heating element are on the order of one micron or smaller.
"As materials shrink, the rules governing heat transfer change as well," explained David Cahill, a professor of materials science and engineering at Illinois. "Our current understanding of nanoscale thermal transport isn't nuanced enough to quantitatively predict when standard theory won't work. This can impact the design of high-power RF devices that are widely used in the telecommunication industry—for example, 4G wireless infrastructure.
Schematic representation of thermal transport for small heater dimensions. Vibrational waves, or photons, that travel parallel to the surface do not help cool the hot region when its dimensions are small because they can traverse its small diameter without interacting with it. The metal-coated surface prevents phonons traveling perpendicular the surface from traversing the heated region without interaction.
Credit: Richard Wilson, University of Illinois
The transistor spacing in RF devices is rapidly approaching length-scales where theory based on the diffusion of heat won't be valid, and the engineering models currently used won't accurately predict the operating temperature of the device. The temperature is a key factor for predicting mean-time to failure"
"Our research focuses on understanding the physics of thermal transport on submicron length-scales in the presence of an interface," explained Richard Wilson, lead author of the study published in Nature Communications. "Our study focused on a variety of crystals that have controlled differences in thermal transport properties, such as Si, doped Si, and SiGe alloys," Wilson said.
"We coated these crystals with a thin metal film, heated the surface with a laser beam, and then recorded the temperature evolution of the sample.
"On length-scales shorter than the phonon mean-free-paths of the crystal, heat is transported ballistically, not diffusively. Interfaces between materials further complicate the heat-transfer problem by adding additional thermal resistance."
Researchers found that when the radius of the laser beam used to heat the metal coated crystals was above ten microns, the predictions made by assuming heat is transported diffusively matched the experimental observations. However, when the radius neared one micron, diffusive theory over-predicted the amount of energy carried away from the heated surface.
"We discovered fundamental differences in how heat is transported over short versus long distances. Fourier theory, which assumes heat is transported by diffusion, predicts that a cubic crystal like silicon will carry heat equally well in all directions. We demonstrated that on short length-scales heat is not carried equally well in all directions. By measuring the temperature of the sample surface as a function of distance from the center of the heated region, we were able to determine how far heat was traveling parallel to the surface, and deduce that, when heater dimensions are small, significantly less heat is carried parallel to the surface than Fourier theory predicts," Wilson stated.
Wilson and Cahill also studied the effect of interfaces on nanoscale thermal transport.
"It's been well known for 75 years that the presence of a boundary adds a thermal boundary resistance to the heat-transfer problem, but it's always been assumed that this boundary resistance was localized to the interface and independent of the thermal transport properties of the underlying material," Cahill added. "Our experiments show that these assumptions aren't generally valid. In particularly for crystals with defects, the boundary resistance is distributed and strongly dependent on the defect concentration. "
Wilson and Cahill also provided a theoretical description of their results that can be used by device engineers to better manage heat and temperature in nanoscale devices.
This work was supported by the Air Force Office of Scientific Research and was carried out, in part, in the Frederick Seitz Materials Research Laboratory at Illinois.
David G. Cahill | Eurek Alert!
Stretchable biofuel cells extract energy from sweat to power wearable devices
22.08.2017 | University of California - San Diego
Laser sensor LAH-G1 - optical distance sensors with measurement value display
15.08.2017 | WayCon Positionsmesstechnik GmbH
Whether you call it effervescent, fizzy, or sparkling, carbonated water is making a comeback as a beverage. Aside from quenching thirst, researchers at the University of Illinois at Urbana-Champaign have discovered a new use for these "bubbly" concoctions that will have major impact on the manufacturer of the world's thinnest, flattest, and one most useful materials -- graphene.
As graphene's popularity grows as an advanced "wonder" material, the speed and quality at which it can be manufactured will be paramount. With that in mind,...
Physicists at the University of Bonn have managed to create optical hollows and more complex patterns into which the light of a Bose-Einstein condensate flows. The creation of such highly low-loss structures for light is a prerequisite for complex light circuits, such as for quantum information processing for a new generation of computers. The researchers are now presenting their results in the journal Nature Photonics.
Light particles (photons) occur as tiny, indivisible portions. Many thousands of these light portions can be merged to form a single super-photon if they are...
For the first time, scientists have shown that circular RNA is linked to brain function. When a RNA molecule called Cdr1as was deleted from the genome of mice, the animals had problems filtering out unnecessary information – like patients suffering from neuropsychiatric disorders.
While hundreds of circular RNAs (circRNAs) are abundant in mammalian brains, one big question has remained unanswered: What are they actually good for? In the...
An experimental small satellite has successfully collected and delivered data on a key measurement for predicting changes in Earth's climate.
The Radiometer Assessment using Vertically Aligned Nanotubes (RAVAN) CubeSat was launched into low-Earth orbit on Nov. 11, 2016, in order to test new...
A study led by scientists of the Max Planck Institute for the Structure and Dynamics of Matter (MPSD) at the Center for Free-Electron Laser Science in Hamburg presents evidence of the coexistence of superconductivity and “charge-density-waves” in compounds of the poorly-studied family of bismuthates. This observation opens up new perspectives for a deeper understanding of the phenomenon of high-temperature superconductivity, a topic which is at the core of condensed matter research since more than 30 years. The paper by Nicoletti et al has been published in the PNAS.
Since the beginning of the 20th century, superconductivity had been observed in some metals at temperatures only a few degrees above the absolute zero (minus...
16.08.2017 | Event News
04.08.2017 | Event News
26.07.2017 | Event News
22.08.2017 | Health and Medicine
22.08.2017 | Materials Sciences
22.08.2017 | Life Sciences