Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

All directions are not created equal for nanoscale heat sources

01.10.2014

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!
Further information:
http://engineering.illinois.edu/

More articles from Power and Electrical Engineering:

nachricht A big nano boost for solar cells
18.01.2017 | Kyoto University and Osaka Gas effort doubles current efficiencies

nachricht Multiregional brain on a chip
16.01.2017 | Harvard John A. Paulson School of Engineering and Applied Sciences

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Traffic jam in empty space

New success for Konstanz physicists in studying the quantum vacuum

An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

Im Focus: Studying fundamental particles in materials

Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales

Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...

Im Focus: Designing Architecture with Solar Building Envelopes

Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.

As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Sustainable Water use in Agriculture in Eastern Europe and Central Asia

19.01.2017 | Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

 
Latest News

New Study Will Help Find the Best Locations for Thermal Power Stations in Iceland

19.01.2017 | Earth Sciences

Not of Divided Mind

19.01.2017 | Life Sciences

Molecule flash mob

19.01.2017 | Physics and Astronomy

VideoLinks
B2B-VideoLinks
More VideoLinks >>>