Scientists have discovered and demonstrated a new material, for use in advanced processors, that can conduct heat 150% more efficiently, claims an article published by Lawrence Berkeley National Laboratory. Heat build-up in processors is a big problem for performance, and silicon acts as a natural heat insulator, inhibiting cooling. With new ultrathin silicon nanowire technology applied, it is hoped that chips will be able to get smaller, faster and cooler from a relatively simple change. The key change that has been tested is the use of isotopically purified Silicon-28 (Si-28).
Silicon is cheap and abundant but a poor heat conductor, and this is a problem in tiny microchips with tens of billions of transistors clocked at GHz speeds. Natural silicon is made up of three isotopes: silicon-28, silicon-29, and silicon-30. Silicon-28 is the most naturally abundant, making up about 92% of natural silicon. Moreover, it has long been known that Si-28 is the best conductor of heat. If purified, Si-28 can conduct heat about 10% better than natural silicon. However, the advantage has been judged as not really being worthwhile, until now.
Sometimes technologies are worth re-examining and re-evaluating as complementary technologies emerge, and this seems to be the case here. In brief, the scientists decided to use purified Si-28 to create ultrathin nanowires.
Initially, the scientists confirmed that Si-28 was just 10% better at heat conductance than natural silicon. Moving down to wires of 1mm in diameter, this remained the case. However, when they created 90nm Si-28 nanowires (about a thousand times thinner than a human hair) the heat conductance was 150% better, which was a great surprise to them. They had only expected 10–20% better performance.
Investigations brought to light two main reasons for the excellent thermal conductivity of Si-28 nanowires. Electron microscopy revealed that the Si-28 nanowires had a more perfect glassy finish, so they didn't suffer from the drawbacks of phonon confusion / escape in natural silicon nanowire thermal transmission. Secondly, a natural layer of SiO2 formed on these nanowires, keeping the phonons transporting heat on-track. In summary, two previously observed phonon blocking mechanisms were vastly reduced due to the new material.
So, what is the use of a 150% better heat conducting silicon nanowire? Some state-of-the-art transistor designs already incorporate silicon nanowires. The Gate-All-Around Field Effect Transistor (GAA-FET) uses silicon nanowires stacked together to conduct electricity, but they still suffer from heat build-up. If this new material can be substituted then processor designers can grab a relatively quick and easy win.
Moving forward, the research team involved in Si-28 nanowires want to do more to control, rather than measure, heat conduction in the nanowires. Reading through the source article, there currently seems to be a severe lack of purified Si-28 available for further testing. The samples used in the above experiments came from a former Soviet-era isotope manufacturing plant.
If the benefits are really as good as claimed, then it would be necessary for one or more manufacturers to begin refining Si-28 again. Given the difficulty of further scaling of process technology, even a 50% boost in heat conductance might be reason enough to do so, never mind the claimed 150% improvement.
