by Debra Vogler, Senior Editor

Hot-spots resulting from nonuniform power dissipation over a chip are the bane of manufacturers of high-performance ICs such as CPUs, graphics chips, and DSPs — and as CMOS devices continue to scale and total power dissipation increases, these hot-spots will only become more pronounced. Nextreme Thermal Solutions is readying its nanoengineered films for embedded thermoelectric coolers to help the industry meet the challenges associated with the International Roadmap for Semiconductors (ITRS), which projects high-end CPUs dissipating 200W five years from now.

“Multi-core processors exacerbate the hot-spot issue,” making thermal management more challenging and requiring cooling solutions closer to the die, noted Bob Conner, VP of marketing and business development at Nextreme Thermal Solutions, a 2004 spin-off of RTI International that manufactures nanoengineered films for embedded thermoelectric coolers (eTECs). Cooling close to the die is necessary to handle nonuniform power dissipation, he told WaferNEWS, because there is significant spreading resistance within the IC package itself. Cooling close to the source of the heat keeps that problem to a minimum.

TECs, whether bulk or embedded nanoengineered films, are based on the Peltier effect: when current is applied to two dissimilar materials (e.g., p- and n-type), heat is absorbed at one junction and released at the other junction ¹. Typically, CMOS device manufacturers want to keep junction temperatures below 100°C. Nextreme is targeting the hottest hot-spots — those that are typically 5-10°C above the average die temperature (see figure, below).

Strategically placing an eTEC between the backside of the die and the heat spreader creates a temperature inversion at the site of the hot spot. “Essentially, we are moving heat from a low thermal conductivity material, the Si die and the thermal interface material [“TIM1″ in the figure below, between the die and heat spreader] to high thermal conductivity materials — the spreader and heat sink,” explained Conner. “So we’re cooling the hot spot by 5-10°C, which impacts reliability, performance, yield, and leakage power.” Conner maintains that this small amount of cooling of the hottest hot spots enables CPUs to bin out at slightly higher chip speeds, thus earning more money for the IC manufacturer.

The eTECs are thin (5-10µm thick) nanoengineered films, typically from bismuth telluride (Bi₂Te₃) and antimony telluride (both p-type and n-type materials), using a modified MOCVD reactor and proprietary processes. After processing, these films have the property of retaining their electrical conductivity while incurring a decrease in their thermal conductivity, vs. the more typical situation where a good electrical conductor is a good thermal conductor. “What you want for a TEC is a very good electrical conductor, so you can move a lot of electrons and holes from the heat absorbing side to the heat releasing side, but a very poor thermal conductor to prevent the hot side from heating up the cold side,” explained Conner.

The company believes it is positioned to help the industry meet the challenges associated with the International Roadmap for Semiconductors (ITRS), which projects high-end CPUs dissipating 200W five years from now. Conner added that the eTEC approach complements uniform cooling techniques that apply to the entire chip, such as liquid cooling, refrigeration, and improving existing thermal components (e.g., heat sinks, fans, heat pipes, and heat spreaders). Specifically, eTECs enable designers to avoid over-engineering chip scale thermal solutions by targeting the hot spots. He further maintains that alternative hot-spot cooling approaches such as thermal plugs and bulk TECs in combination with thermal plugs are either too costly, or have CTE mismatch issues. Additionally, jet spray impingement is only in the research stage.

Initially, Nextreme plans to target low-volume/high-value products such as electronics used in oil wells, military applications, and communications infrastructure — basically, high ambient temperature environments. The company is not yet in production but is sampling some early products. “Next year, we’re going to have samples to start product qualification,” said Conner. “There’s an extensive customer qualification process so we’re probably 2-3 years away from starting a production ramp for niche applications, and 3-5 years to move into volume PC applications.” — D.V.

¹ G. Snyder, M. Soto, R. Alley, D. Koester, B. Conner, Hot Spot Cooling using Embedded Thermoelectric Coolers, 22nd IEEE SEMI-THERM Symposium, 2006.