Thermoelectric Cooling Prepares for the Small Stage

Two of the biggest technical challenges the semiconductor industry faces in upcoming technology generations are heat removal and efficient power delivery. Even with the reduced supply voltages that smaller transistors will require, faster clock speeds and increased use of stacked-die packaging make heat removal a key concern. Also, supplying power with adequate quality is more of a challenge when the supply voltage is reduced. At the Next-Generation Thermal Management Materials and Systems Conference hosted by the Research Triangle Institute earlier this year, three companies presented their results of using microscale thermoelectric devices to solve these problems.

Thermoelectric coolers use the Peltier effect (Figure ). A current is passed through a circuit containing dissimilar metals, or semiconductors of different conductivity types, producing a temperature difference between junctions. They are currently used in a variety of applications, including small refrigerators. Their main advantages are that they have no moving parts and do not require phase changes in environmentally unfriendly materials.

The use of thermoelectric devices for cooling electronics has been talked about for decades. There has been little need for them until now, though they have been used from time to time to cool graphics accelerator cards and overclocked CPUs.

Of course, they do not eliminate the need for heat sinks; they simply transport heat to the heat sink more efficiently. In fact, they generate heat because of joule heating. The heat sink must be able to handle this extra load. The amount of this extra load depends on the materials used and geometric factors. The new devices minimize the added load by addressing both of these aspects.

In Peltier effect devices, heat flows in the same direction as majority carriers.

The thermoelectric devices presented were fabricated using thin-film processes, and had a thickness of ~100 µm. The thinnest devices based on bulk thermoelectric materials are a few millimeters thick. The new, thinner devices are particularly attractive for two reasons: They can fit inside a packaged device, rather than on the package; and they can be used for highly localized cooling, such as a hot spot on a die.

Localized cooling is an attractive solution for dealing with hot spots. Eliminating hot spots in the design is preferred, but many times it is either not possible or it might extend the design schedule too far. Cooling the whole die in hopes of keeping the hot spot temperature down can be very inefficient, because most of the die does not require much, if any, cooling. By cooling the hot spot directly, the overall package design can be simpler, reducing material and design cost.

Infineon (Munich) is spinning out a company, Micropelt , to commercialize its microscale thermoelectric technology. Using an optimized sputtering process, n-type bismuth telluride (Bi₂Te₃) posts are deposited on one wafer, and p-type posts are deposited on another. Individual die from each wafer are attached face-to-face to form the cooling element.

It is possible to make Peltier coolers using Si/SiGe, and CMOS-compatible processes already exist for this material set. The Infineon/Micropelt process was developed to provide a CMOS-type of process for Bi₂Te₃.

Nextreme Thermal (Research Triangle Park, N.C.), a spinout company from the Research Triangle Institute, has developed thermoelectric coolers with similar characteristics. Its technology also uses a thin-film Bi₂Te₃ superlattice material. The material has phonon-blocking properties, which ensures that most of the heat transfer is done by charge carrier motion. This makes it less likely for heat to flow in the undesired direction, and therefore more efficient.

Nanocoolers (Austin, Texas) has also developed thermoelectric cooling modules based on thin-film technology, but has not given many details about it so far. The company had been splitting its efforts between thermoelectric coolers and its liquid metal cooling technology, but has recently focused its efforts solely on thermoelectric coolers.

Another application for thermoelectric cooling is laser diode cooling. Laser diodes dissipate a lot of heat, and their output wavelengths are also temperature-sensitive. In addition to being small enough to fit in a laser diode package, a microscale thermoelectric cooler also provides faster response time. It is conceivable for thermoelectric coolers to provide active temperature control in this application.

Another thermoelectric effect can be used to help with power delivery. The Seebeck effect transforms thermal energy to electrical energy. Seebeck effect devices can be made using the same processes used to make Peltier effect devices, and they can scavenge waste heat for reuse as electrical power.

Since waste heat is typically produced by the transistors using the most power, localized heat scavenging could help augment electrical power delivery right where it is needed the most. Realistically, this could, at most, reduce the localized supply ripple, but with supply voltages decreasing as they are, any help reducing supply ripple would be welcome.

As transistors get smaller, it seems that things that used to be outside of the chip are going into it. It also appears that targeted heat removal may become a trend, which requires smaller coolers be brought into the package.