Refrigerator on a chip

Microelectronic and optoelectronic integrated circuits (ICs) develop hot spots that shorten the devices’ lifetimes. To solve this problem, a collaboration led by John Bowers of the University of California, Santa Barbara, and Ali Shakouri of the University of California, Santa Cruz, is developing tiny thermoelectric (TE) refrigerators that sit right on top of the chips. Conventional TE coolers are already used widely to cool semiconductor lasers and other circuitry, but they are manufactured separately from the ICs. The new superlattice microcoolers are grown directly on silicon surfaces, giving them more intimate thermal contact with the semiconductors and simplifying overall fabrication. Earlier versions of the microcoolers made of Si and germanium required buffer layers to ease the strain of matching their lattice structures to the underlying Si substrate. By adding carbon to the lattice, the researchers found they could do without the buffer layers, and thus improve thermal contact and simplify fabrication even further. The 7°C cooling achieved so far is modest, but theoretical calculations show that a single-stage microcooler should attain the tens of degrees of cooling needed for commercial applications. (Xiaofeng Fan et al., Appl. Phys. Lett. 78, 1580, 2001 http://dx.doi.org/10.1063/1.1356455 .)