The cooling units, made by Xiaofeng Fan of the University of California at Santa Barbara and co-workers, consist of 200 alternating layers of two semiconductors stacked atop one another like tiles. Each layer of this 'superlattice' is just ten millionths of a millimetre (nanometres) thick 1.

The layers can reduce the local temperature on a silicon chip by up to 7°C when the chip is operating at a temperature of 100°C. To be commercially useful, these devices will have to perform several times better than this; this should be possible with further improvements, the researchers estimate.

Chips get hot for the same reason as any other electrical device: some of the energy of the electrons that carry the current is dissipated as heat, warming up the wires and circuit components. As integrated circuits get ever smaller and more dense, overheating is becoming more problematic. It is this, as well as the difficulties of ultra-small fabrication, that limits miniaturization.

Currently, some integrated circuits and lasers used in telecommunications are cooled with thermoelectric coolers. These exploit the flow of an electrical current from one semiconducting material to another to extract heat from the junction between them. The current plays the same role as the coolant fluid in a conventional refrigerator.

Unlike fridges, thermoelectric cooling devices have no moving parts, and so are potentially very robust and reliable. But they are not very efficient, and so tend to be used where compactness and lightness are more important -- such as plug-in drinks coolers. In microelectronics, thermoelectric coolers are generally manufactured separately from the chips or devices, and bolted on to cool the entire chip.

The new microcoolers are more efficient to make and to use because they can be fabricated directly onto a chip. They are also so small that they can regulate 'hot spots' that can severely reduce the working lifetime of a chip.

Microcoolers are made from silicon and an alloy consisting of silicon, germanium and carbon. By choosing the right ratios of silicon, germanium and carbon in the alloy, Fan's team have matched its atomic spacing to that of silicon, allowing the layers to fit together at the interface between them.

Previously, superlattices made from only silicon and germanium for thermoelectric cooling had to sit on a thick 'buffer layer' to alleviate the mismatch with the underlying silicon surface. This hindered good thermal contact between the chip and its cooler, reducing efficiency.