New cryogenic refrigerator dips chips into a deep freeze

In a major advance for cryogenics, researchers at the National Institute of Standards and Technology (NIST) have developed a compact, solid-state refrigerator capable of reaching temperatures as low as 100 milliKelvin. The refrigerator works by removing hot electrons in a manner similar to an evaporative air-conditioner or “swamp cooler.”

When combined with an X-ray sensor, also being developed at NIST, the instrument will be useful in semiconductor manufacturing for identifying trace contaminants and in the astronomical community for X-ray telescopes. The device can be made in a wide range of sizes and shapes, as well as readily integrated with other cryogenic devices ranging in size from nano-meters to millimeters.

A report of the work is featured on the cover of the January 26, 2004, issue of Applied Physics Letters. “The idea is to use a solid-state refrigerator for on-chip cooling of these cryogenic sensors,” says Anna M. Clark, the report’s lead author. “We have a working refrigerator that reduces temperatures low enough to be used with highly sensitive X-ray detectors. These detectors require subKelvin temperatures to minimize thermal noise and maximize their resolution.”

Current equipment capable of cooling to 100 milliKelvin is bulky and expensive. By combining an on-chip cooler with an X-ray sensor, the NIST device may reduce substantially the weight and cost of such equipment.

The refrigerator is made from a sandwich of nomal- metal/insulator/superconductor junctions. When a voltage is applied across the “sandwich,” high-energy (hot) electrons tunnel from the normal metal through the insulator and into the superconductor. As the hottest electrons leave, the temperature of the normal metal drops dramatically.