refrigerators capable of reaching temperatures as
low as 100 milliKelvin have been used to cool bulk
objects for the first time, researchers at the National
Institute of Standards and Technology (NIST) report.
The solid-state refrigerators have applications such
as cooling cryogenic sensors in highly sensitive instruments
for semiconductor defect analysis and astronomical
The work is featured in the
April 25, 2005, issue of Applied Physics Letters.*
The NIST-designed refrigerators, each 25 by 15 micrometers,
are sandwiches of a normal metal, an insulator and
a superconducting metal. When a voltage is applied
across the sandwich, the hottest electrons "tunnel"
from the normal metal through the insulator to the
superconductor. The temperature in the normal metal
drops dramatically and drains electronic and vibrational
energy from the objects being cooled.
The researchers used four pairs
of these sandwiches to cool the contents of a silicon
nitrate membrane that was 450 micrometers on a side
and 0.4 micrometers thick. A cube of germanium 250
micrometers on a side was glued on top of the membrane.
The cube is about 11,000 times larger than the combined
volume of the refrigerators. This is roughly equivalent
to having a refrigerator the size of a person cool
an object the size of the Statue of Liberty. Both
objects were cooled down to about 200 mK, and further
improvements in refrigerator performance are possible,
according to the paper.
The refrigerators are fabricated
using common chip-making lithography methods, making
production and integration with other microscale devices
straightforward. The devices are much smaller and
less expensive than conventional equipment used for
cooling down to 100 mK, a target temperature for optimizing
the performance of cryogenic sensors. These sensors
take advantage of unusual phenomena that occur at
very low temperatures to detect very small differences
in X-rays given off by nanometer-scale particles,
enabling users such as the semiconductor industry
to identify the particles. The work was supported
in part by the National Aeronautics and Space Administration
and NIST's Office of Microelectronics Programs.
*A.M. Clark, N.A. Miller, A. Williams, S.T. Ruggiero,
G.C. Hilton, L.R. Vale, J.A. Beall, K.D. Irwin, J.N.
Ullom. Cooling of Bulk Material by Electron-Tunneling
Refrigerators. Applied Physics Letters. April 25,