Almost three years ago a team of scientists at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) was performing an experiment in which layers of gold mere nanometers (billionths of a meter) thick were being heated on a flat silicon surface and then allowed to cool. They watched in surprise as peculiar features expanded and changed on the screen of their electron microscope, finally settling into circles surrounded by irregular blisters.
The circles varied in diameter up to a few millionths of a meter, and in the center of each was a perfect square. The mysterious patterns were reminiscent of nothing so much as so called "alien" crop circles.
Until recently the cause of these strange formations remained a mystery. Now theoretical insights have explained what's happening, and the results have been published online by Physical Review Letters at http://prl.aps.org/abstract/PRL/v108/i9/e096102.
Eagerly melting alloys
When two solids are combined in just the right proportions, changes in chemical bonding may produce an alloy that melts at a temperature far lower than either can melt by itself. Such an alloy is called eutectic, Greek for "good melting." The eutectic alloy of gold and silicon 81 percent gold and 19 percent silicon is especially useful in processing nanoscale semiconductors such as nanowires, as well as device interconnections in integrated circuits; it liquefies at a modest 363˚ Celsius, far lower than the melting point of either pure gold, 1064C, or pure silicon, 1414C.
"Gold-silicon eutectic liquid can safely solder chip layers together or form microscopic conducting wires, by flowing into channels in the substrate without burning up the surroundings," says Berkeley Lab's Junqiao Wu. "It's particularly interesting for processing nanoscale materials and devices." Wu cites the e
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory