WEST LAFAYETTE, Ind. Researchers at Purdue University have tapped an inexpensive wet-chemistry method to deposit nanoscale patterns of gold, platinum or palladium directly on top of semiconductor chips. While limited at present to gallium arsenide and germanium substrates, the work could enable a low-cost alternative to current metal deposition methods in chip manufacturing. It may also hold promise for the development of single-chip biohazard sensors to combat terrorism.
The technique takes low-purity metal-salt compounds and chemically extracts their pure metal by bonding it to a patterned chip's surface. The gold nanoparticles could also be functionalized with organic molecules for sensing biological agents, said professor Jillian M. Buriak. "Our discovery of a rapid, cost-effective method for forming tiny particles of high-purity metals on the surface of semiconductors should have economic benefits for chip makers," she said. "In addition, we think our nanoparticles can be useful in sensors."
Last year, Buriak demonstrated an inexpensive photolithography process that enabled nanocrystalline "porous" silicon to be stabilized for chip making without significant loss of the material's photoemissive qualities. Since then Buriak has been working with chemistry graduate student Lon Porter and others in her group to find a cost-effective method for precipitating nanoparticles of high-purity metals on the surface of advanced semiconductor materials. A bonus was that the gold surface proved to be so rough that it could easily be functionalized with organic molecules that change its response enough to sense specific biohazards.
"We have found a way to connect a computer chip to the biological world using organic molecules that help us detect biohazards such as bacteria, nerve gas or other chemical agents," said Buriak. The method could result in single-chip solutions for detecting biohazards. Chips would be patterned with a large array of nanoscale gold pads (50 nanometers square), each treated with a different organic coating and all connected to individual, 30-nm wires that would fan up to connect with the microcontroller portion of the chip.
Gold is chosen to connect chips to their packages because it does not corrode in air as copper and silver do. Most of the gold in the world is dissolved in the oceans as metal ions. But an industrial by-product metal salt called tetrachloraurate is packed with wasted gold ions. Buriak's group has come up with an approach that mines that gold in a simple "dip, rinse and dry" method.
"Our method only works for gallium arsenide and germanium chips, unfortunately, not silicon. The gold metal ions are triply charged. GaAs and germanium will give up three electrons to make the metal solid again," said Porter.
The sharing of electrons between the former gold ions and the substrate makes for a strong bond to the surface, according to Porter. The molecule-by-molecule nature of this de-ionizing process also ensures that the gold deposited on the substrate is in tiny nanoparticles that form an exceedingly rough porous surface.
"The reaction sustains itself; you don't need any special training or equipment,"said Porter. "It's a real money saver from both a manpower and a technical perspective, and the rough surface gives us a lot of nooks and crannies in which to secure organic molecules that react in the presence of other chemicals."
In the Purdue process, nanoparticles form gradually on a surface of bare germanium (top) after the semiconductor is dipped into a solution of metal salt.
The group has experimented extensively with using traditional photolithographic techniques to pattern the surface of its GaAs and germanium wafers before exposure to tetrachloraurate. The group also explored advanced techniques beyond photolithography, including microcontact printing and dipped nanolithography patterning.
Currently the group is "downsizing" its chip-patterning techniques to direct writing by atomic-force microscope (AFM).
"With the AFM we can write impossibly small circuitry much smaller than anything you will ever get with lithography. The smallest feature we can make this way is by dipping the tip into tetrachloraurate and just touching it to the chip," said Porter.
The tetrachloraurate is deposited wherever gold is desired, and the gold ions are attracted to the GaAs or germanium. Then a washing step removes all the other compounds in the tetrachloraurate, leaving only the gold bonded to the surface.