Atomic layer deposition uses metal-ion containing precursors (1) which chemisorb to a surface (2) in a layer one molecule thick. An activating agent (3) reacts with the precursors and converts the ions to elemental metals (4). A team at Carleton University has developed a new, more efficient precursor for copper and the world's only effective precursor for gold.
By Tyler Irving
Posted January 2012
If a microchip is a city, copper interconnects are like the streets between different buildings. As those chips continue to shrink, scientists must find ways of laying down ever-thinner layers of copper. A new technique developed at Carleton University could hold the answer.
Professor Sean Barry and PhD candidate Jason Coyle of Carleton’s Department of Chemistry work on a technique called Atomic Layer Deposition (ALD). This involves treating the desired surface with a gas-phase, metal-ion-containing complex called a precursor. Because the precursor complex can bond to the surface but not to other precursor molecules, a one-molecule-thick layer is deposited. That layer is then reacted with an activating agent such as hydrogen plasma that causes the metal within the complex to revert to its elemental form and bond to the surface. The process is then repeated a few thousand times, building up a layer of metal only nanometres thick.
ALD is decades old, but the difficulty lies in finding the right precursor molecules. Many candidate compounds break down at the temperatures in the reaction vessel, which can reach 250 C. Others have high boiling points and thus require too much energy to put into the vapour phase. The copper precursor developed by Barry and Coyle is unique: it’s stable to 500 C, evaporates at relatively low temperatures and deposits thin, electrically conductive layers of copper. Even better, the same chemistry works to deposit thin layers of gold. “Gold is a tricky metal centre to work with,” says Barry. “There’s a few ALD processes for copper, but until now there were none that worked for gold.”
The work was supported by a grant from GreenCentre Canada, which has licensed the technology and is working with Barry and Coyle on securing a patent. The copper process could lead to faster, smaller microchips, while the gold version could lead to better coatings for medical implants and opto-electronic sensing platforms. Best of all, the process allows manufacturers to be more efficient with each metal atom they deposit. “It’s exceedingly slow, but exceedingly careful,” says Barry. “And it’s green in the sense that it improves on a more messy technique.”
Photo Credit: GreenCentre Canada
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