PORTLAND, Ore. -- Today's sub-22-nanometer chips are placing a strain on the ability to connect their billions of transistors with traditional copper interconnects. As the number of metallization layers escalates from nine to fifteen or more, the fineness of the copper lines is causing open circuits from voids during manufacturing and electromigration after the chips are deployed. New barrier materials have been proposed to solve the problem, but Applied Materials Inc. of Santa Clara, Calif., claims what is needed is a new technique that encapsulates copper interconnect lines in cobalt, thereby nixing electromigration and extending copper interconnects to the 10-nanometer node.
"Today chips have more than a thousand interconnects within the width of a human hair," Kavita Shah, global product manager for Applied Materials' Endura Volta platform tells EE Times in an interview. "At higher nodes we had room for redundant lines, but at sub-22-nanometer nodes there is no room for redundancy -- and a single void can render a chip useless -- lowering yields."
Through years of research Applied Materials has come up with a solution that it believes can extend copper interconnects to the 10-nanometer node -- namely, encapsulating the copper lines in cobalt. Today Applied Materials Endura platform is used to manufacture copper interconnects on chips by first performing a cleaning step to remove particles left over after etching, then using physical vapor deposition (PVD) to add a tantalum nitride/tantalum (TaN/T) barrier layer to prevent copper diffusion into the dielectric. Then it uses PVD to deposit a copper seed -- a thin continuous layer of copper alloy -- onto†the TaN/T barrier. Finally the wafer goes to electroplating and polishing.
The new Endura Volta CVD Cobalt tool encapsulates copper features in cobalt to prevent voids and electromigration.
By integrating Applied Materials' Endura Volta CVD chamber into the process flow, cobalt is deposited atop a TaN barrier before a pure copper seed†layer is deposited. Then after electroplating and polishing, a selective CVD metal deposition process caps the copper with cobalt so that it is completely encapsulated, ensuring reliability and mitigating electromigration.
"Cobalt adheres well to both the TaN barrier and to the copper and works to confine it and prevent electromigration," Shah tells EE Times. "And it should work all the way to the 10-nanometer node. We have measured a 10-fold improvement in electromigration, and some of our customers have reported 100-fold improvement."
Applied Materials has already qualified cobalt encapsulation at multiple customer sites. In the last 24 months it has already shipped more than 75 Endura Volta CVD chambers.
"CVD cobalt technology represents the biggest material change to copper interconnections in the last 15 years," Shah tells EE Times. "We advocate complete encapsulation of copper lines for the best reliability.
Cobalt encapsulation adds two new steps to the copper interconnection process and requires an Endura Volta CVD Cobalt tool.
(Source: Applied Materials)
"We are the industry's first selective CVD cobalt technology that is ready for high-volume manufacturing. And we are also the only integrated conformal CVD system for the interconnect application."
In typically applications encapsulation in cobalt will add two additional processing steps to the five most critical interconnection levels.
— R. Colin Johnson, Advanced Technology Editor, EE Times