Discover Copper Online

Spring 1992

Copper to Replace Aluminum in New Chip Technology

Reprinted with permission from Rensselaer, copyright Sept. 1991, Rensselaer Polytechnic Institute.

Shyam Murarka holds gleaming silicon wafers for new chips that will contain 3-D copper pathways. Shyam Murarka holds gleaming silicon wafers for new chips that will contain 3-D copper pathways.

Photo courtesy of RPI

If copper makes you think of pennies, you may not consider it valuable. But researchers in the Sematech Center of Excellence (SCOE) at Rensselaer are demonstrating that copper can be the key to powerful new three-dimensional computer chips.

Two years ago, the SCOE received a five-year, $5 million contract from Sematech, a consortium of U.S. semiconductor firms, to develop a four-level metal interconnect system for use in 1993 and beyond. SCOE researchers at Rensselaer and at the State University of New York at Albany are collaborating with Colorado State University, the University of North Texas and Sandia National Laboratory. They are reporting extremely encouraging research results, especially in the technology that will let manufacturers replace aluminum connections with copper.

Shyam P. Murarka, professor of materials engineering and co-director of the SCOE, said four-level computer chips will be faster and will have far greater memory than current chips. At present, most chips are laid out in two dimensions - devices are arranged on a flat substrate. At best, they contain two levels.

"This requires a lot of real estate on the chip," Murarka said. "Interconnections become too long, which slows down the device. Our goal is to use the third dimension to reduce the length of the interconnections and save area on the chip."

Current technology has pushed materials now in use to their physical limits, he said. Aluminum, now used for the connections between chips and between the devices on a chip, has problems in four-level chips.

Its relatively high electrical resistance inhibits the flow of electrons. The resistance also creates heat that becomes increasingly troublesome as the devices on a chip are packed more tightly. In addition, the low melting point of aluminum makes it less useful due to diffusion-induced instabilities.

Copper has lower resistance and will function well even if lines are thinner. But copper reacts chemically with silicon and causes electrical problems. To use copper, chip designers must learn to prevent this reaction, and they must develop practical manufacturing techniques that will use copper on dielectric layers away from the semiconductor.

SCOE researchers are studying barriers that can be used between copper and silicon dioxide, the preferred dielectric layer. In a major breakthrough, they have developed a way to lay down extremely pure layers of copper through chemical vapor deposition (CVD) at temperatures of 200 - 400C They also have developed ways to etch thin lines of copper on chips, and they have demonstrated the possibility of laying down thin lines of copper selectively, eliminating expensive etching.

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