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Columbia-IBM Team Is First To Measure Stresses In Microcircuits From Electromigration

Columbia University

As the tiny electric wires in computer chips grow ever smaller and the current they carry proportionately greater, the wires' atomic structure becomes increasingly prone to breakdown.

A team of materials scientists from Columbia University and IBM is the first to measure the huge forces created as electric currents dislodge atoms from the microwires, causing gaps that may disable a chip or an entire computer.

Called electromigration, the phenomenon probably won't cause failure in existing computers, but will present a mounting problem to chip designers and manufacturers, say the researchers, Slade Cargill, professor of materials science at Columbia's School of Engineering and Applied Science, and I.C. Noyan, a research staff member with the Materials and Processing Science Group at IBM's T. J. Watson Research Center.

"We have gone up in current density by an order of magnitude, and that's the problem," Professor Cargill said.

The researchers focused a fine beam of X-rays to measure changes in the atomic structure of a wire as current flowed through it. They showed that one end of the wire -- about a sixth of its length -- was stripped of metal, and that a buildup of atoms at the other end caused large stresses, of as much as 50,000 pounds per square inch, that eventually damaged the wire and its insulation. Professor Cargill presented a paper on the work, "X-Ray Microprobe Study of Electromigration," March 17 at a meeting of the American Physical Society in Kansas City, Mo.

Electric current consists of electrons moving inside a metal wire. Electrons also dislodge atoms of metal from their positions in the wire, then carry them along and deposit them further downstream. Rearranging atoms in this fashion can create gaps where atoms are removed and can also create local pressures where atoms pile up, squeezing metal out of the wire much like toothpaste from a leaking tube.

Electromigration is not a problem in households or most industries because moving a few thousand atoms won't affect wires millions of atoms in diameter. But in microelectronics, many wires are now less than one micron in diameter, one-one-hundredth the diameter of a human hair, and moving thousands of atoms around can have far more dramatic effects.

If electromigration occurred in a working computer, what would happen-- Your computer would stop," Professor Cargill said. "The screen might go dark and it would cease functioning."

Computer failures because of electromigration actually occurred in the 1960s, before hardware engineers were fully aware of the problem. They solved it by using new combinations of metals, by limiting current and the wires' length, and by encapsulating circuits in rigid insulating materials.

During the last 30 years, however, the size of microelectronic circuits has decreased by almost 40 times, Professor Cargill said, while current in those circuits has decreased less rapidly, from the range of 10 to 50 milliamperes to about a tenth of that level now. As a result, current density, the measurement of electric current per unit of area, has increased by a factor of ten.

Once microwires shrink to a quarter of a micron in diameter and smaller, chip designers will have to discover ways to limit current sent through such wires, or find other ways to counter electromigration. Presently, other factors limit the current that can be carried in computer chips, Professor Cargill said.

"These problems can be solved by changing the configuration and metallurgy of the chip," Dr. Noyan said. "However, new solutions will have to be found for each generation of chips, since electromigration is like an allergy. It can be mitigated but never fully cured."

The joint research team from Columbia and IBM Research, working at the National Synchrotron Light Source at Brookhaven National Laboratory on Long Island, concentrated X-rays to a 10-micron beam by focusing them through a tapered glass capillary. That X-ray source provided the fine resolution needed to examine the microwire as electromigration took place.

For 70 hours, the researchers sent current through a flat aluminum wire half a micron thick, 10 microns wide and 200 microns long, covered with a layer of rigid silica insulation. As current passed through the tiny circuit, the team made X-ray measurements of changes in the wire's atomic structure. At the end of the wire where current was applied, about 30 microns of aluminum was stripped away, creating a gap.

At the wire's other end, researchers measured a continuous increase in stress as additional atoms were forced into the region. The maximum stress measured was 50,000 pounds per square inch, which is more than five times the usual load-bearing capacity of aluminum. After about 15 hours, they observed an abrupt decrease and then a more gradual increase in stress. The decrease in stress is thought to represent a deformation of the aluminum and a resulting break in the silica insulating material.

The work was supported by the National Science Foundation, Brookhaven National Laboratory and IBM.

This document is available at http://www.columbia.edu/cu/pr/.

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