Sat, Aug 23, 2008 page4

• Taiwanese paper published in ‘Science’

  COPPER FOILS Researchers at National Tsing-hua University increased the durability of the wiring in IC chips tenfold with the use of a new nano-manufacturing process

  • By Meggie Lu / STAFF REPORTER
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  The lifespan and durability of copper foils, which are widely used to conduct electric currents in integrated circuit (IC) chips, may increase tenfold using a new manufacturing process, so that even with the reduction in the size of foils in future computers, the foils would still be able to sustain usage, the National Science Council (NSC) said yesterday.

  The breakthrough, which is highly applicable in industry, was published in Science magazine yesterday, making it the first Taiwanese paper in materials science to receive such recognition.

  “For the past half century, it has been agreed that damage occurs in metal foils [wires] in IC chips because of the ‘electromigration’ phenomenon, which creates voids in the foils,” said Liao Chien-neng (廖建能), an associate professor at National Tsing-hua University’s department of materials science and engineering.

  “With time the voids grow in size, until eventually the foils cease to conduct electricity and a malfunction in the chip occurs,” Liao said.

  Though the phenomenon may seem complex, it can be easily explained with a metaphor, Liao said.

  “If electric current is water in a river, then atoms in copper grains are like pebbles in the river — with time, pebbles in the outer rim of the river would be washed away, creating a void of space where the pebbles used to exist,” he said.

  When enough “pebbles” are diffused outward, the “river” would cease to flow smoothly, he said.

  The atomic diffusion along copper grain boundaries is especially problematic because computers today are becoming smaller and smaller, meaning the copper wires used in IC chips have to be thinner and thinner, he said.

  Using a highly specialized high-resolution transmission electron microscopy (HRTEM) and ultra-high vacuum, the team not only observed for the first time ever the electromigration phenomenon, but also found a way to retard it, so that the durability of conducting wires in IC chips can be enhanced, he said.

  “Today, the copper foils used in IC chips are nano-scale and single-grain — the structure looks like bamboo, with each compartment of the bamboo being a ‘grain’ of copper,” said Chen Kuan-chia (陳冠嘉), Liao’s doctoral student and the first author of the paper.

  Chen said that, traditionally, atomic crystals in all of the copper grains go in the same direction.

  “However, if we employ a pulsed electro-deposition to synthesize a copper foil, instead of continuous electro-deposition, a ‘nanotwin’ foil can be created,” he said.

  A nanotwin foil is a foil for which adjacent copper grains carry mirror-image crystalline orientations — meaning the copper grains in the foil would have two orientations that are placed in intermittent order.

  “We saw that at twin-modified grain boundaries, atomic diffusion is slowed because moving atoms cannot travel as smoothly when they have to ‘hop’ over a ‘twin boundary’ to go onto the next grain, which has oppositely oriented crystalline,” said Wu Wen-wei (吳文偉), a former Ph.D. student in the project who is now an assistant professor at National Chiao-tung University.

  When the diffusion is slowed, the rate of void formation speed is also slowed, he said, adding that “with the twin-width of 5 nanometers, the void creation rate can be slowed by ten-fold.”
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