Friday, January 30, 2009

Super-Charged Metal Ion Generator

It is rare indeed that a justifiably "new" thin films deposition technique comes along. In my lifetime, I have not seen such a new technology, other than cathodic arc in the 1970s, that has the potential to significantly advance the field.
BERKELEY, CA – In the electronics industry, thin metal films are deposited on silicon wafers with a sputter gun, which uses ionized argon gas to knock the metal atoms off a target. Scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory have now developed a powerful new kind of sputter process that can deposit high-quality metal films in complex, three-dimensional nanoscale patterns at a rate that by one important measure is orders of magnitude greater than typical systems.
Called “self-sputtering far above the runaway threshold,” the new method “is an extraordinarily prolific generator of metal ions,” says Andre Anders, a senior scientist in Berkeley Lab’s Accelerator and Fusion Research Division, where he leads the Plasma Applications Group. Anders and his colleague Joakim Andersson, now at Uppsala University in Sweden, based their new "Self-Sputtering" method on the existing technique of High Power Impulse Magnetron Sputtering (HIPIMS). The new method uses much higher power in short pulses in order to keep the average power low enough to avoid overheating the sputtering target.
Self-sputtering occurs when target atoms that have themselves been ionized return to the target to knock out yet more target atoms. Some of the sputtered atoms remain neutral and may fly straight to the substrate; others are ionized and may return to the target, producing yet more ions and yet more free electrons (secondary electrons).
Once self-sputtering gets started, if enough new atoms get ionized and enough new ions return to the target, it becomes self-sustaining. The magnetic field lines near the target grow thick with spiraling electrons, the plasma is dominated by metal ions instead of gas, and the sheath becomes a potent source of a large flux of energetic electrons that produce still more “excess” plasma – the system runs away, until it finally reaches a new equilibrium at a much higher peak-power level than before.
Beyond the semiconductor industry Anders sees a wide range of applications for the efficient new process, some of which may sound exotic. Because a sustained, self-sputtering plasma can operate in pure vacuum, the new method could also be used for coating materials in space, or even for ion thrusters whose fuel consists of a low-cost, noncombustible metal target, making it unnecessary to carry bottled gases or liquids into space.
For now, Andersson and Anders’s demonstration of a 250-ampere current of copper ions to a substrate – far higher than any ever achieved in a magnetron system – stands as an achievement with the potential to revolutionize some of the semiconductor industry’s most important manufacturing processes.