by J. William Bell

Ask the stereotypical packrat--just because you replace something doesn't mean you get rid of it. The new television sits on top of the old one, a TV Guide mountain abuts the recliner, and someone might use that old bicycle someday. Room for a life amongst the clutter constantly diminishes.

As a silicon wafer is etched to become a microchip, it suffers a similar fate. The silicon is bombarded with boron, and the boron shoves its way into the bulk silicon's crystal structure. This boron, properly implanted, forms transistors and the other minute electronic devices on the chip.

Click to enlarge different views of a tri-interstitial defect moving through crystalline silicon.

But for every boron atom that is introduced, a silicon atom is knocked loose. These silicon atoms can sit relatively inert in the open spaces of the crystal structure. Unfortunately, loose silicon atoms can also combine with one another and form large "self-interstitial defects."

"Clever reactions take place at low energies in the structure, and the silicons have no reason to leave," explains NCSA material scientist Jeongnim Kim. Instead, the interstitial defects twist the bonds between surrounding silicon atoms. This influence can alter the properties of the structure--leaving you with electronic switches that don't switch or resistors that don't resist. In other words, a microchip that doesn't work as it should.

During implantation, the silicon is commonly heated, or annealed, to allow most defects to relax and improve chip performance. However, this process can introduce new complications of its own. As an alternative to implantation, the chip can also be built from the ground up, one atomic strata at a time. The cost of this approach is usually prohibitive.

The fabrication of microchips and other electronic devices goes on, and continues to improve, in the face of these difficulties. But a group of NCSA users isn't satisfied, recognizing that a clearer vision of how defects form will ultimately yield better products. Researchers from the Ohio State University, High Performance Technologies, Inc. in Aberdeen, MD, and NCSA are identifying a series of previously unknown silicon defect structures and modeling the defects' evolution from singleton silicons to large-scale havoc wreakers. The simulations are led by David Richie of HPTi, Richard Hennig and John Wilkins of Ohio State, and NCSA's Kim. They rely on NCSA's Titan Linux cluster and IBM p690 supercomputer.

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Access Online | Posted 8-10-2004