Luminous Bugs
Like the flashing amber light at a roadworks site, glowing molecular markers could help computer chip makers avoid imperfections on super-thin templates. This would speed up the manufacturing process considerably. Mary J. Wirth, Professor of Chemistry and Biochemistry at the University of Delaware, presented her research at the April 1, 1998 meeting of the American Chemical Society.
"We used a fluorescent molecule to find defects on the surface of materials used as templates for integrated circuits," she explained. "Our goal is to optically measure surface planarity on a molecular scale - as fast as we can."
The work is not finished yet but seems promising. It could be a starting point for new optical polishing techniques, allowing chip makers to correct imperfections on photomasks in real time, says Wirth's collaborator Daniel W. van der Weide, director of the university's new Center for Nanomachined Surfaces. "A tiny scratch on the surface of a photomask is like a smudge on a copier," he said. "You never get a clean copy."
As computer chips or integrated circuits become more complex with smaller and smaller components, even molecular-scale defects can pose greater problems. Photomasks are chromium-on-synthetic-quartz structures that protect certain parts of silicon dies when exposed to ultraviolet light. The masks must be given the finishing touches to give them a surface that is completely smooth, even at the atomic level. A scratch just one micron thick could significantly affect the quality of the photomask, Wirth said.
The traditional method of detecting defects on photomasks is to scan the surface with an atomic force microscope, which measures the topography of the surface. However, the technique is extremely time-consuming. Wirth's glowing tracer molecules quickly probe much larger surfaces.
Wirth explained the principle of their method: "To take measurements in the size range of a molecule, you should also use a molecule." First, silica is washed with nitric acid and water to remove any surface contamination. Then a small amount of fluorescent dye with a high affinity for silanols-silicon compounds with hydroxyl groups found in scratches on silicon oxide-is applied to the sample. "Individual fluorescent molecules of an indocarbocyanine dye bind tightly to points along these superficial scratches because of electrostatic attraction," says Wirth's collaborator, graduate student Derrick J. Swinton.
In this way, atomic-scale scratches are converted into bright fluorescent lines that are visible with a high-quality optical microscope. Wirth and Swinton attached a specially designed camera to the microscope to capture real-time images of the dye's fluorescence. Compared to scanning the surfaces with a miniature tip, Wirth's method "can quickly find small scratches spread over large areas," says van der Weide. "The dye molecule acts as an intensifier of the defects so that they can be detected fairly easily with conventional microscopes."
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