Lone glowing molecule points out minute imperfections
Vol. 17, No. 27April 16, 1998

Lone glowing molecule points out minute imperfections

Mary J. Wirth and doctoral candidate Derrick J. Swinton

Like the flashing yellow sign on a road under construction, glowing molecular markers might help computer-chip makers avoid 'potholes' on super-flat blueprinting materials, a UD researcher reported April 1 during the American Chemical Society meeting.

"We deploy a lone fluorescent molecule to measure imperfections on the surface of materials used as templates for integrated circuits," explained Mary J. Wirth, chemistry and biochemistry, winner of a 1994 National Science Foundation Creativity Award. "Our goal is to measure surface flatness optically, on the molecular scale, as fast as we can."

The work is still preliminary, but promising, and it may set the stage for new optical polishing techniques, allowing chip makers to correct photomask imperfections in real time, said Wirth's collaborator, Daniel W. van der Weide, director of UD's new Center for Nanomachined Surfaces and one of only 28 researchers in 1998 to receive a Young Investigator Program Award from the U.S. Office of Naval Research.

"A tiny scratch on the surface of a photomask is like having a speck of dirt on a copy machine," said van der Weide, also one of 20 scientists in 1997 to win a National Science Foundation Presidential Early Career Award for Scientists and Engineers. "You never get a clean reproduction."

As computer chips or integrated circuits (ICs) become increasingly complex, with ever-smaller components, even molecular-scale flaws can create big problems. Each photomask-a blueprint made of chromium on synthetic quartz, which exposes selected sites on a silicon wafer to ultraviolet light- must be polished to an atomically smooth finish. A scratch no larger than a thousandth of a micron- much slimmer than the wavelength of light, and more than 50,000 times thinner than a human hair- could result in serious photomask limitations, Wirth said.

Traditionally, van der Weide said, one way to spot flaws on photomasks has been to scan the surface with a miniature tip that measures the topography of the surface. The technique, known as atomic force microscopy, is extremely time-consuming, he noted. Wirth's glowing molecular markers rapidly illuminate much larger surface areas.

The UD strategy, Wirth said, is simple: "If you need to measure something the size of a molecule," she said, "you ought to use a molecule." First, silica is washed with nitric acid and water, to remove any contaminants on the surface. Next, a small amount of fluorescent dye with a high affinity for silanols-groups of silicon, oxygen and hydrogen that comprise scratches on silica-is placed on the sample.

"Individual fluorescent molecules of an indocarbocyanine dye tightly stick to points along these shallow scratches because of an electrostatic attraction," said Wirth's collaborator, doctoral candidate Derrick J. Swinton.

In this way, scratches of atomic dimensions turn into bright fluorescent lines-visible through a high-quality optical microscope, which is positioned beneath the silica sample.

Wirth and Swinton attach a specially designed camera to the microscope to capture real-time images of the dye fluorescence. Compared to existing tip-based techniques for probing surfaces, Wirth's approach "lets us quickly find small scratches over large areas," van der Weide said. "The dye molecule amplifies the presence of these imperfections, so that they can be detected quite easily, with conventional microscopes."

Wirth's research is supported by the National Science Foundation and the state of Delaware Advanced Technology Center program.

-Ginger Pinholster