Versatile Masks of Dye Speed the Chip-Making Process
By ANNE EISENBERG
OOD dye, it turns out, is not just for coloring Easter eggs and adding roses to birthday cakes.
A young researcher at the University of Washington has placed this homiest of materials at the center of a decidedly high-tech process: photolithography, the dominant microfabrication technology in the semiconductor industry.
Albert Folch, an assistant professor in the bioengineering department, used minute amounts of the brilliantly colored dyes, flowing in tiny, transparent-walled channels, to create a new kind of photomask, one of the basic tools of photolithography.
Photomasks are the pattern-makers in photolithography, functioning as stencils that either block or let through the ultraviolet light that projects an image of the chip patterns onto the photoresist, the photosensitive material below the stencil.
Conventional photomasks are flat, metal and stationary. But Dr. Folch's masks have none of these qualities because they are based on microfluidics, the manipulation of small quantities of fluids flowing in miniature channels.
Dr. Folch's microfluidic photomasks are made by filling hollow silicone pathways with different colored dyes to create the patterns that are projected onto the photoresist. The patterns formed by the dyes can be changed quickly, for example, to test chip design prototypes, because to change the design, only the dyes need be altered.
Traditional mask features are permanent, and any alteration in the chip calls for fabricating a new mask, a slow, costly process. But features in the microfluidic mask can be reconfigured within seconds simply by adjusting the concentration and color of the dyes, and thus the amount of light they allow to pass through at a particular point.
"Microfluidic photomasks are such a simple idea," Dr. Folch said. "Liquids are easily made more or less opaque." Make the blue a bit darker, for instance, and it will absorb more light. Dilute the dye, and more light passes through, changing the features of the pattern.
A paper detailing Dr. Folch's techniques was published this month in the Proceedings of the National Academy of Sciences.
The masks, which can be used to create patterns on larger areas by using standard photolithographic equipment, demonstrate the growing sophistication of microfluidic systems, said George M. Whitesides, Mallinckrodt professor of chemistry at Harvard University.
Dr. Whitesides reviewed Dr. Folch's paper for publication in the academy's journal. "This mask takes advantage of a basic property of microfluidics - its lack of turbulence," he said. In the microfluidics world, in which minute volumes of liquid flow in channels that may be one-tenth or even one-twentieth the size of a human hair, fluids mix exceedingly slowly, so that heterogeneous flows can travel along side by side without diffusion. "It's a nonintuitive idea, patterning fluids as Dr. Folch has done," Dr. Whitesides said. "We think that a red fluid together with a blue one will mix. But in a small pipe, they don't."
Because a multitude of dyes can be set flowing next to one another in the tiny channels, with each dye absorbing a different amount of light, the microfluidic mask offers a different way to do gray-scale photolithography, in which each part of the photoresist receives a different exposure.
Microfluidic masks may lead, for instance, to the fabrication of microscopic curved features like those in microlenses. "The height of the exposed photoresist is approximately dependent on the light the region receives," Dr. Whitesides said. "Because of this, it's possible to do a kind of sculpting of the photoresist by the amount of light."
Fabrication of the masks is simple and inexpensive, based on replication of tiny troughs made of a supple polymeric material that can be pulled from the original mold. The troughs are then closed by bonding them to a thin layer of flexible material, Dr. Folch said. Because the replicas are soft, they can be removed from the template without damaging it, allowing the pattern to be used repeatedly.
Stephen R. Quake, an associate professor of applied physics at the California Institute of Technology who works extensively with microfluidic devices, said that Dr. Folch's work was likely to be useful in academic research. "It can be used for inexpensive, low-overhead fabrication of scientifically interesting structures," he said. This is an advantage for academics, who lack the resources available to researchers in the commercial sector, he added.
David J. Beebe, a fluidics expert and associate professor of biomedical engineering at the University of Wisconsin, said that the speed with which the new masks can be reconfigured was an advantage. "He's used a fluid to create the gray scale, so that allows one, in theory, to change the mask more rapidly," he said. "You don't need to make a new mask. All you have to do is put in a new fluid."
In his paper, Dr. Folch discusses the creation of photoresist patterns with features of multiple heights, showing, for instance, five concentrations of dye that produced five lines of different heights when they were projected and then etched. In one figure, he shows a microchannel in which blue, yellow and red dyes flow side by side because of the lack of turbulence.
Dr. Whitesides expects microfluidics to fill a growing need in microfabrication as its properties become better known. "Microfluidics is a funny field in that its basic physics is not, per se, new," he said. But it has yet to wend its way into many areas of science.
This situation will change, he said, as scientists learn how differently fluids behave in small pipes versus big ones. "The really interesting part of Folch's work is that he is generating a pattern by using flowing streams with different colors rather than using a stationary mask," he said. It might lead to ways to sculpture many three-dimensional shapes.
This sculpturing ability is not a characteristic of standard photolithography, with its all-or-nothing stencil-like masks meant to produce features of exactly the same height. The perfectly vertical sides that emerge in the process are the basis for all photoelectronics manufacture and photolithography, Dr. Whitesides said.
"Perfectly vertical sides are a good feature for a binary world," he said. "But having the ability to contour in three dimensions is far more interesting."