Harvard chemist george Whitesides believes the electronic industry’s most basic technology is long overdue for an overhaul. Specifically, Whitesides is interested in improving photolithography, the rather complex process that etches circuit designs onto computer chips. The drawback of photolithography, he says, is that the etching process must be repeated for each chip. It’s as if the Treasury Department had to engrave individually every single coin it mints rather than cast them from one master. And besides being repetitive, photolithography works only on silicon and a few other materials.
Whitesides and his colleagues have come up with a new approach to making not only microchips but virtually any piece of micro machinery. They have found a way to make microscopic molds from which thousands of chips or other miniature components may be cast. Although they still have some significant technical problems to work out, their methods may form the basis of micro-scale manufacturing in the next century.
Whitesides’ ideas were inspired by watching biologist colleagues prepare specimens for viewing beneath electron microscopes. To study tiny living creatures, the biologists must typically kill the organisms and make plastic casts of their bodies. The casts are much more durable than the actual specimens and can withstand repeated scrutiny.
About three years ago, Whitesides started devising a similar technique for manufacturing silicon chips. He begins by carving up a chip using the conventional techniques of photolithography.
First he places a stencil, or mask, over the chip and then shines ultraviolet light through the openings in the mask. The ultraviolet light hardens the exposed parts of the silicon, which are coated with photo- sensitive film. Then he removes the mask and washes the silicon with a chemical similar to those used in developing photographic film. The chemical dissolves the coated silicon that had been protected by the mask. These dissolved areas make up the skeleton of the chip’s electronic circuit.
The next step would normally be to deposit various metals onto the etched chip. But Whitesides instead pours onto the chip a polymer called polydimethylsiloxane, often used in plastic surgery; when the polymer hardens, he pulls it off. What’s been created is a polymer mold of the original etched chip, and Whitesides uses it as a stamp, inking it with a chemical that catalyzes reactions between silicon and various metals. Rather than using ultraviolet light and chemicals to repeatedly etch different chips with a circuit pattern, he etches one master and then uses the chemically treated mold to stamp many different pieces of silicon. Finally he sprays vaporized metal over each chip’s surface. The metal bonds to the chemically inked circuit pattern.
This technology, however, isn’t quite ready yet to replace the design of chips by photolithography, says Whitesides. Microchips often consist of many layers of conductive circuitry, and the circuits in one layer have to connect to those above and below for the chip to work. Whitesides hasn’t devised a way to align his stamped chips precisely, but he is optimistic. I put it in the category of a problem that hasn’t yet been solved rather than one that can’t be solved, he says.
In the meantime, his experiments have made him think about using his plastic molds for applications beyond the design of microchips. The flexible molds, Whitesides realized, could open up a whole new field of microscopic manufacturing. Within the past three years, he and his colleagues have come up with, as he puts it, a bag of techniques for designing tiny lenses, machine parts, and other devices.
As with the microchips, Whitesides’ new techniques typically use traditional photolithography to create a master mold. He begins by carving reliefs of various shapes--miniature cogs for use in a micro machine, for example--onto silicon chips. He then pours a liquid plastic over the reliefs and peels it off after it hardens. The plastic can then be used as a mold to mass-produce the miniature cogs.
One of the applications Whitesides is now working on is making microscopic lenses that could be used in fiber-optic communications networks. The lenses would consist of tiny grids or bars created by his molding process. Because light passing through a mesh of fine grids refracts, or bends, the grids can be used as lenses to focus or redirect the laser light that carries information through a fiber-optic system.
The advantage of Whitesides’ technique is that it makes possible the manufacture of grids on a scale beyond the reach of photolithography.
The older method is limited in its ability to etch fine grids because the ultraviolet light doing the etching is itself bent by the mask it passes through, so the spacing between grids can’t be precisely controlled. At best, photolithography can be used to make grids with a spacing of about 300 nanometers, or .0000012 inch. Using his molding process, Whitesides can get down to 30 nanometers.
The method is straightforward: Whitesides first makes a flexible mold of grids, squeezes the mold to reduce the space between the grids by, say, half, and pours a polymer over the squeezed mold. When the polymer hardens, he has a second mold with the same grid pattern but with half the space between the slits of the grating. He repeats this process until he reaches the desired spacing.
If you want to make something smaller than 200 nanometers with photolithography, it’s hard, he says, and if you want to make things below about 100 nanometers it gets to be just excruciatingly difficult.
Another potential use of such squished molds is to increase the amount of information carried by a compact disk. Compact disks are now made by pressing a metal disk covered with micrometer-size bumps into a polymer. The pocked polymer becomes the surface of the cd, and a laser translates the pattern of pits into digital signals that are in turn transformed into sound. Whitesides’ molding process could shrink the pits down to about a hundredth their current size, increasing the storage space on cds.
It’s a very flexible technology, says Whitesides. We’re at the very beginning, and we don’t yet know what the ultimate limitations and opportunities in the field are.
