PRINCETON, N.J. The world's first demonstration of lithographically induced self-assembly (Lisa) enables submicron-sized circular pillars to be created without a mask. The maskless procedure, invented by Princeton University researcher Stephen Chou, promises semiconductor manufacturers a new tool for patterning deep-submicron features on chips.
"I believe that this discovery is a universal principle that, when perfected, will be used with many kinds of materials, not just to pattern microchips," said Chou. Lisa works by placing a solid planar plate above a liquid, without touching it (the exact distance, measured in microns, seems not to be critical).
Chou discovered that if the liquid is left to solidify while the plate is in place, a pattern of finely spaced pillars, measuring about half a micron in diameter, spontaneously form. Why that phenomenon occurs is not yet well-understood, however.
"We've worked on Lisa for two years trying to figure out a suitable theory, and we plan to continue that search, but we feel we need to begin developing applications now," Chou said. Chou kept his discovery secret for those two years while he sought an explanatory theory, but Princeton's technology-licensing office finally persuaded him to release news of the discovery without the supporting theory.
Chou's best guess is that Lisa faces off two universal "forces" in nature: surface tension and electrostatic attraction. Minutely different charges on the plate and the opposing liquid cause them to be attracted to one another, but surface tension opposes that attraction by trying to pull the liquid back down flat. When the two forces balance, the liquid forms a fine pattern of pillars on its surface that solidify as the liquid cools.
"Surface tension is always trying to repair any deformity in the surface of a liquid. When a liquid is flat its surface tension is at its lowest point, because it's proportional to the curvature of the liquid's surface," Chou said. "If surface tension is then opposed by a stronger force, it will try to deform the surface.
"However, when it is very close to an opposing plate, instead of humping up in the middle like liquids do when not facing an opposing plate it does so in a fine pattern of submicron-sized pillars," Chou continued. "'We discovered the effect totally accidentally, when we were trying to stamp out nanostructures with my imprinting technique."
Imprinting, which Chou also invented, stamps out tiny nanomachine parts from partially cooled liquid polymers with a template. Tiny machine parts, with features as small as six nanometers only a few dozen atoms wide have been successfully stamped out with Chou's nanoimprinting technique. The only other way to create such small machine parts is with X-ray lithography, a much more complex process.
Chou recently convinced IBM to create experimental nanocompact disks, called nano-CDs, and "'quantized" magnetic disks with his imprinting process.
Quantized magnetic disks, the other application of imprinting under development by IBM, can pack the theoretical maximum capacity on a magnetic disk. Normal magnetic disks can't pack their maximum information capacity, because adjacent magnetic particles can demagnetize each other. However, the quantized magnetic disk uses nanoimprinting to impose a fine pattern on the previously flat magnetic material, which then locks in place the fields of magnetic dipoles. In this way, each magnetic dipole is free to orient itself up or down regardless of how many oppositely oriented dipoles are nearby. Quantized magnetic disks can thereby pack their maximum capacity, where each magnetic bit requires only a single dipole consisting of just two oppositely charged molecules.
Chou discovered Lisa while he was stamping out nanoparts with that imprinting technique. A few pieces of dust prevented one of his templates from touching the liquid polymer from which it was supposed to stamp out a nanosized part. The result was a fine pattern of pillars, even though the template had never touched the liquid polymer.
"We were trying to create a fine pattern with copolymers two oppositely charged molecules that when chained together form a dipole but the experiment failed because dust got under the template. However, when inspecting our sample we discovered that it had self-assembled these little pillars," Chou said.
Subsequent experiments have determined that the pillars are always round, but their diameter and tallness depends on molecular weight and resulting surface tension of the polymer. And although electrostatic attraction appears to be the force opposing that surface tension, connecting a ground wire between the liquid and the opposing plate thereby equalizing their electrostatic charge seems to have no effect on the pillar formation. "We're not sure that the charge on the top plate is necessary," said Chou. Or "it might just be our timing."
Regardless of how the theory for Lisa finally pans out, Chou is forging ahead with applications. Currently he is experimenting with creating "nanopixel" LEDs where each pixel consists of hundreds of nanopixels, any one of which can fail without affecting the overall display. He is also experimenting with doping the polymer, before Lisa, to modify the shape and size of the resulting pattern of pillars.