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Posted: February 20, 2009
Making the smallest, most perfect polymer films
(Nanowerk News) Like many creative but essentially conservative scientists, Tom Russell of the University of Massachusetts Amherst, in more than 30 years of polymer science research, would rarely use the term “truly transformative” about his work. Until now.
As Russell says of new techniques described in the Feb. 19 issue of the journal Science, “I expect this new method of producing highly ordered macroscopic arrays of nanoscopic elements will revolutionize the microelectronic and storage industries and perhaps others, like photovoltaics.”
Russell, a leading expert on polymer behavior and director of the UMass Amherst Materials Research Science and Engineering Center, with colleagues there and at the University of California Berkeley, have developed a faster, more efficient way to produce defect-free thin polymer films with the smallest domains ever achieved and ordered in the densest way possible for any given size—to dramatically improve storage density.
The new technique for guiding self-assembly of block copolymers—two chemically dissimilar polymers joined together—should not only increase data storage volume, but will save months in manufacturing and open up vistas for entirely new applications, say Russell and Ting Xu, leader of the UC Berkeley team. The density achievable with the technology they’ve developed could allow the contents of 250 DVDs to fit on a surface the size of a quarter, for example, says Xu.
Their work was supported by the Department of Energy Office of Basic Energy Science, the National Science Foundation-funded Materials Research and Engineering Center at UMass Amherst and the university’s Center for Hierarchical Manufacturing.
In seeking to design a new way to guide the self-assembly of layered block copolymers, Russell and Xu recall a specific conversation one day when they saw in a flash of insight that atomic order could be translated to larger scales by using surface ridges of a base crystal to guide the assembly of a copolymer. It’s like using the corrugations in cardboard to direct how closely-packed marbles will order, Russell explains.
For the base layer, Xu, Russell and colleagues used commercially available sapphire wafers, which start out flat. Heating them from 1300 to 1500 degrees Celsius for 24 hours causes the surface to reorganize into a sawtooth topography with an inherent orientation. So when a thin copolymer film layer is applied, the underlying corrugations or crystal facets guide the film’s self-assembly in a highly ordered way to form an ultradense hexagonal or honeycomb lattice.
“We can generate nearly perfect arrays over macroscopic surfaces where the density is over 15 times higher than anything achieved before,” Russell says. “We applied a simple concept to solve several problems at once, and it really worked out. It’s really exciting.”
By varying the annealing temperature, the scientists can change the angle and height of the saw teeth and the depth of the troughs between peaks. Most previous efforts to create a well-ordered substrate or bit-patterned media, as it’s known, were stuck at 15 nanometers for the smallest achievable pattern size, Russell explains. But “we’ve shattered that barrier and I think we can go farther,” he adds. Also, where others have achieved at most one terabit per square inch, Russell and colleagues have generated a copolymer of more than 10 terabits per square inch. A terabit is an information storage unit equal to one trillion bits.
Further, in the past there had not been a way to characterize, or precisely measure, a large area of these structures on the nanoscopic level because each time a measuring device is moved, accuracy is lost. But for the first time, Xu and Russell solved the problem using grazing incidence X-ray scattering. “We’ve essentially established a new technique,” Russell says, for characterizing large-scale arrays with nanoscopic precision.
Certainly, Russell and Xu say, there is more to be explored using this new ordering method, but they say theirs is the first fast, simple, robust and precise way to generate thin films containing arrays of “highly oriented, closed-packed, nanoscopic cylindrical domains that span the entire film thickness and have an exceptionally high degree of long-range lateral order,” as their paper reports.
Another advantage of the new method is that the self-assembly process guided by crystals doesn’t require months of painstaking etching with an electron beam or repeated use of toxic chemicals to prepare the base. Instead, with the UMass Amherst-UC Berkeley approach, self-assembly occurs simultaneously everywhere in a few minutes (is highly parallel) when the polymer film is exposed to a solvent in a sealed environment. The researchers used sapphire in these experiments but note that other single-crystalline off-the-shelf materials can be used.
Finally, Russell says, “because the sapphire wafer is transparent and the elements are so small and densely packed, this method opens the possibility for use in photovoltaics and reveals a pathway to efficient energy-harvesting devices.”