SCIENCE & TECHNOLOGY
MOLECULAR-SCALE LITHOGRAPHY Advances in BLOCK COPOLYMER SELF-ASSEMBLY may lead to applications in microelectronics CELIA HENRY ARNAUD, C&EN WASHINGTON
MOORE’S LAW, named for Intel cofounder
Usually, block copolymer lithography involves only one block copolymer. The UCSB group, however, blends two different block copolymers. One copolymer consists of poly(ethylene oxide) (PEO) and polystyrene. The second copolymer contains polystyrene and poly(methyl methacrylate) (PMMA). Each polymer block performs a specific role in the blend. The PMMA blocks are photodegradable, so they can be easily removed to generate pores. The hydrophilic PEO provides long-range ordering. The polystyrene acts as a matrix for the other polymer blocks. The polystyrene blocks are randomly modified with small amounts of 4-hydroxystyrene in one copolymer and 4-vinylpyridine in the other to promote hydrogen bonding. Adjusting the ratio of hydrogen-bond donors and acceptors causes the block copolymer mixture to assemble differently. Some ratios still form the typical hexagonal array, but a 1:1 ratio of donors and acceptors assembles into square arrays. “There seems to be a delicate balance between the desire of the block copolymers to separate and the desire for the hydrogen bonds to keep the blend together,” Hawker says. COURTESY OF PAUL NEALEY
of features. But Edward J. Kramer, Glenn H. Fredrickson, Hawker, and their coworkGordon E. Moore, predicts that the numers at UCSB recently showed that they can ber of transistors on computer microproget different types of arrays by adjusting cessors will double every two years. Until the hydrogen-bonding properties of a now, standard photolithography, in which blend of designer block copolymers (Scilaser beams are used to generate patterns in masks and transfer those patterns to a substrate, has been sufficient to satisfy the need for tinier transistors. But that might not be the case much longer. “We can’t shrink the features down much further” with standard photolithography, says Craig J. Hawker, a chemistry professor at the University of California, Santa Barbara. “It’s becoming too expensive to make sub-30-nm features.” This is where block copolymer lithography comes in, Hawker FINDING APPLICATIONS for adds. “It is an attempt to keep up block copolymer lithography in with Moore’s law.” the microelectronics and comBlock copolymer lithography puter industries will require more harnesses the power of chemistry than just broadening the range of to reduce feature size even furpatterns that can be made with ther, potentially smaller than 10 the technique. For such applicanm. “It’s not a replacement for tions, features must be uniform photolithography,” Hawker says. and individually addressable. SELF-MAKING PATTERNS The arrays formed by block “It’s an add-on.” “You need to be able to place copolymer self-assembly (bottom) are higher quality and Block copolymer lithography every domain exactly where you resolution than those made with conventional photolithogratakes advantage of spontaneous want it on the substrate,” Nealey phy (top). The arrays on the left have a 1:1 ratio of templated self-assembly processes to create says. Gaining acceptance for to final features; those on the right have a 1:4 ratio. arrays of molecular-scale features block copolymer lithography in whose size is dictated by the microelectronics applications ence, DOI: 10.1126/science.1162950). Their chemistry of block copolymers. Typically, will require reaching levels of perfection modified block copolymers self-assemble the process relies on diblock copolymers, with only one defect in 1 million, 10 million, into square arrays of cylinders. which are covalently linked chains of two or 100 million features, he asserts. The UCSB team harnesses competing different polymers. The two chains that Achieving that level of perfection probforces to control the orientation and patmake up the diblock copolymer would ably will require some type of templating tern of the arrays. “We’re using the fact “separate at a very large length scale if they process. At the moment, block copolymer that the polymers hate each other so they weren’t tied together at the molecular self-assembly forms ordered structures only want to get away from each other when length scale,” says Paul F. Nealey, a chemiover short distances. Before the method blended,” Hawker says. “But then, we’re cal engineering professor at the University can become a practical lithography tool, it using the attractive force of the hydrogen of Wisconsin, Madison. must be capable of producing long-range bonding to keep them together and give us A common outcome of block copolymer order. Two teams recently reported ways to different patterns.” lithography is a hexagonally packed array impose the needed long-range order on the WWW.C E N- ONLI NE .ORG
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hydrogen silsesquioxane on a silica substrate, all coated with a thin layer of poly(dimethylsiloxane) (PDMS) brush. The researchers have found that a PDMS/polystyrene block copolymer selfassembles in a hexagonal pattern around the posts. Each post occupies a spot otherwise earmarked for a PDMS feature. “You end up with a structure that has essentially PINNED DOWN Block copolymers selfSQUARE OFF A square array can be formed perfect long-range order assemble around a physical template of posts by adjusting the hydrogen bonding in a block because it’s pinned at vari(the lighter dots). copolymer blend. The inset is a close-up view ous places by these little pilof the same array. lars,” Ross says. “You’d like to be able to get whatever pattern of block copolymer self-assembly process. ey’s team, in collaboration with scientists block copolymer you want with the sparsCaroline A. Ross, Edwin L. Thomas, at Hitachi Global Storage Technologies, est or simplest template or post array. We Karl K. Berggren, and their coworkers at does this by using an electron beam to draw want to find out how little work we can do Massachusetts Institute of Technology a pattern in a polystyrene brush layer on on the template and still get the polymer to use a physical template to impose order a silica substrate (Science 2008, 321, 936). fill in the gaps the way we want.” on the block copolymers (Science 2008, They then expose parts of the pattern to an Chemical methods can also be used to 321, 939). Their template consists of a oxygen plasma. During self-assembly, the render the surface into a template. Nealtwo-dimensional array of posts made of
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SCIENCE & TECHNOLOGY
PMMA portions of a polystyrene-PMMA block copolymer preferentially interact with the oxidized portions. Polymer self-assembly on top of this kind of chemical template generates many more features than were in the template itself. The same is true with the physical templates used by Ross. With such “density multiplication” of features, there is not a one-to-one correspondence between the features in the underlying template, be it physical or chemical, and those in the block copolymer layer. Density multiplication overcomes one of the biggest challenges for standard photolithography. “It’s always easy to make small features,” Nealey says. “What’s hard when we go to smaller dimension features is getting ones that are packed close together and all exactly the same size.” Ross’s and Nealey’s methods use standard lithography methods to generate the underlying template. Their technology is nonetheless promising, Nealey says, because the templates don’t have to be of the same quality or uniformity that is required for the final products. “The assembly of the block copolymer corrects for many mistakes you could make in the process of chemical patterning” of the template, he notes. Hawker plans to combine his chemical modifications with template-patterning techniques such as those reported by Ross and Nealey. “Registration is really critical for a number of applications,” Hawker says. “You’d like to know where your square array starts and be able to control the registration as you move away from that starting point.”
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[email protected] IT MAY NOT BE long before patterning methods based on block
copolymer lithography find their way into industrial applications. Patterning “is an important new technology for us in hard disk drives,” says Thomas R. Albrecht, manager of the patterned media technology group at Hitachi Global Storage Technologies and leader of the team that collaborated with Nealey’s group. “We need this much sooner than the semiconductor industry will ever get to it.” Hitachi is interested in using such methods to create the templates for patterning tiny magnetic islands to use in hard disk drives. Dissolving the PMMA microdomains creates holes in the surrounding network of polystyrene, which can be used as a mask for transferring the pattern from the polymer into the surface of a template that, in turn, can be used for nanoimprinting. Nanoimprinting allows the pattern to then be replicated on many disks via a process that resembles printing. “The big question has been: How can we perform the lithography to create islands in the size range we need,” Albrecht says. A typical specification would call for islands that are 15 nm in diameter and separated, center-to-center, by 25 nm. That’s beyond the reach of standard lithography methods, he notes. Albrecht acknowledges that he was initially skeptical about selfassembly techniques. “The scenarios people talked about tended to be way too ambitious,” he says. “The dream is that you’d mix some stuff up in a vial, pour it out on a surface, let it go to work, and come back the next day and your pattern is formed.” By creating the template first, the task of self-assembly becomes a matter of filling in the missing dots. “That’s a much easier task than creating the perfect pattern from nothing,” Albrecht says. Although Albrecht is hopeful that the technology will succeed, other hurdles, such as fine-tuning the islands’ magnetic properties, must be overcome first. “We hope that over the next few years we’re able to have the breakthroughs we need in some of the other areas, just as this block copolymer method was a real breakthrough.” ■ WWW.C E N- ONLI NE .ORG
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