NIL Plus Self Assembly Equals Low-Cost Patterning

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NIL Plus Self Assembly Equals Low-Cost Patterning

by David Lammers

Nanoimprint lithography (NIL) and directed self-assembly techniques both appeared unlikely a few years ago, with a too-good-to-be-true aspect. Now, these potentially low-cost techniques are complementing each other, with a possible application in patterned media for next-generation hard disk drives.

Since 2003, Prof. Paul Nealey and colleagues at the University of Wisconsin have pioneered the use of self-assembling block copolymers to enhance and augment the patterning capabilities of lithographic tools, including 193i nm, electron beam, and nanoimprint lithography. The litho tool creates patterns in resists, and these resist structures define the even smaller patterns formed by the block copolymer. The block copolymer self assembles, Nealey said, into patterns which are one-half or one-third the critical dimensions of the resist.

“This approach provides clear pathways to a sub-10 nm pitch. We have strategies to assemble at dimensions down to 3 nm,” Nealey said at the recent Sematech Litho Forum in New York City. “The beauty is that we never have to pattern at the final critical dimension, which is why the SRC (Semiconductor Research Corp.) keeps funding this. They see it as something useful.”

The assembly process is not sensitive to the detailed geometry of the underlying pattern, Nealey explained, because the block copolymer mitigates the line edge roughness (LER) and line width roughness (LWR) of the patterns. In one experiment, which included pattern trimming, the Wisconsin team created 15 nm wide lines from 90 nm resist patterns, which Nealey said provides “an avenue to using 193i tools” for future device technology nodes.One research program focuses on using block copolymer multiplication to form silicon nanowires, another promising research topic for future transistor channels. A more immediate opportunity may be in HDD patterned media with terabit/sq. in. densities.

Block copolymers assemble in patterns, guided by lithographically defined resist patterns that are two or more times larger than the final dimensions. (Source: Paul Nealey, University of Wisconsin; presentation at the Sematech Litho Forum)


Thomas Albrecht, a research manager at the Hitachi Global Storage Technologies (HGST) San Jose Research Center, said the HDD industry has two R&D paths to reach Tbit/sq. in. densities, after discrete track recording runs out of steam. One approach is called thermally-assisted media, which improves the magnetic sensitivity by raising the temperature gradient. The thermal approach, which requires less capital investment than using NIL and self-assembling block copolymers, has yet to be fully proven out.

The other avenue is to combine NIL machines optimized for high throughput patterning of the HDD media with directed self-assembling materials to achieve a 4:1 multiplication of the imprint patterns. Making patterned media requires a low-cost method of defining magnetic islands that are 7 nm in diameter and about 10 nm thick. The goal is media bit densities an order of magnitude higher than today’s discrete track media.

HGST has been working with Nealey and other University of Wisconsin researchers, using lithography

equipment developed by Molecular Imprints Inc. (MII, Austin, Texas). “By combining these two techniques, we believe we can be cost-effective. The cost has to be $1.50 per patterned disk, but the volumes are such that is a big business: about $1 billion per year,” Albrecht said.

Doug Resnick, vice president of strategic development at MII, said MII has 40 of its imprint tools in the field for both semiconductor and HDD patterned media development. The Imprio 300 is a 5X generation (semiconductor) tool with 20 wafer-per-hour (WPH) throughputs and 15 nm overlay capabilities. The company plans to introduce a mask replication tool by the end of this year capable of sub-32 nm variable-shaped beam (VSB) mask processing with low defects and much-improved mask write times. MII also is working on improved mask handling procedures, as well as control of soft and hard particles.

Both HDD patterned media and flash chips are good targets for NIL, partly because redundancy techniques are well understood, Albrecht said.

Progress is needed on throughput and overlay for 28 nm IC patterning with Nanoimprint Lithography. (Source: Doug Resnick, Molecular Imprints Inc.; presentation at the Sematech Litho Forum)