DSA: High Stakes Game Of Alphabet Soup

By Mark LaPedus
Directed self-assembly (DSA) is making progress for potential use in semiconductor production, but the industry must make some major advances in a sometimes forgotten and unsung segment—materials.

DSA is a complementary patterning technology that makes use of block copolymer materials to enable fine pitches in chip designs. But today’s block copolymers based on poly (MMA-co-styrene), or better known as PS-b-PMMA materials, do not scale beyond 11nm.

To bring DSA from the lab to the fab, Arkema, AZ, Dow, IBM, JSR, SEH, TOK and others are separately developing next-generation DSA copolymers that promise to scale beyond 11nm. But for the most part, the commercial vendors are keeping their R&D work close to the vest.

To date, most of the research in the public domain has originated at universities. Given that the universities provide a clue of what’s coming down the pike, there are several DSA materials candidates on the table, including high chi copolymers, blended compounds, and even one based on simple sugar.

Each approach, which is still in R&D, consists of complex chemical compounds that resemble a mix of letters in a bowl of alphabet soup. The candidates have various trade-offs, but there is no clear leader in the next-generation copolymer race right now.

“The field is much more wide open than it was for the first generation PS-b-PMMA materials,” said Ralph Dammel, chief technology officer for AZ Electronic Materials, a supplier of materials for DSA and other applications. “There certainly are a number of possible high chi materials. It may be that one single high chi polymer technology will evolve, or several ones may co-exist, possibly for different applications. It is too early to say.”

DSA to the rescue?
DSA is one of several lithographic options for chip production. In the current lithography landscape, chipmakers are expected to use 193nm immersion and double patterning at 20nm. Then, at 14nm and 10nm, the industry would like to insert extreme ultraviolet (EUV) lithography. If EUV misses those windows, the industry will extend 193nm immersion and move to a more complex multi-patterning scheme.

The wild card is DSA, which is still in R&D. Technically, DSA is a complementary scheme. When used in conjunction with a pre-pattern that directs the orientation for patterning, DSA is said to reduce the pitch of the final printed structure. Used in conjunction with 193nm immersion, DSA could extend 193nm lithography beyond 10nm, eliminate expensive multi-patterning steps and push out EUV.

In the lab, DSA has demonstrated the ability to devise features down to 5nm. The likely insertion point for DSA is the 10nm node, said Moshe Preil, manager of emerging lithography and tools at GlobalFoundries. “If it was only about the chemistry, we would know how to implement DSA at 14nm,” Preil said. “We still don’t know how to design a chip using DSA.”

Design enablement and defectivity are among the challenges with DSA. And to put DSA in a production fab, chipmakers must make some complex choices. First, there are two main types of DSA methods: graphoepitaxy and chemical epitaxy. In chemical epitaxy, self-assembly is guided by a sparse chemical pre-pattern. Graphoepitaxy is based on a topographical pre-pattern.

Chipmakers must come up with the right DSA materials, which is also a challenge. “The design of high chi block copolymers has a number of pitfalls. It is not easy, but there has already been a lot of progress, enough that we can say there will not be a technical issue with availability of high chi materials for <10nm patterning,” said AZ’s Dammel.

A polymer is a molecule that consists of repeating structural units. A copolymer is a polymer compound derived from two or more structural units. Block copolymers consist of dissimilar structural units or chains linked by covalent bonds.

In DSA, the dissimilar units are able to phase separate into distinct domains and then self-assemble into controllable dimensions. The trouble is today’s block copolymers based on PS-b-PMMA materials have a chi factor of about 0.04. If the chi N factor is less than 10.5 in certain mixes, the blocks will not phase separate. N denotes the degree of polymerization, while chi represents the interaction parameter.

So, the industry must find a suitable material that scales. In this area, there are two basic approaches: organic-organic versus organic-inorganic. Academia has focused on organic-organic compounds, while commercial vendors are narrowing their options down to organic-inorganic materials, said Mark Slezak, vice president of sales and technology at JSR Micro, the U.S. sales arm for JSR.

IBM and JSR have been co-developing a technology that blends a copolymer and organosilicate material. This blended approach can scale and consists of fewer process steps using sidewall image transfer technology, according to JSR and IBM. “This has opened Pandora’s Box,” said Yoshi Hishiro, director of R&D at JSR Micro. “The blended approach enables several possibilities. The chi for the blended system can be defined.”

IBM also is working on other solutions, including non-blended copolymers with AZ Electronic Materials. “Both polymer blends and block copolymers phase separate on annealing. Polymer blends are very easy to make, and they also are easier to process. Usually, they require lower annealing temperatures and it is easier to achieve short annealing times,” AZ’s Dammel said. “But the blended approaches are restricted to 2x frequency duplication. For 3x or more, one has to use the longer range self-assembly capability of block copolymers.”

Frequency duplication is key for DSA. “Let’s say we have 20nm lines with 160nm pitch. Let’s say we also use a copolymer with a natural period L0 of 40nm. After the process, we have 20nm equal lines and spaces, or 4x the lines that were there before. That’s what is called 4x frequency multiplication,” Dammel explained.

Today’s block copolymers have scaling limitations. “So, one needs to make block copolymers with higher chi factors. One problem is that some types of block copolymers with high chis are difficult to phase separate by thermal annealing. The blocks repel each other so strongly that they have a hard time moving past each other. One has to use solvent vapor to soften them enough to get them to phase separate.” Dammel explained.

The problem is that solvent annealing may not be the right solution, because the technology is incompatible with today’s semiconductor manufacturing processes, he added.

The contenders
At present, there are several promising next-generation, high chi copolymer candidates on the table. “All of them have their strong attributes. They also have their shortcomings,” said Christopher Ober, professor in the Department of Materials Science and Engineering at Cornell University.

For years, Cornell has been developing block copolymers. The other pioneers in the DSA materials field include the University of Massachusetts Amherst (UMASS) and the University of Wisconsin.

In a more recent effort, Intel and the University of Queensland in Australia described a diblock copolymer, based on poly(styrene)-b-poly(DL-lactide) or PS-b-PDLA materials. Using a graphoepitaxy process flow with 4x frequency multiplication, the entities demonstrated 8nm lines and spaces with PS-b-PDLA materials.

“The new high chi systems such as p(S-b-DLA) have even better selectivity than p(S-b-MMA),” Dammel said. “The p(S-b-DLA) system also has much lower annealing temperatures than the p(S-b-MMA) system.”

In another effort, the Massachusetts Institute of Technology (MIT) recently disclosed that self-assembly can form 3D structures. MIT used a poly (styreneb-dimethylsiloxane) or PS-b-PDMS block copolymer. PS-b-PDMS has a chi factor of 0.27. MIT also HAS devised higher chi materials based on poly(2-vinylpyridine-b-dimethylsiloxane) or P2VP-PDMS.

“I would not treat the chi parameter as the only figure of merit,” said Caroline Ross, professor in the Department of Materials Science and Engineering at MIT. “Chi is one consideration, but there are others that are relevant to microphase separation, such as the molecular weight, volume fraction, surface energy of the blocks and relative etch resistance.”

Hitachi and CNRS-CEA/LETI also are pursuing PS-b-PDMS. In a separate effort, CNRS and the University of Texas at Austin have recently described the synthesis and self-assembly of block copolymers “composed of naturally derived oligosaccharides” or sugar.

These block copolymers enable 5nm feature sizes. “However, that system can never be annealed thermally,” said C. Grant Willson, professor of chemical engineering at the University of Texas at Austin. “It has to be annealed with a solvent. I personally do not think that long solvent annealing processes are compatible with semiconductor manufacturing.”

Willson and his associates are working on other materials. “We have designed a new block copolymer that has all of the attributes of the sugar polymer, but can be annealed thermally with a top coat. This new material looks very promising,” he added.

Meanwhile, UMASS and others are developing copolymers based on polystyrene-block-poly(ethylene oxide) or PS-b-PEO materials. UMASS is also exploring tri-block copolymers based on poly(ethylene oxide) or PEO-b-PPO-b-PEO materials.

In another effort, Pohang University of Science and Technology, Tokyo Institute of Technology, and the University of Wisconsin-Madison, Wisconsin recently described an organic-inorganic diblock copolymer, dubbed poly(styrene-b-methacrylate) or PS-b-PMAPOSS.

Cornell has been developing the concept of orthogonal processing of block copolymers. By using a semi-flourinated photoresist/solvent system, researchers are able to selectively pattern or lift-off and then recover the block copolymer film intact. This relates to “orthogonal lithography,” a new patterning process for organic semiconductors. The process is based on a novel photoresist that is compatible with sensitive organic systems. The specially designed photoresist is composed of polymeric or molecular glass that enables the use of fluorous solvents, thus not damaging the organic material while enabling pattern formation.