Self-Assembling Resists Improve CD Control, LER

Line edge roughness (LER) is a significant concern for the lithography community, a loss of definition that stands to worsen with continuing feature shrinks. Along with those shrinking CDs, dimensional control is also becoming a problem. On the latest version of the International Technology Roadmap for Semiconductors (ITRS) , both factors are showing yellow at the 90 nm node, but they are quickly slipping into the red, showing the inability of resists to handle the demands being placed on them.

According to Paul Nealey, a professor of chemical and biological engineering at the University of Wisconsin-Madison and director of the Nanoscale Science and Engineering Center , there is a technology gap between advanced lithography tools and the resists. "A lot of resources have been put toward exposure tools, especially EUV lithography and others, so they have the 20 nm resolution, and they have the overlay requirements," he said. "But there haven't been the same resources for the development of resist materials."

CD control and LER are of particular concern. Although some materials may emerge as being suitable for improved LER — such as molecular glasses — it's unclear how they will achieve the sub-1 nm dimensional control required beyond the 32 nm node, Nealey said. Even one of the polymer molecules that typically make up resists is significantly larger than 1 nm. Molecular glass makes molecules smaller, he said, which will help with LER, but probably not dimensional control.

Nealey and others at the University of Wisconsin and the Paul Scherrer Institut (Villigen, Switzerland) have teamed up to develop self-assembling resists that could give the control needed for 22 nm node and beyond patterning. They will present their findings at this year's IEEE International Electron Devices Meeting (IEDM) in December, in a paper titled, "Self-Assembling Resists for Nanolithography." The technique uses advanced lithography tools to create a thin (1-3 nm) patterned substrate that then directs the assembly of lamellae-forming block copolymer films into patterns for device manufacturing. The block copolymer domains assemble perpendicular to and in precise registration with the underlying substrate.

Controlling the molecular weight of the polymer enables dimensional control. LER is also well served by the separation of the polymers within the copolymer block. "The interfacial energy makes those interfaces want to be atomically smooth," Nealey explained. Funded by Semiconductor Research Corp. , the initiative was focused on meeting all the demands of mainstream lithographic processes. With those boiled down into three main attributes, the team has demonstrated the resist's viability:

• It can pattern virtually perfectly over huge areas (300 mm wafers). "What we've been able to show is that, under the right conditions, we can get the block copolymer to assemble absolutely perfectly," Nealey said.
• Patterns can be perfectly registered with underlying and overlying layers. "This is part of our strength," Nealey said. "Very few of those working with block copolymers are thinking about registration."
• It can achieve free-form design. Block copolymers want to make periodic structures, Nealey noted, but they need to have the flexibility to make varying patterns and shapes (Figure ).

Directed self-assembly of block copolymers can be achieved in a variety of geometries, including 60° bends, arcs, lines that terminate at a fixed position, and T-junctions. Each of these structures was fabricated using a lithography technique and block copolymer blend with a periodicity ofLS=LO=70 nm. (Source: Mark P. Stoykovich and Paul F. Nealey, University of Wisconsin)

For some lithographic processes, the researchers developed an interferometer that uses EUV radiation, patterning lines and spaces of 20-30 nm, Nealey said. In fact, these self-assembling resists are well suited for EUV lithography. "Currently, there are no resist materials that would be suitable to use at 20 nm or even 30, for that matter. Maybe 50 nm," Nealey asserted.

Resolution at those dimensions may be achievable, but image depth is another issue. The depth of the pattern is related to the absorption of the wavelength into the resist material. To get more uniform exposure, there needs to be a certain amount of transparency. Because EUV radiation is so readily absorbed, it has been difficult to come up with resist materials that fit the bill. "It's unclear if there could be a material that will be adequately transparent," Nealey said. "We have no problem with that because we use the tool only to pattern a molecularly thin layer." The imaging process is decoupled from the chemistry of the block copolymer. So the imaging layer can be optimized to pattern efficiently, while the block copolymers are optimized for specific applications, such as etch resistance.

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