Post-Litho Shrink Breaks Advanced Barrier

No matter the technology node or the future techniques being considered, one thing is generally true: The current solution remains the best until it just won't work anymore. So, while lithographers may be pursuing immersion lithography or even extreme ultraviolet (EUV) lithography for future generations, they'll stick with dry lithography for as long as they can.

Lam Research Corp. (Fremont, Calif.) recently released a tool to help extend lithography's process window, enabling device makers to make further use of existing lithography techniques. The Motif post-lithography pattern enhancement system, based on the company's 2300 platform, uses a proprietary plasma-assisted process to deliver controlled photoresist hole and space critical dimension (CD) shrinks of up to 100 nm. The system can create features as small as 10 nm, demonstrating extendibility to the 22 nm node and beyond. Competitive post-lithography pattern enhancement systems are typically limited to shrinks of <30 nm, according to Lam.

Looking at the lithography roadmap, there's a gap that needs to be filled between immersion lithography and EUV, noted Jeff Marks, Lam Research's vice president of new business. “Basically, we're looking at a capability to be able to extend the use of current lithography, improve some of the OPC, and actually simplify some of the OPC that you might need to use on some of the devices by being able to improve the lithography profile, and then basically extend the use of the etch tools,” he said. “We would actually allow some customers to make some features and feature sizes that are 2-3 generations ahead of lithographic capability.”

The Motif process occurs after lithography and before etch (Figure ). To shrink feature sizes, it deposits a thin-film coating on printed photoresist holes and spaces. The film is typically the thickness of the desired feature shrink. Using current lithography and mask technology, the photoresist holes and spaces are printed at a large enough size to optimize exposure latitude and minimize distortion. The shrink process — based on plasma technology — is applied after lithographic patterning to reduce printed features to the desired size prior to etching. After etch, the film deposited by Motif is removed during the photoresist strip step.

The 2300 Motif post-lithography pattern enhancement system extends the limits of conventional lithography by shrinking final CDs to as small as 10 nm. (Source: Lam Research)

Like chemistry-based shrinks currently available, Lam's approach only shrinks features, and does not enable pitch shrinks. But unlike previous solutions, which are limited to shrinks of 20-30 nm, Marks said, Motif is able to do much larger shrinks — down as small as 10 nm contacts.

Some key applications are able to benefit from such feature shrinks. “The most interest we're seeing is in the memory technologies — both in the DRAM and in the flash. There are typically contact-level applications where they can benefit from shrinking the contact hole size,” Marks said. “Holes are generally more difficult to pattern than line/spaces. So typically we are seeing most of the interest today in hole shrink.”

Some chipmakers are applying the technology to deposit removable spacers for implant applications, such as with strained silicon. Lam's process is much simpler, and can be done at essentially room temperature, Marks noted. Lam is also working with customers on integrating Motif into double patterning schemes, both with and without CD shrink. The industry is showing a lot of interest in the double patterning schemes, he said, but this application is targeted more at the 32 nm node.

Working jointly with Lam, IMEC (Leuven, Belgium) has been able to achieve 30-60 nm CD shrinks, including contacts down to 40 nm with sub-3 nm across-wafer uniformity and wide exposure latitude, according to Serge Vanhaelemeersch, department manager of IMEC's Silicon Process and Device Technology Division. “These impressive results clearly demonstrate Motif's effectiveness in extending current lithography with the process control and performance required for production,” he said.

The film deposited by Motif enhances etch plasma resistance, resulting in reduced line roughening and distortion during pattern shrinking and transfer, providing excellent CD uniformity, typically equivalent to or better than incoming lithography. In addition, the Motif process can be tuned for a range of feature sizes in the pattern.

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