Industry Takes Further Steps Into Advanced Lithography

As we went to press with this issue, the lithography community was gearing up for this year's SPIE Microlithography conference, pulling together last-minute announcements of their latest achievements. Although we'll certainly have more detailed news to report coming out of the show, here are a few announcements that have come across recently with regard to achievements in leading-edge immersion and extreme ultraviolet (EUV) lithography.

For one, Nikon Corp . (Tokyo) has shipped what it says is the world's first production immersion lithography system. The NSR-S609B, an ArF immersion scanner with a lens with a numerical aperture (NA) of 1.07, was shipped in January to a major IC manufacturer, Nikon announced recently. The system is targeted for mass production of 55 nm memory products and development of 45 nm devices.

Nikon points to its Local Fill Technology as a key deterrent of defects in immersion lithography, eliminating scanner-induced immersion defects with no bubbles, water spots, or backside wafer contamination. The technology also eliminates evaporation of the immersion fluid, which helps to prevent immersion-related overlay problems.

Within the past year or so, Nikon introduced its Tandem Stage, which is also part of the NSR-S609B. It uses two stages with different functions to optimize the performance of the tool for immersion lithography. The exposure stage is designed to process wafers at high accelerations and scan speeds, and the calibration stage is used to calibrate the tool between each wafer exchange. Overlay accuracy has been reduced to ≤7 nm with the Tandem Stage, and the frequent calibration checks help to eliminate fluctuations or variations over time.

Rohm and Haas Electronic Materials, Microelectronic Technologies (Marlborough, Mass.), has made its own announcement with regard to immersion progress. The materials supplier announced recently that IMEC (Leuven, Belgium) has named as a process of record Rohm and Haas's 193 nm photoresist optimized for immersion contact hole imaging. The Epic 2330 resist will be used as a baseline material for 65 nm immersion lithography.

"Investment and extensive research in our Epic series photoresists has paid off with impressive performance characteristics, including excellent hole circularity and pattern smoothness with leading-edge depth of focus for both dense and isolated imaging," said Robert Kavanagh, 193 nm marketing manager for Rohm and Haas, in a statement. "And this resist exhibits minimal difference when patterned with either conventional dry or immersion exposure."

In immersion applications, Epic 2330 can be used without a topcoat, but is also fully compatible with topcoats. This second-generation resist is also optimized to meet or exceed water leaching specifications set by scanner manufacturers.

Rohm and Haas Electronic Materials also recently introduced its fourth-generation immersion photoresist, the Epic 3000 series, which is geared toward 45 nm immersion production. In studies on industry-leading exposure systems, the new resist showed no watermark defects, the company said.

Meanwhile, progress is also being made in EUV lithography. Sematech North (Albany, N.Y.) announced last month that its researchers have achieved an important breakthrough in cleaning mask blanks.

Engineers used a proprietary cleaning regimen on quartz substrates to remove all particles as small as 43 nm. For EUV masks to be used in microchip production, none can have a substrate defect larger than 40 nm for pilot lines and 25 nm for volume manufacturing. "This achievement is a critical and necessary step in generating a zero-defect mask blank," said David Krick, program manager for Sematech North's Mask Blank Development Center (MBDC), in a statement.

The significance of the MBDC's cleaning milestone lies in the science of producing usable masks for EUV manufacturing, Krick explained. First, the blank must be made almost perfectly free of non-removable defects, such as pits or scratches. Second, it must be cleaned of removable defects, such as airborne particles, down to 25 nm. This cleaning process paves the way to the final step of depositing a multilayered reflective coating on the substrate, allowing the resulting photomask to effectively reflect EUV energy.

"Cleaning of defects is a critical and necessary step in generating a zero-defect mask blank, because defects in the substrate become defects in the multilayer, which ruins the mask blank," Krick noted. A related challenge involves working with suppliers to produce mask substrates with no non-removable defects larger than 25 nm, he added.

Krick said the cleaning project was challenged by metrology limitations, since even the most powerful commercial mask inspection microscopes cannot reliably detect particles smaller than 50 nm. The Sematech team used an upgraded confocal microscope from Lasertec Corp. (Kanagawa, Japan), compensated with a repetitive cleaning and overlay methodology that effectively "enlarged" the sub-50 nm particles so that they could be detected.

Although much work still remains for EUV mask blanks, this latest achievement has taken them one step further, Krick said. "We are well positioned for the tasks ahead."

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