New hyper-NA tools aim to boost throughput over ASML’s Twinscan
By J. Robert Lineback, Senior Technical Editor
With no “showstoppers” identified yet in immersion lithography, rival scanner makers ASML, Canon, and Nikon are accelerating efforts to take 193nm “wet” exposure tools to the next level, quickly pushing numerical aperture lenses to their feasible limits in systems using water to boost depth of field and resolution for 65nm and 45nm processes. All three say they now plan to launch high-volume immersion scanners with “hyper-NA” optics – up to and slightly over 1.3 NA – between 2Q06 and Jan. 2007.
To go beyond that, researchers and material suppliers now are beginning to narrow the options for high-index immersion fluids, which will eventually replace water and support next-generation ArF lithography with lens systems in the 1.5-1.6 NA range for 32nm and beyond process nodes.
“It is now conceivable that some version of ArF could lead down to 20nm half-pitch,” said Gene Fuller, principal engineer for Nikon Precision Inc., during a panel discussion held in conjunction with the annual SPIE International Symposium on Microlithography (Feb. 28-March 4) in San Jose, CA.
In interviews and presentations at the SPIE conference, Canon Inc. and Nikon Corp. said they also were preparing to introduce new multistage scanner platforms, which are designed to increase immersion lithography throughput up to 130 wafers/hour. Both Japanese lithography suppliers believe their new platforms will offer significant productivity improvements over ASML Holding NV’s Twinscan series, which first introduced the dual-stage scanner concept at the start of this decade.
A number of factors are beginning to favor multistage scanner platforms, including difficulties in measuring wafers for focus exposures using high-NA lenses, said Phillip Ware, lithography strategy manager and senior fellow at Canon USA Inc.
“As we go to immersion systems, we are dealing with the potential of ‘hyper-NA’ lenses that are so huge the focusing mechanisms would be far away from the actual critical areas. You also must map more to get the degree of accuracy that’s needed in the future,” he told WaferNews, explaining Canon’s decision to launch a dual-stage platform – called the 7000 – in Jan. 2007. The 7000 AS7 model will perform 193nm immersion exposures with a lens system >1.3 NA, targeting 65nm and 45nm volume production as well as 32nm development.
Canon believes it has significantly improved the way dual stages are driven in its 7000 platform, compared to ASML’s Twinscan system. “It does not have the overhead of swapping the stages between drive motors,” Ware explained. Similar to ASML’s Twinscan system, Canon’s new platform will perform full-focus calibrations and produce an alignment map on a wafer while another is being exposed.
While ASML and now Canon are betting that the two full stages are better than one, Nikon has a much different concept in multistage platforms. During Nikon’s LithoVision briefing prior to SPIE, officials disclosed the first details about the company’s new “tandem-stage” platform, which will combine a moving wafer-exposure stage with a dedicated calibration stage. Unlike ASML’s Twinscan design and Canon’s upcoming 7000 platform, Nikon’s tandem-stage system performs local alignment measurements and checks focus and dose (light intensity) using various sensors while the lens and immersion fluid reside on the calibration stage during wafer transfer.
When the wafer is fully exposed, the calibration stage pushes against the wafer stage in what is described as a “rugby scrum” movement. This movement eventually pushes the wafer out from under the projection lens and moves the puddle of immersion liquid onto the calibration stage. When the new wafer is in place, the main wafer stage pushes the calibration stage out of the way, reclaiming the projection lens and the immersion liquid for a new series of exposures. Since there is no need either to fill the wafer/lens gap with water or to remove the liquid, the scan cycle can be faster than systems with one or two stages that use liquid fill/recover cycles, said Nikon managers.
“We never stop the flow of water,” explained Chris Sparkes, director of technology at Nikon Precision Inc., based in Belmont, CA. “With our twin tandem stage, only the wafer resides on the exposure stage. The measurement stage never holds a wafer. The purpose of this setup is to allow the water to flow continuously during calibration. One of the things we have discovered [with Nikon’s single-stage engineering evaluation tool] is that good focus control requires temperature measurements of the water,” he told WaferNews.
Nikon will use the tandem-stage platform to introduce an immersion scanner with 1.07-NA optics in 4Q05. The new S609B system “will offer the highest NA in the industry for refractive optics,” said Soichi Owa, immersion lithography manager at Nikon Corp.’s development headquarters in Japan.
Other innovations on the S609B include a 22 x 33mm field at full numerical aperture with the ability to perform 26 x 33mm field exposures at NA=1.0, a third-generation polarization illumination system (which does not use filters), and a method of tilting wafers for what Nikon calls “continuous DOF expansion procedure” (CDP), which can be used in “focus drilling” of contact holes.
In 2H06, Nikon plans to use the tandem-stage platform to offer a 193nm immersion scanner with an NA of 1.30, using 4x catadioptric optics for a 26 x 33mm field size.
Meanwhile, ASML also is aggressively working to roll out high-NA systems with throughputs over 100 wafers/hour. The company leads Canon and Nikon in shipping several early immersion systems to development operations at IMEC, Taiwan Semiconductor Manufacturing Co. (TSMC), and the Albany NanoTech center in New York. The Dutch company now plans to be ready to deliver its Twinscan XT:1400i NA=0.95 tool in 3Q05, and a hyper-NA system in 2Q06.