LuAG, Other High-Index Immersion Elements Get Needed Boost
Aaron Hand, Executive Editor, Electronic Media -- Semiconductor International, 2/29/2008 4:26:00 AM
Although most chipmakers will use immersion lithography at the 45 nm node, it’s unclear what the best option will be at 32 nm. One option being considered as a possible extension of water-based immersion is immersion lithography using high-index materials. Until this week, however, the prospects for really making a go of it were dim.
But the mood has certainly changed this week at the SPIE Advanced Lithography conference, where progress has been reported on Generation 2 immersion fluids (with a refractive index of ~1.65) and their associated recycling and lens contamination issues, and most significantly news has come from Schott Lithotec (Jena, Germany) on its impressive progress on lutetium aluminum garnet (LuAG) materials for high-index lens elements.
Schott made an announcement last Friday that it had made progress on meeting its absorption targets for LuAG, but it wasn’t until Thursday morning that Lutz Parthier, who is responsible for the LuAG project and head of development at Schott Lithotec, presented the specifics in San Jose. In particular, the researchers have achieved an absorption of 0.05 cm-1 — a significant improvement over the 0.11 cm-1 that Parthier reported at the Immersion Symposium in Keystone, Colo., last fall. And although there is still more progress to be made to reach the final goal of 0.005 cm-1, the latest number level was the requirement set by the exposure tool manufacturers for them to feel comfortable pursuing high-index immersion tools.
 |
| Schott Lithotec's LuAG roadmap shows the progress that the optics maker has made thus far. Announcing an absorbance level of 0.05 cm-1 achievement, they have also achieved 0.03-0.04 cm-1 in lab-scale bulk LuAG. The ultimate absorption goal is 0.005 cm-1. View a larger version of the roadmap |
The impetus behind such tools is that higher-index materials will enable higher numerical apertures (NAs) in the exposure lens, effectively reducing resolution. Patterning at the 45 nm node will make use of water-based immersion tools with NAs as high as 1.35. The use of a Generation 2 immersion fluid, however, would enable an NA of up to 1.45, and incorporating a high-index lens such as LuAG would boost NA further to 1.55. The ultimate goal is NA=1.70, which would require a Generation 3 immersion fluid with an index of refraction of ~1.8, as well as similarly high-index photoresists.
Peter Krüll, Schott Lithotec’s vice president, calls high-index immersion lithography the perfect technology to prolong immersion. Inserting extreme ultraviolet (EUV) lithography even at the 22 nm node looks questionable, he said, and it still faces several potential showstoppers. Meanwhile, double patterning, another option for extending water-based immersion lithography, is an expensive alternative.
Although there are certainly risks associated with the development of the high-index lens materials, Krüll said, Schott’s experience with CaF2 (which would’ve been needed in larger quantities and at higher quality levels for 157 nm lithography) has helped them be more prepared to LuAG development. For example, intrinsic birefringence was a completely unknown problem that came up suddenly with 157, but has been a known entity for LuAG. Schott researchers are doing their homework, he said, and they are pushing the team hard to get it done. By around the end of March or beginning of April, Schott plans to make LuAG materials available to anybody who wants them, Krüll said, to help them in further development work.
Other teams are continuing work on high-index immersion as well, tackling various issues surrounding the materials. High-index immersion presentations were also given Thursday morning by toolmakers Nikon and ASML, immersion fluid suppliers JSR and DuPont, and MIT’s Lincoln Labs. JSR and DuPont have both made good progress in Generation 2 (n~1.65) immersion fluids, improving their understanding of and solutions for transmittance change, lens contamination and recycling.
Of considerable concern has been lens photocontamination, but progress has been made on understanding and mediating contamination, and there are now at least a couple promising methods for cleaning the optics. Vlad Liberman presented the work that has been done at Lincoln Labs, showing that the optics could be cleaned with a very minimal amount of hydrogen peroxide.
ASML’s Harry Sewell highlighted several areas in his presentation, including various concepts for improved fluid delivery and a different scheme for cleaning the optics as they become contaminated. Nikon’s Hiroyuki Nagasaka detailed work his company has been doing on optimizing the final lens processing for the LuAG material, showing promising results on polishing’s ability to bring birefringence closer in line with fused silica.
There has also been concern about scan speed capabilities with high-index fluids, which exhibited problems when put into water-type flow systems. But the toolmakers have been working on new flow candidates, and are no longer concerned. In fact, Canon presented a poster at Thursday night’s poster session showing that they have been able to achieve 800 mm/sec scan speeds with high-index immersion fluids.
Sewell seemed upbeat about the prospects of high-index immersion, showing that it was on ASML’s roadmap. But ASML’s review of the technology has been pushed out to the third quarter this year as they wait for the results of Schott’s feasibility study, he said, which is due around the end of March. “Everybody in this branch of the industry is waiting with baited breath, month by month,” Sewell said. He later added, “Progress is slow. We’d like you to go faster. Everybody would like you to go faster.”
In a private conversation after Thursday morning’s session, Krüll and Bryan Rice, immersion lithography program manager at Sematech, reviewed the progress and expressed their happiness with the results so far. “We’re very pleased with the progress that’s been made,” Rice said. “It’s a great accomplishment. It’s really outstanding.”
The toolmakers needed to see progress on the LuAG development in order to be convinced to pursue development of high-index immersion exposure systems. The concern has been that, without LuAG’s ability for high transmission levels, high-index immersion fluids alone would not offer enough NA boost to make the tool changes worthwhile. “They needed to see the progress, and now it’s been achieved,” Rice said. “When the scanner manufacturers agree with that, and follow up with their financial support, Sematech will be the first to congratulate all parties.”
Sematech will be meeting next week as part of its standard post-SPIE evaluation of the technologies, and discussion of high-index immersion will certainly be on the list of topics, Rice noted. But he believes that the focus should now move on to the next challenge in high index. “Now the industry is beginning to turn its eyes away from high-index lenses, to focus on the next generation of fluids,” he said. “Generation 3 progress is lacking, so it’s the new No. 1 goal for Sematech’s high-index program.”
