November 23, 2004, Miyazaki, Japan--Available source power for use in extreme-ultraviolet (EUV) lithography has increased significantly in the past year, reaching nearly half the level required for commercial manufacturing in 2009, participants learned here at the Sematech-sponsored EUV Source Workshop, held during the Third International Symposium on EUV Lithography (EUVL) earlier this month.
Sematech (Austin, TX) conducts an extensive EUV program aimed at developing mask-blank technology and exposure processes, using the mask-blank-development line and microstepper recently installed at Semateech North (Albany, NY). Additionally, Sematech organizes the EUV Source Workshop twice a year.
At the workshop. Uwe Stamm of Xtreme Technologies said his company had used tin-based, discharge-produced plasma (DPP) sources to generate an initial 400 watts of EUV power, estimating that 50 watts of this energy could be collected of use in an exposure system. This compares with a system requirement of 115 watts needed to expose 120 wafers per hour in high-volume manufacturing. Xtreme Technologies is a joint venture between Lambda Physik (Göttingen, German and Jenoptik (Jena, Germany).
Joseph Pankert of Philips Extreme (Aachen, Germany) also reported about 50 watts of estimated collected power in his company's tin-based DPP system. "This was achieved by only 260 watts in original source power," Pankert said. "However, the energy was more efficiently collected because of an optimal match to the optical system."
These announcements were coupled with statements from suppliers Carl Zeiss (Oberkochen, Germany) and Media Lario (Bosisio Parini, Italy) that they are working to increase the lifetime of EUV light collectors, now identified as an issue more pressing than source power. In addition, Cymer (San Diego, CA) revealed its selection of a laser-produced-plasma (LPP) source based on lithium as a likely candidate for EUVL.
"This is very encouraging news for the potential use of EUV lithography in semiconductor manufacturing," said Vivek Bakshi, workshop chair and member of the technical staff at Sematech. "Until now, source power has been the number one challenge for EUVL implementation. This breakthrough now permits us to focus our resources on addressing other critical issues for EUVL."
Bakshi said Sematech played a key role in these advances by providing several source suppliers with metrology calibration and support; materials for source critical components, such as electrodes and insulators; fundamental modeling data and modeling capabilities to optimize EUV yield and to improve component lifetime and feasibility assessments for some key enablers for various EUV approaches, such as tin delivery for DPP sources.
"We plan to continue to play an important role by planning projects on tin-debris mitigation, collector cooling, intermediate focus metrology and by identifying further challenges like spectral purity by collecting accurate data."
At the Third Symposium, advances in source power were so significant that the symposium steering committee reprioritized it within the list of critical issues facing EUVL, Bakshi noted. The steering committee chose three topped issues and three unranked issues as critical for the commercial introduction of EUVL in the benchmark year of 2009:
1. Availability of defect-free masks
2. Lifetime of source components and collectors
3. Resist resolution sensitivity and line edge roughness (LER)
-- Reticle protection during storage handling and use
-- Source power
-- Projection and illuminator-optics quality and lifetime
In addition, approximately 160 workshop participants ranked technical issues specifically affecting EUV sources. These included collector lifetime and debris mitigation; cost of ownership; thermal loading of collectors; source power; spectral purity; and design of efficient collector mirrors.
In EUV lithography, extremely short wavelengths (13.5 nm) of invisible light are used to print very narrow features consistent with the requirements of the 45 nm technology node or below, as defined by the International Technology Roadmap for Semiconductors (ITRS). The EUV light is produced by heating xenon, tin, or other elements to a plasma state, using either magnetic compression or laser bombardment.
Steady progress in source power has made EUV increasingly viable for high-volume manufacturing, Bakshi explained. During the Sematech-sponsored EUV Source Workshop last February in Santa Clara, CA, suppliers demonstrated source power of 15 and 20 watts at the intermediate focus, the point where light can be used for patterning. A year before, sources were producing only 5 watts of power at that point.
The use of tin as a source fuel has received increased attention recently because it is two to three times more efficient than xenon at converting electromagnetic or laser energy to EUV light. However, achieving higher source power from tin comes at a cost of increased debris, which limits the lifetime of the collector that gathers light from the source and transmits it to the exposure tool. In addition, the collector mirrors must withstand high thermal loads generated by the sources. With the tin DPP approach, the focus now shifts to removing debris and increasing collector lifetime.
"We can now say that the photons can be produced," said Pankert of Philips Extreme. "However, the integration of EUV sources with the collector is still an issue."
To increase the lifetime of tin-based DPP sources, suppliers need to reduce the generation of debris, control where the debris goes, and develop materials that can withstand damage from high-energy tin ions. At November's SEMATECH workshop, Carl Zeiss and Media Lario committed to work with source suppliers to develop more effective, tin-based collectors that will address these challenges.
"We are working on scaling the collector lifetime through a proprietary cooling option and EUV (reflective) material optimization, particularly for tin-fueled sources," said Media Lario representative Giovanni Nocerino. "High-accuracy replication by electroforming, which is one of Media Lario's core competencies, inherently offers a considerable cost effectiveness when manufacturing high precision reflective optics."
Piotr Marczuk of Carl Zeiss, which has provided collectors for the industry's first EUV microsteppers, said its initial results "show that our collectors do a very good job. The source suppliers have shown their paths to scale EUV light power up to high-volume manufacturing requirements, and collectors have to follow this development in order to enable high-volume EUV lithography."
In its announcement at the workshop, Cymer cited LPP as the most viable solution for achieving high-volume EUVL. "Over the past year, we have accelerated our work on LPP-based source technology and we are excited with the progress we are making toward commercializing an LPP-based, EUV illumination source," said Dave Myers, Cymer's Vice President of EUV Development. "Our current LPP system shows a conversion efficiency of 3 percent for lithium-based sources, and we are confident in our ability to achieve the power, conversion efficiency and lifetime requirements for high-volume EUV."