Laser-Produced EUV Source Shows Promise

Although the discharge-produced plasma (DPP) source has recently been considered the strongest candidate for extreme ultraviolet (EUV) lithography beta tools, laser-produced plasma (LPP) sources seem to be gaining ground, according to scientists who met recently in Baltimore, Md., for the latest Sematech-sponsored EUV Source Workshop.
 
LPP-based sources, with tin as fuel, can now produce up to 130 W of power for short periods, which could result in >40 W of power at the intermediate focus (IF) with highly efficient collectors, according to source suppliers. If commercial versions of these sources can be integrated into a reliable source-collector module (SoCoMo) on time, that's enough power to meet the minimum requirements of 40–60 W for beta EUV scanners scheduled for delivery in 2009, noted Vivek Bakshi, workshop chair and senior member of technical staff at Sematech (Austin, Texas).

"LPP-based EUV sources, using tin as fuel, are shining brighter and show increasing promise of meeting power requirements for EUV-based lithography scanners," Bakshi said. "This is good news, since tin-based LPP EUV source technology has many positive features that make it an attractive choice for high-volume microchip manufacturing." Bakshi added that the reported power availability is based on suppliers' estimates of the performance of their own in-house prototypes, which will be developed into commercial products to support EUV lithography.

Despite the remaining challenges, EUV lithography is still widely touted as the most promising lithography technique beyond optical lithography. In the latest LPP-based systems, EUV light is produced by bombarding a sliver of tin with a high-power laser. The light is then gathered by special mirrors that focus the EUV beam in the scanner to produce circuit patterns.

Along with LPP-based sources, DPP sources are still being considered. In fact, DPP sources currently generate more power, consume less energy, are less expensive, and are being integrated into alpha-level EUV scanners. The lower-power version of DPP-based EUV sources has been used to support EUV microexposure tools and industry-wide EUV metrology and resist development projects.

The problem with DPP sources, though, is that the heat and debris that are generated from EUV sources, which can seriously damage scanner components, are more difficult to control in a DPP-based system. This, coupled with recent progress in the high-power lasers needed to power the LPP-based sources, are increasing the feasibility of the alternative approach, Bakshi said.

At the Sematech workshop, 12 suppliers and R&D groups described the readiness and development plans of three types of high-power laser systems designed to bombard tin to produce EUV light. Pulsed CO₂ lasers generate the most power (7 kW), but are still being developed and have not been subjected to continuous operation. Nd: YAG laser modules with 1.5 kW power capable of continuous operation are already available as commercial products, with 3 kW modules in development. Fiber-based lasers require less electricity, cost less and can be easily scaled, but are also less developed than CO2 and Nd: YAG lasers.

Although all three of these laser systems fall short of the 10–20 kW needed for commercial EUV scanners, the necessary wattage can be obtained by "multiplexing" less powerful systems. "The feasibility of linking laser modules in this way has been demonstrated for some time," Bakshi said. "And as the power and reliability improve for individual laser modules, fewer modules will be needed to achieve sufficient power in an LPP-based source."

In a separate announcement in May, Powerlase Ltd. (West Sussex, UK) announced the introduction of a high-power Starlase laser. The 2 kW class module is a diode-pumped solid-state (DPSS) laser that combines with the company's spatial and temporal multiplexing technique for LPP-based EUV sources, and is expected to produce the power required for EUV lithography, according to the company.

The workshop also included a panel discussion on the challenges of developing a SoCoMo, which consists of an EUV source, debris mitigation devices, spectral purity filters and collector mirrors, to channel clean EUV photons to an EUV scanner.

"Most of the panelists agreed that the success of SoCoMo depends on collaboration among all parties: source, collector and scanner manufacturers, research institutions, and end users," Bakshi said. "It appears likely that in the near future, instead of source suppliers, there will be suppliers who will provide an optimized SoCoMo developed in collaboration with scanner manufacturers."

"Efficient collaborations among stakeholders will be critical to demonstrate EUV beta tool sources with power levels of 40–60 W at the intermediate focus, and with reliable operation over weeks by the end of 2008," said Stefan Wurm, EUV strategy program manager for Sematech. "This will ensure that the industry can be confident that mature beta tool sources will be ready for integration into beta EUVL scanners in 2009."

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