High-power sources for EUV lithography: state of the art (2004)

The availability of extreme ultraviolet (EUV) light sources, measurement tools and integrated test systems is of major importance for the development of EUV lithography for use in high volume chip manufacturing which is expected to start in 2009. The estimates of cost of an EUV exposure tool in combination with sophisticated throughput models leads to a throughput of 120 wafers per hour necessary for economic use of EUV lithography. Concluding from that light sources are necessary which deliver an EUV output power of 115 W at 13.5 nm at the entrance of the illuminator system. The power requirement in combination with the required lifetimes of source components and collector optics make the source technology the most critical issue to be solved when developing EUV lithography. The present paper gives an update of the development status of EUV light sources at XTREME technologies, a joint venture of Lambda Physik AG, Goettingen, and Jenoptik LOS GmbH, Jena, Germany. Results on both laser produced plasma (LPP) and gas discharge produced plasma (GDPP), the two major technologies in EUV sources, are given. The LPP EUV sources use xenon-jet target systems and pulsed lasers with 500 W average power at up to 10 kHz developed at XTREME technologies. The maximum conversion efficiency from laser power into EUV in-band power is 1.0% into 2π solid angle. 2.0 W EUV radiation is generated at 13.5 nm in 2π sr solid angle. The small source volume of < 0.3 mm diameter will allow large collection angles of 5 sr. The intermediate focus power is estimated to 1 W. Collector mirror lifetime tests showed 5 million pulses lifetime without debris mitigation. With debris mitigation in place lifetimes of more than 1 billion pulses are estimated. For the next generation of higher power EUV LPP sources a laser driver has been tested at 1.3 kW average laser power. This will lead to 5 W EUV power in intermediate focus. The GDPP EUV sources use the Z-pinch principle with efficient sliding discharge pre-ionization. Prototype commercial gas discharge sources with an EUV power of 35W in 2π sr were already delivered for integration into EUV microsteppers. These sources are equipped with a debris-filter which results in an optics lifetime exceeding 100 million discharges at 1 kHz repetition frequency. The same lifetime was achieved for the components of the discharge system itself. The progress in the development of high-power discharge sources resulted in an EUV power of 150 W in continuous operation at 4.5 kHz repetition rate by implementation of porous metal cooling technology. The EUV plasma has a FWHM-diameter of 0.5 mm and a FWHM-length of 1.5 mm. The intermediate focus power is calculated to be in the range of 15 W - 20 W, depending somewhat on the transmission of the optical path to the intermediate focus and on the etendue specification. The typical fluctuations of the EUV energy are standard deviation σ<5% without any active stabilization. Discharge sources with Sn as emitter were investigated as more efficient alternative to Xenon. Estimates regarding Sn sources reveal the potential of achieving 65 W intermediate focus power by using developed porous metal cooling technology. Improvement of cooling could open the path to 115 W of power for high volume manufacturing using EUV lithography. However, Sn-sources are technologically risky und much less advanced than Xe sources, since fuel-handling and debris mitigation is much more challenging in comparison to Xe-sources. GDPP and LPP sources still compete for the technology of high volume manufacturing sources for EUV lithography. Optimization potential of the etendue of the optical system of EUV scanners will certainly influence any technology decision for HVM sources.