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Chapter 22:
Driver Laser, Xenon Target, and System Development for LPP Sources
The Extreme Ultraviolet Lithography System Development Association (EUVA) was established in Japan in July 2002. Association members are the companies Canon, Fujitsu, Gigaphoton, Komatsu, NEC, Nikon, Renesas, Toshiba, and Ushio. EUVA is funded through the Japanese Ministry of Economy, Trade, and Industry (METI). The target of the EUVA project is the development of an EUV light source and of EUV exposure-tool projection optics. The light-source development project includes DPP sources and LPP sources. The EUVA project will end in March 2006, i.e., at the end of the Japanese fiscal year 2005. By that time DPP and LPP EUV light sources with an IF inband power of 10 W will have been developed. More detailed information related to EUVA can be found elsewhere. The “driving force” for EUVL light sources is the aspect of semiconductor evolution that favors smaller feature sizes because they allow for more tasks to be done at higher speeds. Optical lithographic technologies have been used so far for semiconductor production, meaning that the wavelength of the light source must follow the trend to smaller sizes, i.e., to shorter wavelengths. Current high-end light sources are KrF and ArF excimer lasers oscillating at 248 and 193 nm, respectively. These lasers enable the 90-nm node (KrF), and may still be used in the 45-nm node (ArF) if immersion technology can be successfully applied. The status of the F2 laser (157 nm) as a lithography light source is therefore open, because the currently favored technology for the 45-nm node and below is EUVL. EUVL incoherent light sources will be used at 13.5 nm because the current Mo∕Si multilayer mirror (MLM) technology provides optics with about 70% reflectivity at normal incidence at this wavelength, with a FWHM bandwidth of about 4%. EUVL sources will be based on DPPs or LPPs optimized for the emission at the required wavelength. The main constraint on the light source is the increased power requirement that is due to the limited reflectivity (e.g., using six mirrors for the projection optics results in a transmission of only 0.12) and to the low resist sensitivity (5 mJ∕cm2). Whereas currently KrF and ArF laser powers of 20–40 W are sufficient, an EUVL light-source power of 115 W is required at the IF, i.e., the interface point between the light source and the illuminator of the scanner. The required power estimation of 115 W is based on a 300-mm wafer throughput of 100 wafers per hour, which is considered necessary for profitability. Another quite severe specification is due to the critical dimension control, which requires an integrated energy stability of 0.3% (3σ) for a moving average of 50 pulses. Ota et al. present the current EUV source requirements from the lithography system makers ASML, Canon, and Nikon in detail in Chapter 2. This chapter will briefly describe the development of an LPP light source for lithography done at EUVA. First, the driver-laser and plasma-target development will be outlined, then the current system performance will be presented.
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