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Chapter 24:
LPP Source Development and Operation in the Engineering Test Stand
The Engineering Test Stand (ETS) is a fully integrated, alpha-class, full-field (24 × 32 mm) EUVL step-scan tool that was developed for system and process learning in support of commercialization of EUVL. The overall system design is shown in Fig. 24.1, and is described in detail in Refs. 1–4. This chapter describes the high-average-power LPP EUV source that was integrated into the ETS, and the developments that led to this source. The goal of the ETS source and associated collector and imaging optics was to demonstrate EUV optical throughput and associated wafer exposure rates equivalent to ten 300-mm wafers per hour. The EUV illumination-system requirements to support this throughput are listed in Table 24.1. The source is required to produce 15 W of 13.4-nm radiation integrated over a 2.5% spectral bandwidth (the appropriate bandwidth for the mirror configuration in the ETS optical design) and a 2π-sr solid angle. To achieve the required flux at the reticle plane, 4.4 W, or approximately 30% of the emitted radiation, must be collected by the first element of the condenser. The collection solid angle and the etendu of the projection optical system limit the source emission diameter to ≤150 μm FWHM. The repetition-rate requirement derives from the illumination uniformity requirement of the scanned exposure. The illumination uniformity is measured over the full 24 × 32-mm exposure field and is sensitive both to spatial nonuniformity along the length of the scanned exposure arc and to temporal illumination instabilities that lead to nonuniformities in the scanned direction. The tolerance to source motion is extremely tight, due, in part, to the high effective magnification of the condenser; slight source motions cause significant nonuniformities in the reticle plane and unacceptable motion of the illumination beams in the entrance pupil of the projection optic. The condenser lifetime requirement is derived from an intermediate goal of 500 h of operation before significant condenser maintenance is required.
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