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Soon after the first explorations of the potential of EUVL, it became obvious that the wafer throughput of future EUVL systems, and thus the EUV source brightness, was a key factor in the industrial value of this new imaging technology. The need for plasma-based light sources of moderate temperature, though strongly confined in space and critically optimized for emission at 13.5 nm, implied the use of suitable diagnostics, well adjusted for the particular application. Especially in the early days of EUVL development, a variety of source diagnostics were being used by the different source developers, each having backgrounds in different research and technology areas. These diagnostics ranged from uncalibrated grating spectrographs and luminescent detectors to broadband detection systems, such as filtered calorimeters and photocathodes, each detector obviously having its own characteristics. The measurement units applied varied correspondingly, ranging from J·eV−1·J1−1 to percent.
From the data collected with these devices, extrapolations were then made to the most relevant quantity, namely, the true EUV power within the bandwidth and the radiation acceptance angle of the total EUVL system. In order to allow a direct comparison of the performance of the different candidate EUV sources, in 2000 ASML sponsored a source metrology activity by FOM and Philips Research for which a portable, absolutely calibrated set of diagnostics was used. The approach was to assess each of the different sources with one single diagnostics set, circumventing any uncertainty introduced by the specific properties or calibration of the various metrology devices. In addition, the measurement procedure was standardized. Collecting data from sources on different locations worldwide, the activity became known as Flying Circus, or FC for short, and initially resulted in a full benchmark characterization of five candidate EUV light sources and the development of a common diagnostic standard. The follow-up project, Flying Circus 2 (FC2), sponsored by International SEMATECH, had the broader scope of further assessing the progress of EUV source performance, calibrating EUV source metrology equipment from different EUV source suppliers, and continuing the development of globally accepted EUV source measurement procedures. Nowadays, more then a dozen sources are characterized in this way, and the FC-type diagnostic has found permanent use at the majority of the source development laboratories. FC diagnostics are commercially offered by Scientec Engineering (www.scientec.nl). The general acceptance of the method and the continued program of on-site calibration using the portable FC equipment at the different source labs, as well as further-developed FC hardware, considerably assisted in the standardization of source performance representation.
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