The effort to develop EUVL has presented the metrologist with a set of unique challenges both in radiometry and in measuring reflectivity. Since the inception of the National Institute of Standards and Technology's (NIST's) EUV-detector-based radiometry program in 1970, there has been little need for accurate measurements of pulsed EUV sources, and the radiometric techniques in this spectral range were all tailored to continuous wave (cw) measurements. Now, however, EUV sources bright enough to be suitable for EUVL are all based on very high temperature plasmas that must be generated by extremely high power pulses. Recently some experience in pulsed radiometry has been gained through the creation of standards for pulsed radiometry connected with DUV lithography with its pulsed 248- and 193-nm laser sources. However, important differences in the spectral, temporal, and spatial characteristics of the EUV sources as contrasted with the DUV laser sources necessitate specialized approaches, still under development, that are described in Sec. 29.3.
Soon after the demonstration in 1986 of the first practicable EUV mirror in the form of a Mo∕Si multilayer, NIST established a capability to measure the reflectivity of EUV mirrors for such applications as solar astronomy, plasma diagnostics, EUV laser development, and EUV microscopy. At that point, measurement accuracy of order 0.01 nm in wavelength and 2% in reflectivity seemed to provide more than adequate characterization for the applications at hand. However, the advent of EUVL made the demands much more stringent due to the imaging and illumination needs of the process. NIST has continued to perform EUV mirror calibrations since the program's inception, has increased its capabilities to meet many of the reflectometry demands of EUVL, and will continue to expand and improve the program to help meet the future needs of the industry.
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