The routine availability of high-accuracy soft-x-ray radiometry is a prerequisite for the development of EUVL as the next-generation lithography (NGL). A main issue for EUVL is the ability to produce sufficient levels of EUV radiant power. Therefore, well-characterized radiation detectors that work reliably over long periods of time at high irradiation levels are needed. This development has caused an increasing need for radiometric standards and quantitative radiometric measurements. In order to meet these demands, the Physikalisch-Technische Bundesanstalt (PTB) radiometry laboratory at the BESSY II electron storage ring pursues the characterization and calibration of radiation detectors and tools in the VUV and soft-x-ray spectral regions as a major task.
The PTB has been providing EUV calibration services for many years within the European EUCLIDES and MEDEA projects at a dedicated radiometry beam-line at the BESSY I electron storage ring and, from the year 2000, at the BESSY II electron storage ring, exploiting the benefits offered by this third-generation facility. A large reflectometer enables the PTB to characterize EUVL optics up to 550 mm in diameter and 50 kg in mass, meeting the demands for the present designs of projection optics. For the spectral reflectance of a mirror in the EUV spectral region, a relative uncertainty of u = 0.14% is achieved.
The radiometric approach made at PTB to the calibration of detectors in the EUV spectral range is the comparison with a primary detector standard, a cryogenic electrical substitution radiometer (ESR) operated at liquid-He temperature using monochromatized synchrotron radiation. The operating principle of an ESR is that an absorber transforms the incident radiant power into a heat flow directed through a heat link to a heat sink kept at constant temperature. The radiant power equals the change in the electrical heater power required for keeping the absorber temperature at a constant value when the radiation shutter is opened. Photodiodes are calibrated as transfer detector standards with a relative uncertainty of u = 0.26% by direct comparison with the radiometer. The calibration of tools for source characterization is thus directly traced to the SI system of units.
The calibrations at PTB use monochromatized synchrotron radiation, a quasi-dc radiation with a rather low radiant power of about 1 μW. For EUV source characterization, strongly pulsed radiation and very different radiant powers must be measured. High-accuracy calibrations for EUVL therefore require further investigations of the detector performance. Therefore, high-power beamlines for investigations of the linearity of the response and stability under irradiation with EUV were installed by PTB.
This chapter gives an overview of the measurement capabilities of PTB at the BESSY II storage ring and presents the status of the calibration of detectors and tools for EUV source metrology achieved at PTB.
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