Absolute measurement of EUV radiation from an undulator

Driven by the needs for microlithography, powerful sources for extreme ultraviolet (EUV) radiation at wavelengths around 13 nm are currently under development. For absolute photon flux measurements of highly intense and extremely pulsed radiation in the VUV and EUV spectral range, a detector system based on the photoionization of rare gases has been developed. Due to its extended dynamic range, the device can be calibrated with spectrally dispersed synchrotron radiation at low photon intensities, but applied for high power sources like recently at the VUV-FEL of the TESLA test facility at DESY. The detector is free of degradation and almost transparent, and therefore suitable for intensity monitoring. Here we describe the application of the detector for flux measurements at a bean-dine for undispersed, deflected undulator radiation in the PTB Radiometry Laboratory at the electron storage ring BESSY II. This beamline is used for development and lifetime testing of components for EUV lithography, where accurate determination of the incident radiant power is needed, e.g. for exposure dose control. Taking advantage of the characteristic photoionization cross sections for different rare gases in the soft x-ray wavelength range, contributions of higher undulator harmonics could be determined and suppressed for EUV (13 nm) power measurements. While in normal operation the BESSY II radiation can be seen as quasi-continuous (500 MHz repetition rate), in single-bunch operation mode (1.25 MHz) measurements of the power for single EUV pulses have been performed. This demonstrates the capability of the gas detector for a broad range of applications with pulsed EUV sources.