Characterization of the polarization properties of PTB's EUV reflectometry system

The development of EUV lithography is critically based on the availability of suitable metrology equipment. To meet the industry's requirements, the Physikalisch-Technische Bundesanstalt (PTB) operates an EUV reflectometry facility at the electron storage ring BESSY II. It is designed for at-wavelength metrology of full-sized EUVL optics. A total uncertainty of 0.10 % for peak reflectance is achieved with a reproducibility of 0.05 % and a reproducibility of 1 pm for the centre wavelength. Measurements at PTB use almost linearly polarized radiation, whereas EUV lithography machines are operated with unpolarized sources and the status of polarization changes throughout the optical system. Therefore, to transfer these high-accuracy measurements to the EUV optical components under working conditions, it is essential to study the polarization dependence. We present a detailed characterization of the state of polarization for our EUV reflectometry system. We will discuss the uncertainties of PTB's reflectometry for large oblique angles with respect to the influence of polarization. An example of such measurements is the characterization of large 5 sr collector mirrors for LPP EUV sources, the largest EUV optical components presently available.