Polarization dependence of multilayer reflectance in the EUV spectral range

Abstract

The development of EUV lithography depends strongly on the availability of suitable metrology equipment. The Physikalisch-Technische Bundesanstalt (PTB) with its laboratory at the electron storage ring BESSY II is the European centre of EUV radiometry and supports the national and European industry by carrying out high-accuracy at-wavelength measurements in the EUV spectral region, particularly to support the development of EUV lithography, which holds the key to the next generation of computer technology. To meet the requirements of industry, PTB operates an EUV reflectometry facility, designed for at-wavelength metrology of full-size EUVL optics with a maximum weight of 50 kg and a diameter of up to 550 mm and a micro-reflectometry station for reflectometry with sub 10 μm spatial resolution. An absolute uncertainty of 0.10 % is achieved for peak reflectance, with a reproducibility of 0.05 %. For the center wavelength an uncertainty of 2 pm is achieved with a long-term reproducibility of 1.1 pm and a short-term repeatability below 0.06 pm. To transfer these high-accuracy measurements to the EUV optical components under working conditions it is essential to study the polarization dependence. Measurements at PTB use linearly polarized radiation, whereas EUV optics are operated with unpolarized sources and the status of polarization changes throughout the optical system. PTB has therefore investigated and verified the capabilities of the EUV reflectometer for measurements with variable polarization. Taking advantage of all mechanical movements for detector and sample, measurements with arbitrary orientation of the electric field vector can be carried out up to an angle of incidence of 20°. We present representative polarization dependencies obtained on Mo/Si multilayer coatings, including measurements with a 45° orientation of the polarization to the optical plane of deflection to simulate the behavior for unpolarized radiation.