Characterization of the measurement uncertainty of a laboratory EUV reflectometer for large optics

Quality assurance for the production of optical components for extreme ultraviolet lithography (EUVL) strongly requires at-wavelength metrology. Initially, the at-wavelength characterization of mirrors and masks was developed using synchrotron radiation of electron storage rings, e.g. BESSY II. For the production process of EUV optics, however, the immediate access to metrology tools is necessary and the availability of laboratory devices is mandatory. Recently, an EUV reflectometer (EUVR) for large samples has been put into operation. It consists of a laser-produced plasma (LPP) radiation source, a monochromator and a large goniometer system. It can handle samples with diameters of up to 500 mm, thicknesses of up to 200 mm and weights of up to 30 kg. The wavelength can be varied from 10 nm to 16 nm. The spot size on the sample surface is about 2 mm in both directions. The angle of incidence can be varied from 3 deg to grazing incidence. In this paper, we describe the measurement uncertainties achieved in the EUVR with reference to measurements by the Physikalisch-Technische Bundesanstalt at its soft x-ray radiometry beamline at BESSY II. Using appropriate reference points for the wavelength calibration of the monochromator, the absolute wavelength uncertainty is below 4 pm in the spectral range from 11 nm to 15 nm. The measured peak reflectance of multilayer mirrors shows a constant offset of about 1% absolute and is reproduced within 0.2%.