Status of EUV reflectometry at PTB

Abstract

The development of EUV lithography is critically based on the availability of suitable metrological 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 with a maximum weight of 50 kg and a diameter of up to 550 mm. A micro-reflectometry station was installed for reflectometry with high spatial resolution for, e.g., structured masks. A photon beam size of 10 μm FWHM has presently been achieved. To meet the increasing demands of metrology for future lithography production tools, the measurement uncertainty was permanently reduced. For peak reflectance, a total uncertainty of 0.10 % is achieved with a reproducibility of 0.05 %. The uncertainty of 2 pm in the center wavelength is given mainly by the uncertainty for the reference wavelength of the Kr 3d5/2-5p resonance. A long-term reproducibility of 0.8 pm has been demonstrated over a period of about 4 years. We have recently demonstrated repeatability below 0.06 pm. This good repeatability is important for the determination of the coating-thickness gradient in alpha-tool optics. We present a long-term series of measurements at a set of EUV mirrors and discuss our recent results in improving wavelength reproducibility.