Flatness characterization of EUV mask chucks

For future extreme ultra violet (EUV) lithography at a wavelength of about 13 nm, flatness of the mask surface is an issue, since out-of-plane deviations sensitively transfer to in-plane distortions. Electrostatic clamping devices of extreme flatness and high stiffness are required. At Fraunhofer IOF, manufacturing processes for EUV mask chucks made from low thermal expansion materials are investigated. Since the chucking surface is finally structured into a pin array, flatness characterization is not trivial. The paper reports on flatness characterization of a mask chuck prototype at various stages of surface manufacturing. We present measurement results obtained with sophisticated commercial tools based on optical and tactile principles and discuss limitations encountered in both cases as well as possible strategies for improvement.