Electrostatic chuck for EUV masks

摘要

For future extreme ultra violet (EUV) lithography at a wavelength of about 13 nm, out-of-plane deviations sensitively transfer to in-plane distortions and high flatness is required for electrostatic clamping devices. At Fraunhofer IOF technology analysis has started to identify suitable manufacturing processes for EUV mask chucks made from low thermal expansion materials. The paper reports on chuck design and preliminary experimental results obtained in manufacturing. A symmetric bipolar electrode design was chosen and a high chuck stiffness was obtained by appropriate mechanical design and materials selection. The chuck shape is circular with a diameter slightly less than the mask diagonal, to allow for mask gripping at its four corners and thus avoid particle contamination of the chuck surface from mask handling. Further, the quadratic chucking area bears a hexagonal pattern of pins with a height in the μm-range, to reduce direct mechanical contact between mask and chuck to only a few percent. Preliminary analysis indicates that a chuck flatness of about 50 nm within the quality area of the mask can be realized with an optimized optics manufacturing technology. Yet, there are indications of surface relaxation in nm-range from pin structuring. Deformation of chuck surface from gravity load in the three mounting points is estimated to about 14 nm from FEM calculations.