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2 June 2003 Design and development of a novel actinic inspection tool for EUV multilayer-coated mask blanks
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A novel actinic (at-wavelength) inspection tool for detecting critical phase defects in extreme ultraviolet (EUV) multilayer-coated mask blanks is designed and being developed. Block inspection by actinic dark field imaging using a combination of Schwarzschild optics, a CCD, and a laser-produced plasma (LPP) light source is employed to improve detection sensitivity while keeping decent throughput. To optimally configure optics and a mask blank within currently available options, EUV scattering from both Gaussian-shaped multilayer defects and surface roughness is simulated. Intensity of the roughness-induced scattering into the imaging optics is computed assuming 0.15nm root-mean-square (RMS) of the multilayer surface roughness with a typical power spectral density (PSD) function. Possible configuration candidates are compared in detail to detect phase defects with its size down to 30nm. We have modeled noise intensity based on the optimum configuration by integrating shot noise and spatial fluctuation of roughness. Pixel-to-pixel fluctuation of background intensity due to spatial roughness variation is studied using random fractal model to be built into the total noise model. Under the criteria of minimizing nuisance defects and maximizing capture rate of real defects, the required specifications for the power and the brilliance of the light source are computed and discussed.
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Yoshihiro Tezuka, Masaaki Ito, Tsuneo Terasawa, and Toshihisa Tomie "Design and development of a novel actinic inspection tool for EUV multilayer-coated mask blanks", Proc. SPIE 5038, Metrology, Inspection, and Process Control for Microlithography XVII, (2 June 2003);

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