18 March 2016 Scanning coherent diffractive imaging methods for actinic EUV mask metrology
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For the successful implementation of extreme ultraviolet (EUV) lithography in the upcoming technology nodes, a major challenge to overcome is the stable and reliable detection and characterization of mask defects. We have recently presented a reflective mode EUV mask scanning lensless imaging tool (RESCAN) which is installed at the XIL-II beamline of the Swiss Light Source and showed reconstructed aerial images of test patterns on EUV masks. RESCAN uses scanning coherent diffractive imaging (SCDI) methods to obtain actinic aerial images of EUV photomasks and was designed for 20 nm on-wafer resolution. Our SCDI algorithm reconstructs the measured sample by iteratively solving the phase-problem using over-determined diffraction data gathered by scanning across the specimen with a finite illumination. It provides phase and amplitude aerial images of EUV photomasks with high resolution without the need to use high NA (numerical aperture) lenses. Contrary to scanning microscopy and full-field microscopy, where the resolution is limited by the spot size or NA of the lens, the achievable resolution with our method depends on the detector noise and NA of the detector. To increase the resolution of our tool, we upgraded RESCAN with a new detector and algorithms. Here we present the results obtained with the new tool that is capable of up to 10 nm on-wafer resolution. We believe that the realization of our prototype marks a significant step towards overcoming the limitations imposed by methods relying on imaging optics and shows a viable solution for actinic mask metrology.
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Patrick Helfenstein, Patrick Helfenstein, Istvan Mohacsi, Istvan Mohacsi, Rajeev Rajendran, Rajeev Rajendran, Yasin Ekinci, Yasin Ekinci, "Scanning coherent diffractive imaging methods for actinic EUV mask metrology", Proc. SPIE 9776, Extreme Ultraviolet (EUV) Lithography VII, 97761F (18 March 2016); doi: 10.1117/12.2219937; https://doi.org/10.1117/12.2219937

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