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Title:

Impact of line edge and line width roughness on diffraction intensities in scatterometry

Author(s): H. Gross, M.-A. Henn, S. Heidenreich, A. Rathsfeld and M. Baer
Journal: Proc. SPIE 8550
Year: 2012
Month: December
Day: 18
Event name: Optical Systems Design 2012
Event place: Barcelona, Spain
Event date: November 26, 2012
DOI: doi:10.1117/12.981327
Abstract: The characterization of nanostructured surfaces by scatterometry is an established method in wafer metrology. From measured light diffraction patterns, critical dimensions (CD) of surface profiles are determined, i.e., line widths, heights and other profile properties in the sub-micrometer range. As structures become smaller and smaller, shorter wavelengths like extreme ultraviolet (EUV) at 13.5 nm ensure a sufficient sensitivity of the measured light diffraction pattern with regard to the structure details. Obviously, the impact of structure roughness with amplitudes in the range of a few nanometers can no longer be neglected in the course of the profile reconstruction. To model line roughness, i.e., line edge (LER) and line width (LWR) roughness, a large number of finite element (FEM) simulations are performed for domains with large periods, each containing many pairs of line and space with stochastically chosen widths. These structures are composed of TaN -absorber lines with an underlying MoSi -multilayer stack representing a typical EUV mask. The resulting mean efficiencies and the variances of the efficiencies in dependence on different degrees of roughness are calculated. A systematic decrease of the mean efficiencies for higher diffraction orders along with increasing variances are observed. In particular, we obtain a simple analytical expression for the bias in the mean efficiencies and the additional uncertainty contribution stemming from the presence of LER and/or LWR. As a consequence, the bias has to be included into the model to provide accurate values for the reconstructed critical profile parameters. The sensitivity of the reconstructed CDs in respect of roughness is demonstrated by using numerous LER/LWR perturbed datasets of efficiencies as input data for the reconstructions. Finally, the reconstructed critical dimensions are significantly improved toward the nominal values if the scattering efficiencies are bias-corrected.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

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