Towards the optical inspection sensitivity optimization of EUV masks and EUVL-exposed wafers

U. Okoroanyanwu; J. Heumann; X. Zhu; C. H. Clifford; F. Jiang; P. Mangat; R. Ghaskadavi; E. Mohn; R. Moses; O. Wood; H. Rolff; T. Schedel; R. Cantrell; P. Nesladek; N. LiCausi; X. Cai; W. Taylor; J. Schefske; M. Bender; N. Schmidt

doi:10.1117/12.923134

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16 April 2012 Towards the optical inspection sensitivity optimization of EUV masks and EUVL-exposed wafers

U. Okoroanyanwu, J. Heumann, X. Zhu, C. H. Clifford, F. Jiang, P. Mangat, R. Ghaskadavi, E. Mohn, R. Moses, O. Wood, H. Rolff, T. Schedel, R. Cantrell, P. Nesladek, N. LiCausi, X. Cai, W. Taylor, J. Schefske, M. Bender, N. Schmidt

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Proceedings Volume 8352, 28th European Mask and Lithography Conference; 83520V (2012) https://doi.org/10.1117/12.923134
Event: 28th European Mask and Lithography Conference (EMLC 2012), 2012, Dresden, Germany

Abstract

Pending the availability of actinic inspection tools, optical inspection tools with 193 nm DUV illumination wavelength are currently used to inspect EUV masks and EUVL-exposed wafers. Due to strong optical absorption, DUV photons can penetrate only a few surface layers of EUV masks, making them sub-optimal for detecting hidden defects embedded within the sub-layers of the mask, the so-called phase defects. Although these phase defects may not be detected by optical inspection tools, they may print on the wafer. Conversely, false and nuisance defects which may not print on the wafer may be detected by optical inspection tools, and by so doing, degrade the inspection sensitivity of the tool to real and critical defects. This paper discusses approaches to optimizing the optical inspection sensitivity of EUV masks, with a view to overcoming some of the absorption limitations of the inspection wavelength and also with a view to enhancing the imaging contrast of the reflected light between the low reflective absorber/antireflection coating stack and the moderately reflective mirror surface of Mo/Si bilayers, capped with a thin Ru layer, and which serves to protect the mirror surface from damage and contamination during mask fabrication and wafer printing processes. The effects of mask absorber/ARC stack thickness on optical inspection contrast are simulated using rigorous coupled wave analysis (RCWA), and compared to experimental results. EUV masks with thin absorber/ARC stacks are observed to have higher inspection contrast, up to 15 % higher than their thicker counterparts, especially as the feature pitch gets smaller. Blank defect inspection performance of tools such as the Siemens DFX40 tool and KLA 617 Teron tool equipped with Phasur module are compared, and correlated with patterned mask inspection data generated from KLA 617 Teron tool. Patterned mask defect sensitivities to the tune of 40 nm and 90 nm were obtained on thin and thick absorber/ARC stacks, respectively. The defect location accuracy of the Teron 617 tool is better than 250 nm (3σ), while the alignment repeatability of the Teron 617 on the fiducials is better than 60 nm (3σ). Printability of mask blank and patterned mask defects on exposed wafers in terms of what and where the defects print, are also presented. Four masks with different absorber and antireflection coating thicknesses, some with substrate and absorber programmed defects of different types and sizes, were fabricated and used to expose resistcoated SiN substrate wafers on full field ASML EUV scanners.

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U. Okoroanyanwu, J. Heumann, X. Zhu, C. H. Clifford, F. Jiang, P. Mangat, R. Ghaskadavi, E. Mohn, R. Moses, O. Wood, H. Rolff, T. Schedel, R. Cantrell, P. Nesladek, N. LiCausi, X. Cai, W. Taylor, J. Schefske, M. Bender, and N. Schmidt "Towards the optical inspection sensitivity optimization of EUV masks and EUVL-exposed wafers", Proc. SPIE 8352, 28th European Mask and Lithography Conference, 83520V (16 April 2012); https://doi.org/10.1117/12.923134

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