10 May 2016 Error analysis of overlay compensation methodologies and proposed functional tolerances for EUV photomask flatness
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Due to the impact on image placement and overlay errors inherent in all reflective lithography systems, EUV reticles will need to adhere to flatness specifications below 10nm for 2018 production. These single value metrics are near impossible to meet using current tooling infrastructure (current state of the art reticles report P-V flatness ~60nm). In order to focus innovation on areas which lack capability for flatness compensation or correction, this paper redefines flatness metrics as being “correctable” vs. “non-correctable” based on the surface topography’s contributions to the final IP budget at wafer, as well as whether data driven corrections (write compensation or at scanner) are available for the reticle’s specific shape. To better understand and define the limitations of write compensation and scanner corrections, an error budget for processes contributing to these two methods is presented. Photomask flatness measurement tools are now targeting 6σ reproducibility <1nm (previous 3σ reproducibility ~3nm) in order to drive down error contributions and provide more accurate data for correction techniques. Taking advantage of the high order measurement capabilities of improved metrology tooling, as well as computational capabilities which enable fast measurements and analysis of sophisticated shapes, we propose a methodology for the industry to create functional tolerances focused on the flatness errors that are not correctable with compensation.
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Katherine Ballman, Katherine Ballman, Christopher Lee, Christopher Lee, Thomas Dunn, Thomas Dunn, Alexander Bean, Alexander Bean, } "Error analysis of overlay compensation methodologies and proposed functional tolerances for EUV photomask flatness", Proc. SPIE 9984, Photomask Japan 2016: XXIII Symposium on Photomask and Next-Generation Lithography Mask Technology, 99840S (10 May 2016); doi: 10.1117/12.2242282; https://doi.org/10.1117/12.2242282

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