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Journal of Micro/Nanopatterning, Materials, and Metrology
VOL. 23 · NO. 2 | April 2024
ISSUES IN PROGRESS
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The original inverse lithography technology (ILT) commitment of creating a free-form mask from ex nihilo underwent transformation under the pressure of constraints that are imposed by the capabilities of manufacturing and inspection equipment. This pressure became especially noticeable in dealing with sub-resolution assist features (SRAF). The free-form SRAFs are naturally unmanufacturable and hard to inspect. In addition to this complication, the ILT signal for SRAFs is substantially weaker and often ill-defined in comparison to a strong signal from fidelity objectives to print main features on target. So, it is difficult to automatically produce SRAFs that are simultaneously geometrically stable, do not print, MRC-clean, and deliver the best lithographic quality.
Aim
To overcome these obstacles, we propose new concept of structured SRAFs by combining ILT ideas with robust geometrical parameterization to introduce well-behaved and MRC-clean by construction mask decorations.
Approach
We implemented insertion of structured assist features within full-chip calibre ILT framework to probe for runtime, lithographical quality, and MRC compliance.
Results
Runtime is estimated to be 2.3 times faster than the native ILT SRAFs, without the necessity of using post-ILT MRC cleanup. The lithographic quality, as quantified by the process variability bands, was comparable to the native ILT solution (19% difference) and was 43% better than the rule-based Manhattan SRAFs (1.22 nm versus 1.75 nm).
Conclusions
We proposed structured assist features by combining ILT signal and vector field ideas to introduce well-behaved and MRC-clean by construction mask decorations. This technique compares favorably to other SRAF methods by consistency, lithographic quality, and runtime metrics.
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Markus Greul, Astrit Shoshi, Jan Klikovits, Stephan Martens, Ulrich Hofmann, Olga Barahona, Benyamin Shnirman, Leon Starz, Patrick Wintrich, Holger Sailer
A critical factor in the fabrication of complex nano- and microstructures with high quality and reproducibility is the determination of a suitable working point. This applies particularly to lithography, which is the basis for transferring the desired patterns onto the substrate. For this reason, we present a generic process optimization methodology that has been successfully applied to four chemically amplified positive and negative tone electron beam lithography photoresists with different sensitivities. The method is iterative and designed for the best possible results with a minimum use of resources. This is accomplished by identifying the critical key factors in photoresist processing using contrast curves and determining their impact. Starting with the most influential bake parameter, the maximum effect is achieved. The method used is similar to the Bossung plot procedure and aims for a maximum process window. After the bake parameters, the fundamentals of development kinetics are discussed, and a method for determining an appropriate development time is presented. A mask making approach is then used to investigate the ideal exposure conditions. This includes the determination of an appropriate base dose in conjunction with proximity effect correction and sizing. The evaluation of this method is demonstrated by critical dimension linearity plots and scanning electron microscope cross sectional analysis of resist profiles. The results presented demonstrate the universality of the optimization approach.
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