This paper describes the phase shift mask (PSM) effects in viewpoint of production using i-line lithography. We have understood that the difference of transmittance between shifter and non shifter on mask affect resolution, critical dimension, uniformity, and pattern profile and also the optimum process window may be determined by transmittance difference. For the PSM technology, it was also hard to control process because the process condition was in limitation of exposure tool. To fabricate the 256MB DRAM with 0.25 tm minimum feature size (MFS), we evaluated the PSM including attenuated type for conventional patterns and 0.25 tm cell array using positive and negative tone phase shift mask for actual process. Furthermore, we applied various approaches to get a sufficient depth of focus (DOF) and high resolution using i-line system with 0.57 NA, off-axis illumination system, low partial coherence factor, and process in cases of alternating, subresolution, and attenuated type of phase shift mask. As a result, even if pattern delineation was possible, we should optimize design, topology structure, and process to get enough DOF margin, good uniformity, and high repeatability for device fabrication.
We investigated the influence of lens aberration on the lithographic performance according to the phase error and topography effects of phase-shift mask (PSM). Twin-bar and isolated pattern showing high sensitivity to lens aberration were used for this study. The simulation of aberrated images was carried out using the Solid-CTM simulator. Specially, we quantified the relationship between patterning behaviors such as the isofocal tilt, the left-right (L-R) CD difference and the Z7 and Z9 individual Zernike coefficients. Isofocal tilt aberration sensitivity for Z9 was 0.4nm/nm, which resulted in 2nm CD variation using lens with 5nm Z9 value. When using the lens with 5nm Z7 value, the L-R CD difference and its sensitivity are 10nm and 2nm/nm, respectively. Finally, we evaluated the patterning performance by phase error effect, and determined the phase error criteria for PSM. The pattern placement error was increased by increasing phase error as well as Z7 value, while its slope to the defocus was similar regardless of lens aberration. However, it was found that the aberration sensitivity was not affected by phase error. The simulation predicted that the sensitivity of lens aberration could be increased due to mask topography effect. The nominal shift of phase edge attributed to mask topography was measured.
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