11 March 2002 Rigorous 3D simulation of phase defects in alternating phase-shifting masks
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A study of both the printability and inspectability of phase defects in alternating phase-shifting masks is made using rigorous FDTD simulation of mask topography and vector-based models for optical imaging that account for the angular dependence of the mask scattering coefficients and high numerical aperture effects. Aerial images inside the photoresist are calculated through focus under typical 193nm stepper imaging conditions in order to assess the effects of size, phase, location and shape on the printability of phase bump defects. Defects located adjacent to the vertical sidewalls of the phase shifter are found to affect CD more than defects located in the center of the phase shifter. Defect shape does not appear to be a strong factor in printability. Aerial images and difference signals for various defects under mask transmission-imaging inspection conditions are calculated. A study of illumination pupil effects in mask inspection is presented. Defect visibility when varying defect size and position is investigated. Defects close to the phase shifter vertical sidewall are less visible. Defects are observed to be more visible when the mask is defocused. An investigation of illumination effects show that coherent systems can see defects better than incoherent systems, while certain incident plane waves are found to 'see' a particular defect better than others.
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Thomas V. Pistor, "Rigorous 3D simulation of phase defects in alternating phase-shifting masks", Proc. SPIE 4562, 21st Annual BACUS Symposium on Photomask Technology, (11 March 2002); doi: 10.1117/12.458268; https://doi.org/10.1117/12.458268

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