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Most IC manufacturers are considering MoSi to be the material of conventional 6% attenuating phase-shifting masks (attPSM) in hyper-NA lithography (50 nm half pitch node and smaller). However, simulation results show that Cr-based binary-intensity mask (BIM) outperforms the attPSM at dense lines and spaces (LS) patterns in hyper-NA lithography. A reason lies in the transmitted polarization state through the mask. The attPSM is found to be a transverse-magnetic polarizer for hyper-NA imaging, while the BIM acts as a transverse-electric polarizer, which is beneficial for imaging. Using a metal-based absorber of the attPSM has potential for improving the degree of polarization of transmitted light. In our previous work absorber thickness of bi-layer attPSM, i.e. Ta/SiO2, was optimized through three-dimensional
electromagnetic field (3D-EMF) simulations for better imaging performance than the MoSi attPSM.
In this study, the thickness-optimized Ta/SiO2 attPSM was fabricated to compare the imaging performance with the standard Ta/SiO2 and MoSi attPSMs with 6% transmission and 180o phase shift. The thickness-optimized Ta/SiO2 attPSM has 1% transmission due to 50% thicker Ta than the standard, while the 180o phase shift is controlled by SiO2 thickness. The exposure latitude of 45 nm LS delineated by using an NA1.20 full-field scanner with xy-polarized cquadrupole was 15.7%, 13.4%, and 10.1% with depth of focus of 200 nm for the optimized Ta/SiO2, the standard Ta/SiO2, and MoSi, respectively. Line width roughness of the Ta/SiO2 attPSMs was approximately 5.5 nm for the 45 nm
LS, which was comparable to MoSi. Mask-error-enhancement factor (MEEF) of the 45 nm LS was 4.4, 4.9, and 3.8 for the optimized Ta/SiO2, the standard Ta/SiO2, and MoSi, though the simulation expected MEEF values of 4.1, 5.5, and 6.3, respectively. Because the transmission and the phase shift measured by normal incidence are not linked directly with the imaging performance in the hyper-NA lithography with off-axis illumination, the mask materials and structures need to be optimized by using 3D-EMF simulators for the better imaging quality.
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