Depth of focus enhancement in high-NA curvilinear logic metal designs

Abstract

Curvilinear EUV masks have shown significant potential in enhancing Power-Performance-Area-Cost (PPAC) metric, the process window, and Edge Placement Error (EPE) in EUV lithography. Curvilinear routing in standard cell design relaxes the Tip-To-Line (T2L) spacing while maintaining cell size.1 However, curvilinear designs suffer from Depth of Focus (DoF) degradation due to wavefront deformation in EUV systems, especially in high-NA anamorphic system. This work aims to improve the DoF in curvilinear designs in logic metal layers via Source Mask Optimization (SMO) and wavefront correction. We analyze the reflected wavefront from curvilinear and rectilinear designs in both isomorphic NA 0.33 and anamorphic NA 0.55 and compare the deformation. Curvilinear design suffers from wavefront deformation in anamorphic NA 0.55 system more than its rectilinear counterpart. By employing wavefront optimization in the source optimization, we improve the DoF of the curvilinear design compared to standard source without wavefront correction. Our findings indicate that curvilinear design improves overall image fidelity and image contrast. Wavefront optimization increases DoF in curvilinear routing. Additionally, curvilinear routing combined with wavefront optimization allows for smaller T2L spacing, facilitating further reduction in the standard cell size. Phase Mask Wavefront Optimization (PMWO) effectively corrects aberration-like effects and enhances the DoF in both curvilinear and rectilinear designs. Curvilinear routing, when paired with a wavefront-optimized source, achieves lower dose printing with improved Local Critical Dimension Uniformity (LCDU) across the process window, attributed to its higher image contrast compared to rectilinear routing. Experimental SEM images further demonstrate that curvilinear designs exhibit fewer defects at the same dose, which can be attributed to their superior contrast and fidelity. Additionally, the use of a wavefront-optimized source enables tighter T2L spacing, allowing for further reduction in standard cell size while maintaining or improving DoF and Exposure Latitude (EL) compared to nominal rectilinear design.