Lithographic performance and SEM-based geometrical analysis of curvilinear photomasks

Abstract

Curvilinear photomasks are attracting significant attention as a potential enabler for improved lithographic performance in advanced semiconductor manufacturing. While their benefits have been widely discussed, a systematic understanding of their fundamental imaging behavior remains incomplete. In this study, we investigate the lithographic characteristics of simple curvilinear geometries by comparing circular and square contact patterns under controlled conditions. Lithographic performance is evaluated using aerial image simulations with a 193‑nm ArF water immersion system (NA = 1.35) and annular illumination. Ideal design data and real mask geometries extracted from mask SEM contours are both analyzed to clarify the contributions of mask shape and fabrication-induced effects. Key metrics including normalized image log slope (NILS), mask error enhancement factor (MEEF), defocus margin, and edge placement error (EPE) are evaluated. The results show that circular features provide comparable contrast and NILS to square features, while exhibiting marginally reduced MEEF. Under negative-tone imaging conditions, defocus margins are significantly improved due to enhanced phase effects, particularly when combined with high-transmittance attenuated phase-shift masks. Furthermore, curvilinear patterns demonstrate improved EPE stability at intermediate pitches, where higher diffraction orders begin to influence the printed image. These findings provide quantitative insights into the fundamental behavior of curvilinear features and establish a foundation for future optimization of curvilinear mask design and manufacturing.