title: Pathfinding the perfect EUV mask
Creators: name: Mesilhy, Hazem Mohamed Safwat
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024
Subjects: subject: EUV, Lithography, Optical mask, optical lithography, EUVL

description: In the pursuit of more power-efficient electronics, the necessity for smaller gate sizes on Integrated Circuits (ICs) has become imperative. High-NA EUVL technology holds the promise of achieving an unprecedented 8 nm gate size. In high-NA EUVL operating at 13.5 nm wavelength, the interplay between the mask components becomes increasingly pivotal. This significance is further underscored by the imminent integration of the high-NA system into large-scale manufacturing by 2025, as envisioned by ASML for the EXE-5200 tool. This thesis delves into the nuanced effects of the lithographic mask on aerial images, especially as feature sizes approach the system's limits. Heightened sensitivity in imaging metrics is observed, particularly concerning the optical constants of the mask, with pronounced effects noted for low refractive indexes (n) and extinction coefficients (k). Mask materials with lower n and k exhibit heightened sensitivity to geometric variations and mask roughness, necessitating careful consideration in setting manufacturing tolerances. The study investigates fundamental concepts governing light propagation within the mask absorber, which serves as template to transfer the image using the lithographic projection system. Notably, the waveguiding effect within the mask absorber is rigorously explored, elucidating its consequential outcomes. The Bragg-like reflective multilayer's reflectivity curve is dissected into distinct regions, shedding light on their respective impacts on aerial images. Employing a genetic optimization algorithm, the thesis determines optimal multilayer materials and duty ratios between constructing materials based on imaging metrics. The hybrid mask model is instrumental in discerning the individual impacts of the mask absorber and multilayer, a crucial step toward optimizing EUV mask performance. Variations in absorber geometry, including height and sidewall angle, are scrutinized for their effects on aerial images. Various types of mask roughness are meticulously modeled, elucidating their consequences on aerial images. Moreover, the thesis studies the degradation of the multilayer due to heat accumulation, providing insightful models. The results of this research unravel several key phenomena. The waveguiding effect intricately governs light propagation within the mask absorber, with this work offering a comprehensive explanation of the role of excited waveguide modes within mask absorber openings. The coupling between diffraction orders, driven by excited perpendicular waveguide modes, induces a contrast drop in images. Notably, refractive index and extinction coefficient play vital roles in mitigating this coupling effect. Higher extinction coefficients prove advantageous, suppressing the coupling effect and reducing image shift between single pole images. The reflective multilayer significantly influences imaging performance. Contrary to prior literature, it was found that widening the multilayer's reflective bandwidth does not optimally enhance image contrast. The effective reflective plane within the multilayer profoundly influences mask 3D (M3D) effects in images, with RuSi (ruthenium - silicon) multilayers demonstrating lower M3D effects compared to their MoSi (molybdenum - silicon) counterparts. The hybrid mask model emerges as a powerful tool, correlating variations in imaging metrics with mask components and optical constants. It effectively elucidates the double diffraction phenomenon in EUVL, offering profound insights into the effects of the multilayer. The research underscores the heightened sensitivity of low refractive index and low extinction materials. Notably, the study challenges conventional wisdom, revealing that utilizing transmission and phase of a mask absorber can be misleading. Instead, employing n, k, and absorber thickness yields more accurate results. In conclusion, this research highlights the paramount importance of optical constants (n and k) in the quest for the perfect EUV mask. The waveguiding effect, leading to coupling between diffraction orders and subsequent contrast drop, is a critical consideration. Optimal multilayer design necessitates imaging metrics as objectives, a departure from traditional reflectivity bandwidth maximizing objectives. The effective reflective plane within the multilayer significantly mitigates telecentric errors in images. Additionally, the study emphasizes the inaccuracies associated with identifying the absorber using phase and transmission, advocating for a shift toward utilizing n, k, and absorber thickness. These nuanced insights advance the understanding of EUV lithography, paving the way for enhanced mask design and manufacturing processes.