Pathfinding the perfect EUV mask (EUV)
Internally funded project
Acronym: EUV
Start date : 01.09.2021
Project details
Short description
Semiconductor industry is pushing for a smaller gate size on the chip. EUV is already used in high-volume manufacturing and delivers resolution of 13 nm lines and spaces with NA 0.33 system. The high-NA of 0.55 will be used in the high-volume manufacturing by 2023. The high-NA system has a resolution of 8 nm lines and spaces. High-NA system features an anamorphic demagnification of 4× in y-direction and 8× x-direction instead of 4× in both directions in NA of 0.33. The combination of smaller features to print and the anamorphic demagnification makes the system more sensitive to variations in the mask design and to optical constants. This work explores the effect of the optical constants’ variations in the mask absorber materials and different mask components’ effects.
This work aims to investigate the effect of the mask in high-NA EUVL (extreme-ultraviolet lithography) on the resulting image quality, which to be printed on the wafer for producing ICs (integrated circuits) and chips. The mask in EUVL contains two main parts; an absorber and a reflective multilayer that works as a Bragg mirror. The effect of both parts and the interaction between them are the core of this thesis.
Scientific Abstract
Context: The push to miniaturize transistors on the chips has been a direct measure of success for the semiconductor industry since the late 1970s. Smaller transistors have lower energy consumption and provide the option to either increase the transistor count on the same chip size or to reduce the chip size. This progress follows Moore’s Law, which states that the number of transistors on a chip doubles every 18 months. The latest technology in optical lithography is the EUVL (extreme-ultraviolet lithography).
This work focuses on the EUV wavelength of 13.5 nm with high-NA of 0.55. This system is to be used in high scale manufacturing by end of 2023 as planned by ASML for EXE-5200 tool.
Aim: The focus of this thesis is the mask effects on the resulting aerial images. As the printed features gets smaller approaching the limits of the system, the effects of the optical constants of the mask on the image increases. Increased sensitivity in imaging metrics is observed for low refractive indices (n) and extinction coefficients (k).
Approach: The governing physical concepts of light propagation inside the mask absorber are investigated. The waveguiding effect of the mask absorber is proven and its consequences shown. The reflective multilayer reflectivity curve is divided into regions and the effect of the regions on the aerial image. A genetic optimization algorithm is used to find the optimal multilayer material and duty ratio between its constructing materials in terms of imaging metrics. The effects of the mask absorber and multilayer are separated via the hybrid mask model. The interaction between the absorber and the multilayer is important to reach the optimal EUV mask.
Results: Waveguiding effect governs the light propagation inside the mask absorber. The role of the excited waveguide modes in the mask absorber opening is explained. The coupling between diffraction orders due to excited perpendicular waveguide modes causes a drop in image contrast. The effect of refractive index and extinction coefficient in mitigating the coupling effect is detailed. Higher extinction coefficient is favorable for imaging as it suppresses the coupling effect and reduces the image shift between single pole images. The reflective multilayer impacts the imaging performance. widening the bandwidth of the reflective multilayer is not the optimal solution for the image contrast. The effective reflective plane inside the reflective multilayer changes the mask 3D (M3D) effects in the image. RuSi multilayers exhibit lower M3D effects compared with MoSi counterparts. The hybrid mask model is used to correlate the imaging metrics variations with mask components and optical constants. The hybrid model is used to explain the effects of the double diffraction phenomenon in EUVL. The effects of the multilayer are explored. The sensitivity in low refractive index and low extinction materials is shown. The usage of transmission and phase of a mask absorber is proven to be misleading, however the usage of n, k, and thickness of the absorber is more accurate. Same transmission and phase absorber can behave differently according to the corresponding optical constants and thicknesses.
Conclusion: Optical constants (n and k) is more important than phase and transmission of the mask absorber in finding the perfect EUV mask. Waveguiding effect causes a coupling between the diffraction orders, which causes a drop in the image intensity. Optimizing the reflective multilayer require imaging metrics as objectives instead of traditionally used reflectivity bandwidth maximizing objectives. The effective reflective plane inside the multilayer plays a major role in mitigating the telecentric errors in the image. Identifying the absorber using phase and transmission is inaccurate and should be avoided. Dark-field (lines) images have less sensitivity to variation in bias and optical constants compared to bright field (spaces) images.
Involved:
Andreas Erdmann
Project Leader
Jörg Schulze
Project Leader
Hazem Mohamed Safwat Mesilhy
Project Member
