This dissertation focuses on issues related to extreme ultraviolet (EUV) lithography mask technology: mask inspection and mask 3D eﬀects on imaging performance. Actinic (at-wavelength) mask inspection (both blank and patterned mask) is of critical concern for EUV lithography. In this dissertation, systematic studies exploring the optimal optical system design to improve the defect detection sensitivity for both actinic mask blank and patterned mask inspection tools using EUV light are presented. For EUV mask blank inspection, a complete discussion is conducted to compare the conventional bright ﬁeld method and the Zernike phase contrast method on their phase defect detection sensitivity by thin mask simulations and experiments using the SHARP EUV microscope at Lawrence Berkeley National Laboratory (LBNL). The study shows that higher defect detection sensitivity and in-focus inspection capability can be achieved by the Zernike phase contrast method, while the conventional bright ﬁeld method needs through-focus scanning and results in lower defect detection sensitivity. Experimental results show that a programmed defect as small as 0.35 nm in height is detected at best focus with a signal-to-noise ratio (SNR) ≈ 8 by the Zernike phase contrast method. With the considerations of various noise sources and system design, the thin mask simulation results show that the dark ﬁeld method has better detection eﬃciency in inspection mode, while the Zernike phase contrast method is better in review mode (pixel size ≤ 25 nm). Further, the impact of pixel size, EUV source type, and photon collection eﬃciency for a dark ﬁeld based actinic blank inspection tool is discussed by thin mask simulation. The simulation results show the complex correlation between each parameter on defect inspection eﬃciency and also show that 10-watt EUV source power and 100 nm pixel size are needed to capture a phase defect of height 0.5 nm.
For EUV patterned mask inspection, the possibility of using the optimum phase shift in the pupil plane to improve inspection eﬃciency is discussed using a thin mask model. Then the nature of the EUV mask pattern defect is analyzed by its near ﬁeld distribution using a thick mask model. The simulation results indicate that, as a result of 3D eﬀects leading to phase artifacts, pattern defects cannot be simply treated as ideal absorber defects. The results can aﬀect the choice of optimal patterned mask inspection tool design. Moreover, a study of a bright ﬁeld based EUV actinic pattern inspection tool design using a hybrid (2D + 3D) model is presented, showing that the impact of noise sources and optical design on critical pattern defects detection sensitivity. The study shows that introducing a − 50 nm defocus into the inspection system can improve the SNR by 50%.
The impact of EUV sub-resolution assist feature (SRAF) on mitigation of mask 3D eﬀects is discussed by rigorous 3D modeling. The simulation results show that introducing SRAFs in the mask design induces even stronger eﬀective single pole aberration into the imaging system to balance the Bossung curve. Asymmetric SRAFs pattern placement can achieve a 21% improvement of the process window. Moreover, the complex interaction between the main feature and the SRAFs is analyzed by systematic position sensitivity studies. Bossung tilt sensitivity with respect to the relative positions between main feature and SRAFs is shown, which indicates that diﬀerent location precision requirements are needed for SRAFs during the mask-making process.
Key Challenges in EUV Mask Technology: Actinic Mask Inspection and Mask 3D Effects
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