Key Challenges in EUV Mask Technology: Actinic Mask Inspection and Mask 3D Effects

This dissertation focuses on issues related to extreme ultraviolet (EUV) lithography mask/ntechnology: mask inspection and mask 3D eﬀects on imaging performance. Actinic (at-/nwavelength) mask inspection (both blank and patterned mask) is of critical concern for/nEUV lithography. In this dissertation, systematic studies exploring the optimal optical/nsystem design to improve the defect detection sensitivity for both actinic mask blank and/npatterned mask inspection tools using EUV light are presented. For EUV mask blank in-/nspection, a complete discussion is conducted to compare the conventional bright ﬁeld method/nand the Zernike phase contrast method on their phase defect detection sensitivity by thin/nmask simulations and experiments using the SHARP EUV microscope at Lawrence Berke-/nley National Laboratory (LBNL). The study shows that higher defect detection sensitivity/nand in-focus inspection capability can be achieved by the Zernike phase contrast method,/nwhile the conventional bright ﬁeld method needs through-focus scanning and results in lower/ndefect detection sensitivity. Experimental results show that a programmed defect as small/nas 0.35 nm in height is detected at best focus with a signal-to-noise ratio (SNR) ≈ 8 by/nthe Zernike phase contrast method. With the considerations of various noise sources and/nsystem design, the thin mask simulation results show that the dark ﬁeld method has better/ndetection eﬃciency in inspection mode, while the Zernike phase contrast method is better in/nreview mode (pixel size ≤ 25 nm). Further, the impact of pixel size, EUV source type, and/nphoton collection eﬃciency for a dark ﬁeld based actinic blank inspection tool is discussed/nby thin mask simulation. The simulation results show the complex correlation between each/nparameter on defect inspection eﬃciency and also show that 10-watt EUV source power and/n100 nm pixel size are needed to capture a phase defect of height 0.5 nm./nFor EUV patterned mask inspection, the possibility of using the optimum phase shift in/nthe pupil plane to improve inspection eﬃciency is discussed using a thin mask model. Then/nthe nature of the EUV mask pattern defect is analyzed by its near ﬁeld distribution using/na thick mask model. The simulation results indicate that, as a result of 3D eﬀects leading/nto phase artifacts, pattern defects cannot be simply treated as ideal absorber defects. The/nresults can aﬀect the choice of optimal patterned mask inspection tool design. Moreover, a/nstudy of a bright ﬁeld based EUV actinic pattern inspection tool design using a hybrid (2D/n+ 3D) model is presented, showing that the impact of noise sources and optical design on/ncritical pattern defects detection sensitivity. The study shows that introducing a − 50 nm/ndefocus into the inspection system can improve the SNR by 50%./nThe impact of EUV sub-resolution assist feature (SRAF) on mitigation of mask 3D eﬀects/nis discussed by rigorous 3D modeling. The simulation results show that introducing SRAFs/nin the mask design induces even stronger eﬀective single pole aberration into the imaging/nsystem to balance the Bossung curve. Asymmetric SRAFs pattern placement can achieve/na 21% improvement of the process window. Moreover, the complex interaction between the/nmain feature and the SRAFs is analyzed by systematic position sensitivity studies. Bossung/ntilt sensitivity with respect to the relative positions between main feature and SRAFs is/nshown, which indicates that diﬀerent location precision requirements are needed for SRAFs/nduring the mask-making process.

Advisor: Neureuther, Andrew