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

Field: Materials Science (Physical Sciences)

This dissertation focuses on issues related to extreme ultraviolet (EUV) lithography masktechnology: mask inspection and mask 3D eï¬ects on imaging performance. Actinic (at-wavelength) mask inspection (both blank and patterned mask) is of critical concern forEUV lithography. In this dissertation, systematic studies exploring the optimal opticalsystem design to improve the defect detection sensitivity for both actinic mask blank andpatterned mask inspection tools using EUV light are presented. For EUV mask blank in-spection, a complete discussion is conducted to compare the conventional bright ï¬eld methodand the Zernike phase contrast method on their phase defect detection sensitivity by thinmask simulations and experiments using the SHARP EUV microscope at Lawrence Berke-ley National Laboratory (LBNL). The study shows that higher defect detection sensitivityand in-focus inspection capability can be achieved by the Zernike phase contrast method,while the conventional bright ï¬eld method needs through-focus scanning and results in lowerdefect detection sensitivity. Experimental results show that a programmed defect as smallas 0.35 nm in height is detected at best focus with a signal-to-noise ratio (SNR) â 8 bythe Zernike phase contrast method. With the considerations of various noise sources andsystem design, the thin mask simulation results show that the dark ï¬eld method has betterdetection eï¬ciency in inspection mode, while the Zernike phase contrast method is better inreview mode (pixel size â¤ 25 nm). Further, the impact of pixel size, EUV source type, andphoton collection eï¬ciency for a dark ï¬eld based actinic blank inspection tool is discussedby thin mask simulation. The simulation results show the complex correlation between eachparameter on defect inspection eï¬ciency and also show that 10-watt EUV source power and100 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 inthe pupil plane to improve inspection eï¬ciency is discussed using a thin mask model. Thenthe nature of the EUV mask pattern defect is analyzed by its near ï¬eld distribution usinga thick mask model. The simulation results indicate that, as a result of 3D eï¬ects leadingto phase artifacts, pattern defects cannot be simply treated as ideal absorber defects. Theresults can aï¬ect the choice of optimal patterned mask inspection tool design. Moreover, astudy of a bright ï¬eld 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 oncritical pattern defects detection sensitivity. The study shows that introducing a â 50 nmdefocus into the inspection system can improve the SNR by 50%.The impact of EUV sub-resolution assist feature (SRAF) on mitigation of mask 3D eï¬ectsis discussed by rigorous 3D modeling. The simulation results show that introducing SRAFsin the mask design induces even stronger eï¬ective single pole aberration into the imagingsystem to balance the Bossung curve. Asymmetric SRAFs pattern placement can achievea 21% improvement of the process window. Moreover, the complex interaction between themain feature and the SRAFs is analyzed by systematic position sensitivity studies. Bossungtilt sensitivity with respect to the relative positions between main feature and SRAFs isshown, which indicates that diï¬erent location precision requirements are needed for SRAFsduring the mask-making process.