Inspecting EUV mask blanks with a 193-nm system Stan Stokowski*a, Joshua Glasser a, Gregg Inderhees a, Phani Sankuratri b a KLA-Tencor Corp., One Technology Drive, Milpitas, CA 95035, USA; bPhani Sankuratri ,46958 Masonic Terrace, Fremont, CA 94539 Abstract Data and simulation results characterizing the capability of a DUV system to inspect EUV mask blanks and substrates are reported. Phase defects and particles on multilayer (ML) surfaces, ARC-coated absorber, and substrate material are considered. In addition to the previously reported results of inspecting phase defects on multilayer surfaces, phase defects on a quartz substrate surface are shown. The principle of phase detection is described. Simulations show that the 22-nm node requirement for phase defect detection should be met, assuming a reduction in the multilayer roughness. Initial inspections of deposited SiO2 spheres show sensitivities of at least 40 nm on ML and quartz; however, the availability of calibrated spheres of smaller diameters has limited testing below this value. Simulation results show relative sensitivities for detecting SiO2 spheres of different diameters on various EUV materials. Keywords: EUV blank inspection, phase defects, particles, DUV inspection, EUV multilayer roughness
1. INTRODUCTION EUV mask blank development requires inspection and metrology equipment to be available now. The industry desires to have an actinic (13.5 nm) inspection tool; however, it will be some number of years before a production-worthy tool is available at this wavelength. In the meantime we must use DUV mask inspection systems to enable rapid development of EUV mask blanks with low numbers of defects. At the 2009 International Symposium on Extreme Ultraviolet Lithography in Prague, Czech Republic, 18-21 October 2009 we reported1 on using a 193-nm system (TeronTM 600) to detect phase defects on multilayer-coated (ML) EUV blanks. We compared our DUV images with those from the actinic Advanced Inspection Tool (AIT) at Lawrence Berkeley National Laboratory (LBNL) and showed substantial similarity of the through-focus imagery between the two tools. Measuring signal and noise levels on the Teron 600 allowed us to determine the signal-to-noise ratios (SNR) of bumps and pits. International SEMATECH provided defect sizes measured with an atomic force microscope (AFM). Comparing the measured signal levels with those calculated by a thin-mask simulation showed good agreement. From these data we estimated that we should be able to detect a 1 nm high x 80 nm FWHM defect with >99% capture probability and 1-10 false defects/mask (142 x 142 mm²) with the Teron 600. Our goal now is to improve the sensitivity so as to detect 1 nm x 50 nm phase defects in future tools. We also reported at the Prague conference that exposure to 193-nm light equivalent to 20-50 inspection scans showed no measurable reflectivity change on Ru-capped or Si-capped ML, either at 193 nm or 13.5 nm, and no ML structural change as measured by X-ray reflectivity (XRR).2 Particle detection at various stages of EUV mask manufacture is also important. Particle detection does require a somewhat different approach than that for phase defects because they, in general, are objects with a combination of phase and amplitude contrast, whereas shallow bumps and pits are almost pure phase objects. In Section 2 we present data on an inspection of a programmed-defect mask, estimates of future sensitivities from simulations, and the principle underlying phase object detection. We also demonstrate that we can inspect quartz (and presumably LTEM) substrates. In Section 3 we discuss preliminary inspections of SiO2 spheres on ML and quartz, commenting on the necessity of obtaining calibrated sphere sizes <40 nm diameter. Using an existing dark-field scattering program we show predicted relative sensitivities for particle detection on ML, ML with an absorber and a 257nm anti-reflection coating (ARC), and quartz.
*stan.stokowski@kla-tencor.com
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Extreme Ultraviolet (EUV) Lithography, edited by Bruno M. La Fontaine, Proc. of SPIE Vol. 7636, 76360Z · © 2010 SPIE · CCC code: 0277-786X/10/$18 · doi: 10.1117/12.850825
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