This paper describes our initial investigation into building a greater understanding of the complex mechanism occurring during extreme ultraviolet (EUV) exposure of resist materials. In particular, we are focusing on the number and energy of photoelectrons generated and available for reaction with photoacid generators (PAGs). We propose that this approach will best enable the industry to develop resists capable of meeting resolution, line width roughness (LWR), and sensitivity requirements.
The ability to predict the rate of reflectivity loss of capped multilayer mirrors (MLMs) under various conditions of
ambient vacuum composition, intensity, and previous dose is crucial to solving the mirror lifetime problem in an EUV
stepper. Previous measurements at NIST have shown that reflectivity loss of MLMs exposed under accelerated
conditions of dose and pressure can be a very complicated function of these variables. The present work continues this
effort and demonstrates that reflectivity loss does not scale linearly for accelerated exposure doses over the range of
0-350 J/mm2 either for partial pressures of MMA in the range 10-8-10-7 Torr or acetone in the range 10-7-10-6 Torr. We
suggest that this nonlinear scaling may be the result of a varying damage rate as the surface of the growing
contamination layer moves through the EUV standing wave created by exposure of any MLM to resonant radiation. To
further investigate the potential influence of these resonance effects, we report new measurements showing large
variations of the secondary electron yield as a function of thickness of carbon deposited on top of a MLM.
Metallic ruthenium capping layers ~2 nm thick protect and extend the lifetimes of Mo/Si multilayer mirrors
used in extreme ultraviolet lithography (EUVL) applications. However, Ru-capped mirrors experience a loss of
reflectivity after prolonged exposure to EUV radiation. In the present work, we use ultrahigh vacuum surface science
methods to address several aspects of Ru surface chemistry that may impact on Ru capping layer stability and mitigation
processes. (1) We characterize the composition and stability of Ru surfaces that simulate surfaces of Ru-capped
multilayer mirrors, under exposure to different background gases (water, methyl methacrylate (MMA)) and to electron
irradiation. Evidence for some mitigation of carbon accumulation during electron bombardment in MMA + water vapor
is found. (2) We report the photon-energy dependence of secondary electron yield (SEY) measurements for clean Ru,
O-dosed and C-dosed Ru, and Ru-capped multilayer mirrors using synchrotron radiation near 13.5 nm at Brookhaven
National Synchrotron Light Source (NSLS). Much of the radiation-induced chemistry on the surfaces of capping layers
is induced by low-energy secondary electrons rather than direct photoexcitation, so the SEY is an important parameter
affecting mirror lifetimes in EUVL.
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