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28 May 2004 Scattering in liquid immersion lithography
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We have measured the intrinsic scattering of water with an eye toward its potential impact on immersion lithography. Quantitative measurements of the elastic Rayleigh scatter agree well with theory and show a loss of 0.001 cm-1. Qualitative measurements of the inelastic Raman scattering show a strong peak at 206 nm, consistent with the O-H stretch present in water. Both are expected to contribute flare of < 10-6 of the incident intensity. We have also examined the possibility for bubbles in the immersion liquid, and in particular those which form near the resist surface. We have measured scattering from single bubbles and estimate that bubbles as small as 5 μm should be detectable in this fashion. In addition, we have measured the potential for bubbles due to laser induced resist outgassing by direct imaging. In 2500 resist images (~235 mm2 of surface), we have seen only one bubble candidate which, due to its persistence in the water, we do not believe represents a true outgassing-induced bubble. Finally, using a technique borrowed from biology, rapid cryofixation/freeze fracture, we have examined nanobubbles which form spontaneously on hydrophobic surfaces and found that degassing the water prevents their formation.
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Michael Switkes, Theodore M. Bloomstein, Roderick R. Kunz, Mordechai Rothschild, J. W. Ruberti, T. A. Shedd, and Michael S. Yeung "Scattering in liquid immersion lithography", Proc. SPIE 5377, Optical Microlithography XVII, (28 May 2004); doi: 10.1117/12.534194;

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