18 August 2005 Remaining challenges in microlithographic optical design
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Rayleigh scaling equations for resolution and the control of computer chip critical dimensions (CD) within a finite depth of focus (DOF) have always indicated that resolution is better improved by reductions in wavelength of exposure light rather than by increasing the numerical aperture (NA) of the projection optics, particularly as it approaches the physical limit in air of 1.0. However, liquid immersion of the image increases the physical NA limits and presents new optical design challenges, while postponing the necessity for drastic reductions in the wavelength.
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David M. Williamson, "Remaining challenges in microlithographic optical design", Proc. SPIE 5874, Current Developments in Lens Design and Optical Engineering VI, 58740N (18 August 2005); doi: 10.1117/12.620615; https://doi.org/10.1117/12.620615

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