12 March 2008 Uniaxial crystal last optical element for second- and third-generation immersion lithography
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In this paper, we show that uniaxial crystals and sapphire in particular, can be used as a lens material for high-index immersion lithography. Although uniaxial crystals are birefringent and anisotropic for unpolarized light, under certain conditions they are fully isotropic for transverse electric (TE) polarizations. The strong birefringence of the uniaxial crystal defocuses the residual TM polarization relative to the TE mode so that its image does not reach the photoresist but rather creates a small background. To manifest this property, a combination of pupil-fill polarization constraints, mask design rules and crystal alignment requirements must be satisfied. Sapphire is a well-known industrial optical material featuring high optical quality and homogeneity. Its refractive index of 1.928 at 193 nm, along with its high thermal conductivity and moderate absorption, are close to industry requirements. An implementation of sapphire under these conditions is proposed, suggesting a path to the continued scaling of 193 nm lithography numerical aperture.
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Gabriel Y. Sirat, Gabriel Y. Sirat, Michael Goldstein, Michael Goldstein, } "Uniaxial crystal last optical element for second- and third-generation immersion lithography", Proc. SPIE 6924, Optical Microlithography XXI, 69242T (12 March 2008); doi: 10.1117/12.790828; https://doi.org/10.1117/12.790828

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