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Birefringence Paramters

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Birefringence of polarized light in crystalline materials

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The move to stepper or scanner systems using a 157nm wavelength of light was dealt a setback when unexpected problems with the lens materials were found. Since traditional amorphous lens materials like glass (fused silica) have very high absorption at Deep Ultra Violet (DUV) wavelengths, new crystalline materials like Calcium Fluoride (CaF2) were expected to take over. Unfortunately in 2001, CaF2 was reported by the National Institute of Standards and Technology to have intrinsic birefringence that was 10X higher than acceptable for high Numerial Aperture lenses. With birefringence, a material has two indexes of refraction so that light waves with vertical and horizontal polarization or with left and right circular polarization travel at different speeds through the material. Since lithography requires tight phase control of light, lens that exhibit birefringense are not acceptable.

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Amorphous materials, like glass, don’t exhibit birefringence since the index of refraction is the same for all types of polarized light. But crystalline materials like Calcium Fluoride CaF2 and Barium Fluoride BaF2 have a property where left circular polarized (LCP) travels slightly slower through the material than right circular polarized (RCP) light which gives rise to two indexes of refraction in the material called birefringence.

Figure 2 shows a material with birefringence. When the light waves enter the lens, the two polarization waves are exactly in phase but when they leave the lens, one wave has lagged the other wave by 26O. Since lithography requires the tight phase control of light during exposure, a lens with 5% - 10% bifringence parameter was unusable. The magnitured of the birefringence problem was largely unexpected since it was not experience at higher wavelengths as shown in Figure 1.

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