= Solution
Increasing the angular acceptance can collect higher spatial frequencies, but rays at large angles and away from the optical axis are harder to bring to the same focus. <Spherical aberration>, coma, astigmatism and field curvature can degrade contrast across a large exposure field. The <depth of focus> also decreases approximately as $\lambda/\mathrm{NA}^2$, making wafer height errors more important. A larger geometrical aperture is therefore not automatically better over the whole wafer field.
\b[<Immersion lithography> raises $\mathrm{NA}=n\sin\theta$ by increasing the refractive index of the medium between the final lens and wafer.] A suitable UV-transparent liquid permits a larger numerical aperture without increasing the ray angle by the same factor, easing the purely geometrical demand on the lens. Lens design and focus control remain necessary; immersion does not abolish aberrations. The liquid must also be compatible with the resist and free of bubbles and contamination, and absorption at $193\,\mathrm{nm}$ must be low.
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