Numerical aperture (NA): definition

Beginner

A number that says how wide a cone of light a lens can gather. A bigger number means sharper detail.

Novice

A measure of how wide a cone of light a lens collects: $\mathrm{NA} = n \sin\alpha$ , where $\alpha$ is the half-angle of the cone and $n$ is the refractive index of whatever fills the gap between lens and wafer (1 for air or vacuum). Filling the gap with water lets NA go above 1.

Expert

Resolution improves as $1/\mathrm{NA}$ , but depth of focus shrinks as $1/\mathrm{NA}^2$ . Doubling NA therefore halves the smallest printable feature but cuts the focus window to a quarter, which demands flatter wafers and thinner resist.