Rayleigh criterion: definition

Beginner

A rule for the smallest detail an optical system can print or see.

Novice

A rule for when two points of light can just be told apart: the bright center of one falls on the first dark ring around the other. For a lens it gives a smallest resolvable distance of $0.61\,\lambda/\mathrm{NA}$ . Chipmakers write it as $\mathrm{CD} = k_1 \lambda / \mathrm{NA}$ , where CD (critical dimension) is the width of the smallest feature.

Expert

$k_1$ bundles everything except wavelength and NA: the illumination shape, mask type, resist and optical proximity correction (pre-distorting mask shapes so they print as intended). Its physical floor is 0.25 for a dense pattern of lines and spaces, so a single exposure can’t print a pitch below $0.5\,\lambda/\mathrm{NA}$ . Anything finer needs multiple patterning or a new tool.