Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2021 / Paper 1 / 4 / iii / Solution

For a spatially coherent, on-axis source, the coherent spatial-frequency cutoff isA line-space grating must place at least its first diffracted orders within the pupil. Its limiting pitch is therefore $\lambda /\mathrm{NA}$, and its equal line/space half-pitch isThe corresponding full pitch is 143 nm. Explicitly distinguishing pitch from half-pitch avoids a factor-of-two ambiguity in the phrase “feature size.” This is an ideal optical cutoff estimate; finite resist contrast and process requirements can make the usable feature size larger.