Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2018 / Paper 1 / Section B / 8 / i

A useful scale estimate is $d\sim k_1\lambda /\mathrm{NA}$, where $\mathrm{NA}=n\sin \alpha$. Five complementary developments are immersion optics, illumination shaping, phase-shifting masks, computational proximity correction and multiple patterning. They address different parts of the process rather than repeal diffraction.

Immersion lithography raises numerical aperture by putting a compatible high-index liquid between the final lens and wafer. Off-axis, annular or dipole illumination places useful diffraction orders within the projection pupil and improves contrast for selected pattern directions.

A phase-shift mask uses controlled optical phase differences to sharpen destructive interference around selected features. Optical proximity correction modifies mask edges and adds assist features to compensate pattern-dependent image formation, resist and etch biases.

Multiple patterning divides a dense final layout into more widely spaced optical patterns, or uses spacer-based pitch division after an optical definition step. Overlay, etch control and pattern decomposition are then critical. For example, with $\lambda =193\mathrm{nm}$, $\mathrm{NA}=1.35$ and $k_1=0.25$, a single-exposure scale is roughly $36\mathrm{nm}$; obtaining dense features of order $20\mathrm{nm}$ commonly requires pitch multiplication rather than attributing everything to one exceptionally sharp aerial image.

These resolution enhancements also require suitable resist contrast, focus control and metrology. The manufacturer's description explains the numerical-aperture benefit of immersion; no claim about a particular 2018 production process is inferred from its current product pages.^[ref]