Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2020 / Paper 1 / Section A(ii) / 3 / i / Solution

A high-pressure Mercury arc lamp is a conventional source for proximity photolithography. An electric discharge excites Mercury vapour; radiative transitions produce strong spectral lines, selected by filters and delivered through collimating optics. Common lines are $365\mathrm{nm}$ (i-line) and $436\mathrm{nm}$ (g-line). The finite mask-wafer gap gives near-field Fresnel diffraction, rather than the far-field approximation. Diffracted light spreads into nominally dark regions, limiting the minimum printable feature.