Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2019 / Paper 1 / Section A(ii) / 3 / ii / Solution

On a graph of normalized remaining thickness against ${\log }_{10}D$, both positive photoresist curves remain near unity below $D_0$ and fall to zero by $D_c$. Their transition widths are ${\log }_{10}(D_c/D_0)=1/\gamma$: $0.752$ decades for the lower-contrast material and $0.177$ decades for the higher-contrast material. The latter therefore has a much steeper transition. The curves can be aligned at a common illustrative clearing dose; their absolute horizontal placement cannot be inferred from photoresist contrast alone.

The higher-contrast material distinguishes bright and dark regions even when diffraction has reduced their intensity modulation. The lower-contrast material requires a much stronger aerial-image modulation and loses small features earlier. Finite resist thickness, standing waves, acid diffusion, development conditions and photon statistics can limit actual performance beyond this threshold model.