Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2017 / Paper 1 / Section A(ii) / 4 / ii

In the intended additive-deposition model, A originally contributes $0.35\times 2.2=0.77\mathrm{nm}{\mathrm{s}}^{-1}$. At equal incorporated contributions, B also supplies $0.77\mathrm{nm}{\mathrm{s}}^{-1}$, giving $1.54\mathrm{nm}{\mathrm{s}}^{-1}$. Equivalently, the new total incorporated flux is $0.35+0.35=0.70$ times the calibrated total.

There is a composition-convention qualification. If the percentages represent mass flux and the mass density stays fixed, the preceding thickness calculation follows directly. If they instead represent atomic fractions, a constant atomic number density would justify the same result, whereas constant mass density alone does not. With atomic masses $M_A,M_B$ and constant mass density, the ratio would beThe PDF does not specify atomic versus mass percentages or the two elements. Thus 1.54 nm/s is the intended common flux-to-thickness approximation, and the formula above states the additional dependence if atomic fraction and constant mass density are meant literally.