Past exam of the MPhil in micro- and nanotechnology enterprise course of the University of Cambridge / 2023 / Paper 1 / Section A(ii) / 4 / ii

At fixed reactant partial pressures and temperature, their ideal-gas bulk concentrations $C_i=p_i/(RT)$ remain approximately fixed. Lowering total pressure means removing diluent rather than automatically lowering the reactant concentration.

In the continuum molecular-diffusion regime, gas diffusivities scale approximately as $D\propto 1/P_{\mathrm{total}}$. Delivery through a comparable boundary layer is therefore faster, increasing $h_m$ relative to $k_s$. The system tends toward reaction-limited operation, where concentration differences between substrate locations are smaller. This commonly improves thickness uniformity and access to recessed surfaces.

The qualification is important: changing pressure can also change flow velocity and the boundary layer, gas-phase chemistry and precursor depletion. In narrow structures at low pressure, Knudsen transport limits the simple $1/P$ improvement. Improved transport does not correct a large substrate-temperature gradient in a strongly temperature-sensitive reaction-limited process.