  Area-selective atomic layer deposition is a key technology for modern
microelectronics as it eliminates alignment errors inherent to conventional
approaches by enabling material deposition only in specific areas. Typically,
the selectivity originates from surface modifications of the substrate that
allow or block precursor adsorption. The control of the deposition process
currently remains a major challenge as the selectivity of the no-growth areas
is lost quickly. Here, we show that surface modifications of the substrate
strongly manipulate the surface diffusion. The selective deposition of TiO$_2$
on poly (methyl methacrylate) and SiO$_2$ yields localized nanostructures with
tailored aspect ratios. Controlling the surface diffusion allows to tune such
nanostructures as it boosts the growth rate at the interface of the growth and
no-growth areas. Kinetic Monte-Carlo calculations reveal that species move from
high to low diffusion areas. Further, we identify the catalytic activity of
TiCl$_4$ during the formation of carboxylic acid on poly (methyl methacrylate)
as the reaction mechanism responsible for the loss of selectivity, and show
that process optimization leads to higher selectivity. Our work enables the
precise control of area-selective atomic layer deposition on the nanoscale, and
offers new strategies in area-selective deposition processes by exploiting
surface diffusion effects.
