Surface recombination of plasma radicals is generally considered to limit film conformality during plasma-assisted atomic layer deposition (ALD). Here, we experimentally studied film penetration into high-aspect-ratio structures and demonstrated that it can give direct information on the recombination probability r of plasma radicals on the growth surface. This is shown for recombination of oxygen (O) atoms on SiO2, TiO2, Al2O3, and HfO2 where a strong material dependence has been observed. Using extended plasma exposures, films of SiO2 and TiO2 penetrated extremely deep up to an aspect ratio (AR) of â900, and similar surface recombination probabilities of r = (6 ± 2) × 10-5 and (7 ± 4) × 10-5 were determined for these processes. Growth of Al2O3 and HfO2 was conformal up to depths corresponding to ARs of â80 and â40, with r estimated at (1-10) × 10-3 and (0.1-10) × 10-2, respectively. Such quantitative insight into surface recombination, as provided by our method, is essential for modeling radical-surface interaction and understanding for which materials and conditions conformal film growth is feasible by plasma-assisted ALD.