For plasma-assisted atomic layer deposition (ALD), reaching conformal deposition in high aspect ratio structures is less straightforward than for thermal ALD due to surface recombination loss of plasma radicals. To obtain a detailed insight into the consequences of this additional radical loss, the physical processes in plasma-assisted ALD affecting conformality were identified and investigated through Monte Carlo simulations. The conformality was dictated by the recombination probability r, the reaction probability s, and the diffusion rate of particles. When recombination losses play a role, the saturation dose depended strongly on the value of r. For the deposition profiles, a minimum at the bottom of trench structures was observed (before reaching saturation), which was more pronounced with larger values of r. In turn, three deposition regimes could be identified, i.e., a reaction-limited regime, a diffusion-limited regime, and a new regime that is recombination-limited. For low values of r, conformal deposition in high aspect ratio structures can still be achieved, as observed for several metal oxides, even for aspect ratios as large as 30. For high surface recombination loss probabilities, as appears to be the case for many metals, achieving a reasonable conformality becomes challenging, especially for aspect ratios > 10. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3491381] All rights reserved.