A comparative theoretical analysis of the atomic-layer deposition is implemented for trenches and cylindrical holes. The precursor insertion duration necessary for the conformal coating of the walls of three-dimensional structures is studied at different values of the structure aspect ratio and the sticking coefficient of the precursor molecules. The critical duration (the minimum time required for a conformal surface coating) has been estimated on the basis of the kinetic model of particle flows to the walls of the microstructure. Analytical expressions have been derived, which allow estimate the critical duration as a function of the parameters of the technological process (pressure, temperature).