Chemical vapor deposition can afford conformai films with step coverage > 90% in via or trench features with aspect ratios ≥ 30:1 when growth proceeds at a substrate temperature ≤ 300°C under a precursor pressure > 0.1 Torr. These conditions correspond to a near-saturation of the film growth rate vs. precursor pressure, such that the reactive sticking probability is ≤ 10-5 per collision with the film surface. We model this in terms of a self site-blocking effect in which the growth surface is covered with a dynamic layer of adsorbed precursor molecules that inhibit the adsorption of impinging precursor molecules. We derive an analytical equation that relates the step coverage, growth rate, precursor pressure and substrate temperature for a given aspect ratio. For precursors that cannot achieve conformai coverage due to insufficient vapor pressure, conformai coverage can be achieved by adding an inhibitor species that completes the site blocking.