Modeling the Conformality of Atomic Layer Deposition: The Effect of Sticking Probability

The key advantage of atomic layer deposition (ALD) is undoubtedly the excellent step coverage, which allows for conformal deposition of thin films in high-aspect-ratio structures. In this paper, a model is proposed to predict the deposited film thickness as a function of depth inside a hole. The main model parameters are the gas pressure, the deposition temperature, and the initial sticking probability of the precursor molecules. Earlier work by Gordon et al. assumed a sticking probability of 0/100% for molecules hitting a covered/uncovered section of the wall of the hole, thus resulting in a stepwise film-thickness profile. In this work, the sticking probability is related to the surface coverage θ by Langmuir's equation , whereby the initial sticking probability is now an adjustable model parameter. For , the model predicts a steplike profile, in agreement with Gordon et al. , while for smaller values of , a gradual decreasing coverage profile is predicted. Furthermore, experiments were performed to quantify the conformality for the trimethylaluminum ALD process using macroscopic test structures. It is shown that the experimental data and the simulation results follow the same trends.