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

Abstract

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 by Langmuir’s equation s = s01 − , whereby the initial sticking probability s0 is now an adjustable model parameter. For s0 100%, the model predicts a steplike profile, in agreement with Gordon et al., while for smaller values of s0, a gradual decreasing coverage profile is predicted. Furthermore, experiments were performed to quantify the conformality for the trimethylaluminum TMA/H2O ALD process using macroscopic test structures. It is shown that the experimental data and the simulation results follow the same trends.