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

Abstract

Atomic layer deposition (ALD) is known to be an excellent technique for conformal coating. In this work, two models, a kinetic and a Monte Carlo model, are developed to predict the deposited film thickness as a function of depth inside a hole. 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 coverage profile after a single ALD cycle [1]. However, experimental studies indicate a gradual decrease of film thickness instead of a stepwise drop (figure) [2, and references therein]. It has been argued that the gradual slope may be related to (i) the increasing aspect ratio (AR) during deposition and (ii) the sticking probability, which is less than 100%. The first explanation seems reasonable for the case of microscopic trenches, because during each cycle the deposition of new material results in a decrease of the diameter of the hole, and consequently the effective aspect ratio increases during deposition. However, our experiments using macroscopic structures (~0.1x5x20mm) also show a gradual decrease (figure), suggesting that the sticking probability is an important parameter for predicting the conformality. Therefore, in both models, we related the sticking probability s to the surface coverage θ by Langmuir's equation s(θ)=s 0 (1-θ), whereby the initial sticking probability s 0 is now an adjustable model parameter. For s 0 ≈100%, the models predict a steplike profile, in agreement with Gordon et al., while for smaller values of s 0 , a gradual decreasing coverage profile is predicted. The two models show a good correspondence and follow the same trends as the experimental data (figure).

Conformality of Plasma-Assisted ALD: Physical Processes and Modeling

“…Our Monte Carlo model is similar to those reported earlier for thermal 1,[6][7][8][9][10] and plasma-assisted ALD. 29,30 For simplicity and fast computation, we have chosen a two-dimensional ͑2D͒ Monte Carlo simulation.…”

Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges

“…This has only recently been addressed qualitatively by simulations and experimentally. 42,304,336,337 In many other reports, the impact of the surface recombination of plasma radicals is generalized and the poor conformality of plasma-assisted ALD films is often claimed. However, the conformality achieved by plasmaassisted ALD under certain conditions depends strongly on the value of the recombination probability, r, which itself depends on (a) the type of radicals responsible for film growth in a certain plasma-assisted ALD process and (b) the material being deposited (see Table II).…”

Modeling precursor diffusion and reaction of atomic layer deposition in porous structures

“…Some of the models use Monte Carlo simulations to describe the ALD process, [18][19][20][21] other models are designed only for high-aspect-ratio structures, 22,23 partly very complex 24 and partly simplified. 25 Some studies focus only on single parts of ALD, like the sticking coefficient, [26][27][28] the growth mode, or the growth per cycle (GPC). [29][30][31][32][33] A highly useful and practical model, however, was developed by Yanguas-Gil and Elam describing ALD in viscousflow tubular ALD reactors 34 as well as in porous materials.…”