Conformality of Al2O3 deposited by thermal, plasma-enhanced and ozone-based atomic layer deposition

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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 theta by Langmuir’s equation s(theta) = s0(1−theta), 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

Optimization of the annealing conditions for thin VO2 ALD films

Vanadium dioxide (VO2) is an intriguing material due to its semiconductor-metal transition (SMT). During this transition, which occurs near 67°C, electrical as well as optical properties change drastically. Possible applications include thermochromic windows, and memories or switches in micro- and optoelectronics. Although atomic layer deposition (ALD) is gaining importance for some of these applications, the growth of VO2 with this technique is not obvious, since in most cases V2O5 is obtained. In our previous work we presented ALD growth of VO2 by using Tetrakis[EthylMethylAmino]Vanadium and ozone at a temperature of 150°C [1]. XPS revealed the 4+ oxidation state of vanadium, indicating growth of VO2. Post-ALD thermal processing proved essential to crystallize the VO2 in the desired tetragonal phase (R). In this work we present the influence of the oxygen partial pressure on phase formation during such thermal processes. Additionally the influence of film thickness and annealing temperature on the post-annealing properties were studied, including morphology and SMT characteristics. During thermal processing a minimum oxygen partial pressure of approximately 1 Pa is indispensable to form crystalline VO2 (R) (figure 1). Oxygen partial pressures above 2 Pa show an intermediate monoclinic phase (B), which transforms to VO2 (R) at higher temperatures. At a value of 35 Pa this VO2 (B) phase finally transforms to V6O13 instead of VO2 (R). For very thin films, the thermal post-processing may result in agglomeration of the VO2 layers on the SiO2 substrate. Samples with a film thickness above 20nm show a typical resistivity ratio during the SMT of more than 2 orders of magnitude when annealed in the range 450°C to 500°C. For thinner films or higher annealing temperatures the resistivity ratio is suppressed and an overall increased resistivity is observed due to agglomeration (figure 2)