Reaxff MD Analysis of Substrate Material Properties into Oxide Films during Thermal Oxidation Process of Group IV Semiconductor Materials
Authors: Sekiguchi Kenta , Uene Naoya , Ohori Daisuke , Endo Kazuhiko , Tokumasu Takashi 
Published: December 2024

In a rapidly changing world, the evolution of semiconductors and information processing technology is unstoppable and will continue to be a source of international competitiveness in the future. Semiconductor devices are becoming increasingly miniaturized for higher integration. Therefore, the manufacturing process is becoming more complex and delicate. Oxidation is the first step in semiconductor fabrication, and the formed oxide film serves as a protective film against impurities, thus affecting the accuracy of subsequent microfabrication processes. An experimental oxidation model for Si, a commonly used group IV semiconductor substrate material, has been clarified by B. E. Deal et al. 1. In recent years, device design based on material properties has become mainstream, and Ge is attracting attention as a next-generation semiconductor material. However, the effects of parameters such as atomic species and surface orientation of the substrate material on oxide film quality and oxygen diffusion still need to be investigated. Therefore, a systematic and atomistic understanding of the reaction-diffusion phenomenon of oxygen into the substrate and a better understanding of the oxide film formation process will contribute to the optimization of actual production conditions.
In this study, the influence of substrate material properties on oxide film quality and oxygen diffusion during the oxidation process was simulated using the Reactive force-field Molecular Dynamics (ReaxFF MD) method, which can simulate the behavior of atoms by considering dynamics and chemical reactions. The parameter set of the Si/Ge/O/H system developed by Nayir et al.2 was used in this study. In the calculation system, a substrate consisting of single elements of Si or Ge was placed in the lower part of the system, while the upper part of the system was vacuumed and O2 was injected at regular time intervals.
The radial distribution function (RDF) analysis between O-O atoms on Si (100) and Ge (100) substrates at 500 K and 1500 K is shown in Figure 1. The RDF is denoted by g(r) and is a quantitative function of the probability of the presence of other atoms at a certain distance from an atom. Figure1. (a) is the RDF for a Si substrate. Large peaks appeared at 1.32 Å and 2.52 Å, indicating a specific strong interaction force between oxygen atoms on the Si substrate. From the snapshot of the calculation system in the graph, oxygen atoms are deposited more on the substrate surface, confirming the same trend as in the model of B.E. Deal et al. Since the calculations were validated by obtaining qualitatively the same trend as the existing oxidation model for the Si substrate, the calculations for Ge were also performed using the same simulator. The RDF results for the Ge substrate are shown in Figure 1. (b) shows that the oxygen atoms are relatively more deeply buried than in the Si substrate since the peaks are located at positions where the distance r between the oxygen atoms is large. The second peak at Ge 500 K in (3) changed from 2.52 Å to 3.72 Å due to the difference in bond length between Si and Ge. In (4), oxygen atoms are present to some extent at any distance, suggesting that oxygen diffusion is more intense on the Ge substrate than on the Si substrate.
As the temperature was increased for each substrate material, the peak at 1.32 Å at 500 K became smaller at 1500 K. In (1) and (3), 1.32 Å is the distance between atoms of O2 molecules bonded to the substrate, indicating that the O-O bond is preserved at 500 K. At 1500 K in (2) and (4), the peaks at 2.52 Å and 3.72 Å (O-substrate-O bond) become larger, and the peak at 1.32 Å becomes smaller. This indicates dissociation of O2 at the surface with increasing substrate temperature, and it is more likely to occur on Ge substrates than on Si substrates. Thus, differences in the reactive diffusivity of oxygen for each substrate material were confirmed. The results suggest that the reaction and diffusion of oxygen atoms are slow on Si substrates and that multiple oxygen atoms move in layers, while on Ge substrates, the incident O2 dissociates one after another, and oxygen atoms may diffuse one after another.
References

B. E. Deal and A. S. Grove, J Appl Phys, 36, 3770–3778 (1965).

N. Nayir, A. C. T. Van Duin, and S. Erkoc, Journal of Physical Chemistry C, 123, 1208–1218 (2019).

Figure 1