(Invited) Silicon Emission Mechanism for Oxidation Process of Non-Planar Silicon
Authors: Kageshima Hiroyuki , Shiraishi Kenji , Endoh Tetsuo 
Published: February 2020

Recent aggressive progress of the device technology demands development of three-dimensional non-planar channel MOSFETs. The oxidation process of non-planar silicon is one of the most important keys.

Silicon oxidation is the fundamental but very important process for fabricating the gate dielectrics/silicon interface, which governs the performance of MOSFETs. Historically it had been believed that this process consists of only two elementary processes, the oxygen diffusion through the oxide and the oxygen reaction at the interface between the oxide and the silicon as explained by Deal and Grove in 1965. Afterwards, it has been revealed that this process also consists of the additional process of silicon emission from the interface into the oxide. Our theoretical studies combining the first-principles calculation approach with the rate-diffusion equation approach clearly showed that the inclusion of the silicon emission process can consistently explain the unaccounted initial enhanced oxidation. The theory can also naturally explain the oxidant pressure dependence and the substrate orientation dependence of the oxide growth kinetics. In addition, the experimental study with HRBS clearly detected silicon atoms diffused from the interface through the oxide after the thermal oxidation. Therefore, the silicon emission mechanism is inevitable for the atomic-level control of the gate-dielectrics-fabrication process. Even if the surface is covered with high-k dielectric films, thermal treatments promote silicon diffusion through the films towards the surface because the oxygen atoms in the films naturally react with the silicon at the interface. The silicon emission also causes interfacial defects, which become the charge traps. The emission further causes density fluctuation of the grown oxide, which leads to the leakage current paths for the stress-induced dielectric break down. The emission thus plays important roles for the performance of gate dielectrics.

The situation could not be changed even when the channels of MOSFETs becomes non-planar from planar. For the typical non-planar MOSFET such as the vertical BC-MOSFET, the channel has the shape of a pillar. The experiment on the oxidation of the silicon pillar reported unaccounted missing silicon. 50% in maximum of silicon atoms in the initial pillar disappear after the thermal oxidation. This phenomenon can also be explained by the silicon emission mechanism if almost all of the silicon atoms diffused through the oxide evaporate from the oxide surface as SiO gas. Because the area of the interface is smaller than that of the oxide surface, the interface density of the emitted silicon is geometrically larger than the surface density. Therefore, the silicon diffusion through the oxide is enhanced more than that in the planar case. This means that the missing silicon is inevitable for the oxidation process of non-planar channels. Careful fabrication approaches are necessary to achieve high-performance non-planar MOSFETs.

This work has been supported by a grant from "Three-Dimensional Integrated Circuits Technology Based on Vertical BC-MOSFET and Its Advanced Application Exploration" of ACCEL under JST.