To fabricate a metal oxide semiconductor field-effect transistor (MOSFET) with a three-dimensional (3D) structure, several problems arise in the conventional thermal oxidation (TO) process, such as leakage current, shape nonuniformity, stress concentration, and the dependence of the oxidation rate on the lattice plane of Si. To overcome these problems, we propose low-temperature (<300 C) neutral beam oxidation (NBO) as an alternative oxidation process. We found that an oxide film grown by NBO (NBO film) exhibits performance characteristics of a gate dielectric film that are as high as those of thermal oxide films in terms of the relationship between equivalent oxide thickness (EOT) and leakage current. The rate of NBO is also independent of the lattice plane of Si, while the oxidation is beam orientation dependent. Therefore, growing oxide films by NBO is advantageous, in that the method can be applied to gate dielectric films for the 3D fin structure of field-effect transistors (FinFETs) and surrounding gate transistors (SGTs).

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PURPOSEIn this study, the degradation mechanism of dielectric properties of hafnium dioxide thin films was investigated by using quantum chemical molecular dynamics. Effects of point defects such as oxygen vacancies and carbon interstitials and residual stress in the films on their local band gap were analyzed quantitatively. Drastic decrease of the local band gap from about 5.7 eV to about 1.0 eV was caused by the formation of a defect-induced site in the band gap. Though this defect-induced site was recovered by additional oxidation, the remaining interstitial oxygen deteriorated the quality of the interface with tungsten electrode by forming new oxide between them. The estimated changes of the band gap and the interface structure were confirmed by experiments using synchrotron-radiation photoemission spectroscopy. ANALYTICAL MODEL AND RESULTSThe point defects in gate oxide thin films in the miniaturized transistors play an important role in the degradation of the transistor performance because the relative volume ratio of weak spots caused by point defects has become larger and larger. The residual stress in the film caused by the intrinsic stress in the gate electrode and interconnection films has been also increasing due to strained silicon technology.[1] Such point defects and stress often deteriorate the electronic quality of the gate oxide film. For example, it was found that oxygen vacancies generate donor sites in the band gap of the film and tensile stress in the film decreases its band gap. [2][3]To make clear the formation mechanism of the point defectinduced impurity states in the gate oxide film, quantum chemical molecular dynamics was applied. In this analysis, an extend Hückel approximation was used to solve the electronic state. The empirical parameters used in Hamiltonian were optimized on the basis of density functional theory (DFT) calculations and the experimental results. In this study, the structural and electronic properties of monoclinic HfO 2 with oxygen vacancies and carbon interstitials were analyzed by using a three-dimensional atomic model as shown in Fig.