BOSTON UNIVERSITY COLLEGE OF ENGINEERING Dissertation DENSITY RELAXATION AND IMPURITY INCORPORATION IN THIN SILICON DIOXIDE FILMS AND THEIR EFFECTS ON BORON DIFFUSION

Integrated circuit device geometries require precise control of thickness and electrical properties in thin gate dielectric. This work studies the evolution of excess density generated during thermal oxidation of silicon and the properties of the resulting oxide films. This work also investigates impurity incorporation in thin oxides and its effect on boron penetration leading to changes in threshold voltage. Due to differences in molecular volume between silicon and SiO2, the oxidation process results in the generation of stress near the Si/SiO2 interface. This stress substantially reduces the diffusivity and solubility of oxidant species (e.g., O2) in the oxide, modifying oxidation kinetics. To be able to model diffusion in the oxide as a function of stress, one is required to first model the stress generation and relaxation in the oxide. We have proposed a model that accounts for mechanical behavior of silicon dioxide. In the model, the newly created oxide is assumed to be highly compressed, relaxing as oxidation (or annealing) proceeds. The model was matched to both one and two-step oxidation kinetics to extract stress-dependent diffusivity and solubility for O2 in SiO2. We also investigated the influence of oxide density on boron diffusion through thin gate dielectrics. Capacitor structures were fabricated by depositing polysilicon on top of 54–60Å oxide films grown at 800 C in dry O2. A subset of these high density oxides (approximately 3–4% excess density) were then annealed at 1100 C, to relax the grown-in stress. Following ion implantation of boron into the polysilicon, the capacitor structures were annealed in an inert ambient at 950 C to allow boron penetration. Capacitance-voltage measurements revealed that boron penetration was greatly enhanced for the films that saw the high temperature relaxation anneal, leading to the conclusion that boron diffusion in SiO2 is retarded in the presence of excess density. This work also examines boron penetration from p polysilicon through 50–70Å gate dielectrics following B or BF2 implantation. Gate oxides were grown in N2O/O2 mixtures with average nitrogen contents varying from 0 to 1.4%. A series of capacitance-voltage measurements were used to determine the amount of boron penetration, and SIMS measurement were carried out to measure the depth profiles of incorporated nitrogen and fluorine. In addition, to better understand the role of fluorine, experiments were carried out to