Effect of film density on boron diffusion in SiO2

Oxides grown at low temperatures exhibit higher density than those grown or annealed at high temperatures. This work investigates the role of oxide density on boron diffusion through thin gate dielectrics. Capacitor structures were fabricated by depositing polycrystalline silicon on top of 54–60 Å oxide films grown at 800 °C in dry O2. A subset of these high density oxides 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.