Physical models of boron diffusion in ultrathin gate oxides

Based on a network defect model for the diffusion of B in SiO2, we propose that B diffuses via a peroxy Linkage defect whose concentration in the oxide changes under different processing conditions. We show that as the gate oxide is scaled below 80 Angstrom in thickness, additional chemical processes act to increase B diffusivity and decrease its activation energy, both as a function of the distance from the Si/SiO2 interface. For a 15 Angstrom oxide, the B diffusivity at 900 degrees C would increase by a factor of 24 relative to diffusion in a 100 Angstrom oxide. The role of nitridation of SiO2 to create a barrier to B diffusion is modeled by assuming the N atoms compete with B for occupation of diffusion-defect sites. The model predicts that nitridation is ineffective in stopping B penetration when BF2 implants are used to dope the polysilicon gate, and similarly for B implants when the gate oxide thickness decreases below approximately 30 to 40 Angstrom.