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 Å 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 Åoxide, the B diffusivity at 900°C would increase by a factor of 24 relative to diffusion in a 100 Å 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 Å.

Duke Scholars

Author Richard B. Fair Electrical and Computer Engineering