A physical model for boron penetration through thin gate oxides from p+ polysilicon gates is presented. Based on numerical device and process simulation, it is shown that enhancement of the boron diffusivity by as much as 300x in the thin gate oxide results in a very shallow exponential p-type profile in the underlying silicon substrate. The effect of fluorine and phosphorus co-implantation into the p-type polysilicon gate is modeled by changes in the boron diffusivity in the gate oxide and segregation at the polysilicon/oxide interface, respectively. An inverse PMOS short-channel behavior in which the threshold voltage becomes more negative with decreasing channel length is modeled by twodimensional boron segregation effects caused by the poly gate oxidation.
It is shown that fluorine plays a major role in the penetration of boron into and through the gate oxides of P·channel MOSFETs which employ P+ doped polysilicon gates. Boron penetration results in large positive shifts in V F B, increased P-channel subthreshold slope and electron trapping rate, and decreased low-field mobility and interface trap density. 'Inclusion of a phosphorus co·implant or TiSi2 salicide is shown to minimize this effect. The boron penetration phenomenon is modeled by the creation of a very shallow, fully·depleted P·type layer in the silicon substrate close to the Si02/Si interface. Elemental boron is shown to be superior to B F2 as an implant species for surface channel submicron PMOS devices.
We propose that a severe anomaly among the parameters of the gold acceptor state in silicon can be resolved by taking into account the temperature dependence of all the parameters. The temperature variation of most consequence is that of the trap energy level with respect to the band edges. We show that if the trap maintains a constant relative position in the gap, the resulting corrections to the activation energies place the trap nearly 0.1 eV higher in the gap at 300°K than was supposed, and all of the available data are brought into agreement.
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