The influence of fluorine on threshold voltage instabilities in p/sup +/ polysilicon gated p-channel MOSFETs

It is shown that fluorine plays a major role in the penetration of boron into and through the gate oxides of p-channel MOSFETs that use p/sup +/ doped polysilicon gates. Boron penetration results in large positive shifts in V/sub FB/, increased p-channel subthreshold slope and electron trapping rate, and decreased low-field mobility and interface trap density. Inclusion of a phosphorus coimplant or TiSi/sub 2/ 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 SiO/sub 2/-Si interface. Elemental boron is shown to be superior to BF/sub 2/ as an implant species for surface channel submicron PMOS devices.<
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