The effects of nitrogen implant into gate electrode on the characteristics of dual-gate MOSFETs with ultra-thin oxide and oxynitrides

Nitrogen implantation into the polysilicon gate has been proposed as a means of suppressing boron penetration through the thin gate oxide in surface-channel PMOSFETs. However, a detailed study of the process tradeoffs among various performance parameters such as oxide reliability and transistor performance, and the physical mechanism of how nitrogen suppresses boron diffusion are still not well developed. This paper investigates the effects of nitrogen implantation condition (dosage and energy), gate dielectrics (O₂, vs. N₂O), dopant species (B, BF₂ and As) and gate microstructure (as-deposited amorphous vs, polycrystalline) on the characteristics of ultra-thin gate oxide (45 Å) MOSFETs. It is found that in addition to suppressing threshold voltage shifts due to boron diffusion through the thin gate oxide, nitrogen implantation improves the oxide reliability in devices with p+ poly gates. The nitrogen within the polysilicon increases the poly depletion effect as well as non-silicided gate sheet resistance. However, with proper optimization, PMOSFET transconductance is unchanged, while oxide reliability is greatly enhanced