Effect of thin metal layers on the work function of polysilicon-metal electrode stacks

The impact of thin metal nitride layers on the effective work function (EWF) of poly-Si/metal gate stacks has been investigated. The electrode stacks studied include very thin (0.5-2.0 run) TaN_x and MoN_x metal layers sandwiched between the poly-Si gate and the gate dielectric. Both n and p-type polysilicon electrodes were evaluated. The results indicate that when the metal nitride layers are as thin as 0.5 nm, they can have significant effect on the polysilicon effective work function. The observed results are explained by reactions between poly-Si and the metal nitrides leading to the formation of Ta_xSi_yN_z in the case of TaN _x inter-layers. As the metal nitride inter-layers become thicker, the work function is controlled by the metal nitride EWF. Preliminary quasistatic C-V analysis shows minimal poly depletion with the metal inter-layers. Gate leakage current and fixed charges comparable to conventional polysilicon electrodes were obtained.