WSix/WN/polysilicon DRAM gate stack with a 50Â Ã WN layer as a diffusion barrier and an etch stop

A new DRAM gate stack of WSix/WN/polysilicon with a 50 Å WN layer was studied as a gate stack of future technologies. With the 50 Å WN layer inserted between the WSix and polysilicon, the tungsten silicide etch was able to be very selective to the thin WN layer, and the thickness of the polysilicon underneath can be minimized. The etch selectivity of WN to silicide using the Cl2-based chemistry was approximately 1:13.2. The sheet resistivity of the new gate stack could be reduced significantly by using a thicker WSix and a thinner polysilicon while the aspect ratio of the gate stack remains the same. The thin WN layer seemed to work well as a diffusion barrier, as is shown by secondary ion mass spectroscopy analysis. The new gate stack with a low resistance and aspect ratio and without the diffusion problem seems a good candidate as a future DRAM gate and can extend the use of WSix gates to future technologies.