Trapping Properties of Very Thin Nitride/Oxide Gate Insulators

The trapping properties of very thin nitride/oxide (10-14nm equivalent SiO2) composite gate insulators and their dependences on gate materials and process conditions are reported. Electron trapping and flatband voltage turn-around effects are more pronounced in these films than in thermal SiO2. They both appear to be dominated by water-related species in the bottom oxide layer when the top nitride layer is thin, similar to the case of thermal SiO2 only. For VLSI CMOS applications, trapping and instabilities in the nitride/oxide gate insulator can be minimized by (i) reducing the thickness of the top nitride layer, (ii) using polysilicon gates with proper work functions, and (iii) using appropriate high-temperature dehydration steps after polysilicon gate deposition.