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This paper proposes a novel tetraethylorthosilicate (TEOS)/oxynitride stack gate dielectric for low-temperature poly-Si thin-film transistors, composed of a plasma-enhanced chemical vapor deposition (PECVD) thick TEOS oxide/ultrathin oxynitride grown by PECVD N2O plasma. The novel stack gate dielectric exhibits a very high electrical breakdown field of 8.5 MV/cm, which is approximately 3 MV/cm higher than traditional PECVD TEOS oxide. The novel stack oxide also has better interface quality, lower bulk-trap density, and higher long-term reliability than PECVD TEOS dielectrics. These improvements are attributed to the formation of strong Si≡N bonds of high quality ultra-thin oxynitride grown by PECVD N2O plasma, and the reduction in the trap density at the oxynitride/poly-Si interface.