Electrical characteristics and thermal stability of W, WNx, and TiN barriers in Metal/Ta2O5/Si gate devices

Tantalum pentoxide was adopted as a gate dielectric for obtaining a gate oxide with less than 3.0 nm of SiO2 equivalent thickness and low leakage current. Physical vapour deposited (PVD) W, PVD WNx, PVD TiN, or TiCl4-based chemical vapour deposited (CVD) TiN was used as a gate/barrier material on the Ta2O5. The thermal stability and electrical properties of each barrier were evaluated at elevated temperature of 900C. W and WNx barriers show much lower leakage current than TiN barriers at both low and high fields. Leakage current is lower with WNx barrier than with W barrier at high field. The breakdown characteristics of W and WNx barriers are noticeably improved after annealing at 900C because of the recovery from sputtering damage. The breakdown characteristic of WNx is superior to W, possibly because of incorporation of nitrogen into the Ta2O5 layer. Breakdown is not degraded significantly in the samples with PVD TiN barrier. However, samples with a CVD TiN barrier show significant degradation in electrical properties at elevated temperature. Microvoids are formed in the Ta2O5 layer along the TiN/Ta2O5 interface in a PVD TiN sample after annealing at 900C. Ta diffuses into the TiN layer leaving voids behind. Ta2O5 is more severely damaged in the CVD TiN sample. Diffusion of Ta and O becomes significant in this sample after annealing. The Cl content is also much higher in CVD TiN samples than PVD TiN ones. Thus residual Cl is considered to be the main reason for the degradation of Ta2O5 in the metal/Ta2O5/Si devices containing CVD TiN as a barrier material. 23 refs.