Effect of polysilicon gate on the flatband voltage shift and mobility degradation for ALD-Al₂O₃ gate dielectric

Al₂O₃ (EOT=22.7 Å) gate dielectric layer formed by Atomic Layer Deposition (ALD) process have been characterized for sub-100nm CMOS devices. The gate leakage current was 3 orders of magnitude lower than that of SiO₂ and the hysteresis of C-V curve was not observed. However, the negative fixed charge induced the flat band voltage (Vfb) shift and degraded the channel mobility of MOS transistor. The Vfb shift was reduced and channel mobility was improved by applying P+ gate by BF₂ implantation. It is suggested that the phosphorous diffused from gate polysilicon has a role of network modifier in Al₂O₃ film and formation of the Al-O- dangling bond which may be ascribed to negative fixed charge.