A study of the performance and reliability characteristics of HfO₂ MOSFET's with polysilicon gate electrodes
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Aggressive scaling of CMOS integrated circuits requires continuous miniaturization of the MOS transistor structures, including gate dielectric thickness. Conventional SiO2 is reaching its physical limitations as an insulator in terms of gate leakage current, uniformity control, and reliability requirements. High-k gate dielectrics, which have higher dielectric constants (k) than that of SiO2, have attracted a great deal of attention in the past few years, because of their potential for reducing gate leakage current while keeping the equivalent oxide thickness (EOT) thin. HfO2 has emerged as one of the most promising candidates, since it possesses a dielectric constant of 22 – 25, a large band gap of 5.6 eV with sufficient band offsets of larger than 1.5 eV, and is thermally stabile in contact with silicon. In this research, HfO2 MOS devices with polysilicon gate electrodes were investigated. The HfO2 dielectric was found to be compatible with polysilicon gate electrode. Minimum EOT of ~12 Å was achieved for the control process and it was scaled down to ~10 Å by a surface nitridation (SN) technique with an NH3 annealing. Boron penetration was observed on the control HfO2 devices, but it was improved with the SN technique as well. On the other hand, the SN technique exhibited adverse effects such as the degradations in mobility and negative bias-temperature instability on PMOSFET’s. Although inadequate channel mobility was a concern for the HfO2 MOSFET’s, it was found that high-temperature (500 – 600°C) forming gas annealing (HT-FGA) was effective in improving the mobility as well as reducing interfacial state density. The improvement was further enhanced with surface NO annealing, but it increased the EOT value. With regard to the reliability characterizations, charge trapping due to gate voltage stress caused significant Vt shift on the HfO2 NMOSFET’s and could be a scaling limit of the dielectric. Deuterium (D2) annealing was found to be an excellent technique for suppressing the charge traps while maintaining the similar mobility enhancement as HT-FGA. Overall, HfO2 MOSFET’s with polysilicon gate electrodes demonstrated promising performance and reliability characteristics. The HfO2-based dielectrics deserve to be further investigated as the future high-k gate dielectric.