A study of the performance and reliability characteristics of HfO₂ MOSFET's with polysilicon gate electrodes
Abstract
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.
Department
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