AVS 50th International Symposium
    High-k Gate Dielectrics and Devices Topical Conference Tuesday Sessions
       Session DI-TuA

Paper DI-TuA6
Observation of Bulk HfO@sub 2@ Defects by Spectroscopic Ellipsometry

Tuesday, November 4, 2003, 3:40 pm, Room 317

Session: High-k Dielectric Characterization
Presenter: H. Takeuchi, University of California at Berkeley
Authors: H. Takeuchi, University of California at Berkeley
D. Ha, University of California at Berkeley
T.-J. King, University of California at Berkeley
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HfO@sub 2@ (hafnium oxide) is a promising candidate to replace SiO@sub 2@-based films as the gate dielectric in ultra-scaled MOSFETs, due to its thermal stability in contact with Si, compatibility with a conventional CMOS process flow, and moderately high dielectric constant (20-25). The electrical characteristics of HfO@sub 2@ films, such as equivalent SiO@sub 2@ thickness (EOT), leakage current density, hysteresis in capacitance vs. voltage curves, fixed charge density and resultant field-effect carrier mobilities, have been extensively investigated. However, the physical mechanism for deviation from ideal behavior is not yet well understood. In particular, not much is known about bulk defects inside HfO@sub 2@ and their impact on electrical characteristics and the thermal stability of HfO@sub 2@. In this study, we report a bulk defect in HfO@sub 2@ which can be detected as an optical absorption peak by spectroscopic ellipsometry (SE). 12.5nm-thick HfO@sub 2@ films were formed by oxidation of pure Hf films in a cold-wall rapid thermal annealing (RTA) reactor. Absorption coefficients near the absorption edge were extracted by the data inversion method, in which the optical constants for short wavelength were calculated using the thickness obtained from long wavelength data. The obtained optical bandgap of 5.7eV matches very well with theoretical calculation and VUV measurement reported by other groups, and a shift due to crystallization was also detected. In addition, an extra absorption peak was observed in 4.5~5.0eV range. The energy difference between this absorption peak and the bandgap corresponds well to the trap energy extracted from measurement of the temperature dependence of Poole-Frenkel current. Hence, the peak is associated with electron transition from the valance band to the trap energy level inside the bandgap. This peak reduces with oxidation annealing time, indicating that the defects can be attributed to oxygen vacancies in the HfO@sub 2@ film.