Reliability of ZrO 2 films grown by atomic layer deposition D. Caputo, F. Irrera, S. Salerno Rome Univ. “La Sapienza”, Dept. Electronic Eng. via Eudossiana 18, Rome (Italy) S. Spiga, M. Fanciulli Laboratorio MDM-INFM, via C. Olivetti Agrate Brianza (Italy)

Aim of the work To verify the reliability of ZrO 2 films embedded in MOS structures as gate dielectric Electrical characterization by means of I-V curves and C-V measurements in as grown conditions and after constant current stress (CCS) Extraction of defect density

Why high-k dielectrics? SiO 2 thickness below 2 nm is required in the 2005 technology node Substitution of SiO 2 with oxides with higher dielectric constant Equivalent Oxide Thickness EOT =  SiO2 /  high-k t ox Candidates: Al 2 O 3, Gd 2 O 3, ZrO 2, HfO 2,.... Requirements Good chemical stability, amorphous network, large energy gap and high band offset with silicon and of course … lower leakage current

Silicon substrate SiO 2 V gate ZrO 2 Al Devices under test Silicon substrate is 2-3  cm 1.2 ±0.1 nm thick native SiO 2 layer 19.1±0.3 nm thick ALCVD ZrO 2 layer

About the leakage in ZrO 2 films …..

Fresh Samples Current-voltage measurements Poole-Frenkel Fowler-Nordheim ?

Fresh Samples FOWLER-NORDHEIM regime If m eff = 0.5 m 0 then Al-ZrO 2 barrier is 0.3 eV

Fresh Samples Weak accumulation condition (0<V gate <1 V) Transport dominated by charge trapping and de-trapping t -1 Current (A/cm 2 )

Stressed Samples: Current-voltage measurements Cumulative Constant Current 1nA/cm 2

Stressed Samples: Time behavior of the defect density extracted from PF conduction Scattering Induced Degradation (SID) model

Stressed Samples: Current-voltage measurements Cumulative Constant Current 1nA/cm 2

Stressed Samples: Weak accumulation condition (0< V gate < 1V) I(t)= q. N. A. (2t  ) -1 tunneling front model ** q electron charge N defect density A area  a constant the same defect states are involved in the trapping-detrapping process, i.e the Fermi level remains almost constant with the applied voltage ** S. Manzini, A. Modelli, “Insulating films on semicond.”, Elsevier Science, 112, (1983) D.J. Dumin, J.R. Maddux, IEEE Trans. on Electron Dev., 40, 986, (1993).

Weak accumulation condition (0< V gate < 1V) Defect density greater than cm -3 pins the Fermi level t 1/2

Stressed Samples: Current-voltage measurements Cumulative Constant Current 1nA/cm 2

Strong accumulation condition (V gate > 1V) Unlike in SiO 2 the FN treshold voltage shifts

Capacitance measurements: fresh samples C FB = 168 nF/cm 2 V FB = 450 mVN bulk ≈ cm -3  r ≈ 22

Capacitance measurements: low level stress Negative shift of V FB due to trapping of negative charge A decrease of capacitance in the strong accumulation region due to variation of the interfacial SiO x Zr layer thickness

Capacitance measurements: high level stress Positive shift of V FB due to trapping of positive charge A tendency to saturation in the strong accumulation region

Conclusions Electrical characterization of fresh and stressed ZrO 2 films deposited onto native SiO 2 on Si by ALCVD Densities of native bulk defects were estimated in the order of cm -3 The density of bulk defects follows a square root time dependence (Scattering Induced Degradation model) Leakage current of ZrO 2 films is lower than that of thicker SiO 2 films Deposition technology needs to be improved