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Reliability of ZrO2 films grown by atomic layer deposition D. Caputo, F. Irrera, S. Salerno Rome Univ. “La Sapienza”, Dept. Electronic Eng. via Eudossiana 18, 00184 Rome (Italy) S. Spiga, M. Fanciulli Laboratorio MDM-INFM, via C. Olivetti 2 20041 Agrate Brianza (Italy) Aim of the work • To verify the reliability of ZrO2 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? • SiO2 thickness below 2 nm is required in the 2005 technology node • Substitution of SiO2 with oxides with higher dielectric constant • Equivalent Oxide Thickness EOT = eSiO2/ehigh-k tox • Candidates: Al2O3, Gd2O3, ZrO2, HfO2, . . . . Requirements Good chemical stability, amorphous network, large energy gap and high band offset with silicon and of course … lower leakage current Devices under test Al ZrO2 SiO2 Silicon substrate Vgate Silicon substrate is 2-3 W cm 1.2 ±0.1 nm thick native SiO2 layer 19.1±0.3 nm thick ALCVD ZrO2 layer About the leakage in ZrO2 films ….. 10 -8 ZrO EOT = 4 nm 2 2 inj 2 Current (A/cm ) Q = 1.22 C/cm 10 10 -9 SiO EOT= 5.5 nm 2 -10 fresh 0 0.5 1 Voltage (V) 1.5 2 2 Current (A/cm ) Fresh Samples Current-voltage measurements 10 -6 10 -7 10 -8 10 -9 Fowler-Nordheim 10 -10 10 -11 10 -12 Poole-Frenkel ? -4 -3 -2 -1 0 Voltage (V) 1 2 3 Fresh Samples FOWLER-NORDHEIM regime -42 2 -44 2 ln J/E (A/V ) -43 -45 -46 -47 -48 -6 1 10 -6 1.2 10 -6 -6 1.4 10 1.6 10 1/E (cm/V) 1.8 10 -6 If meff = 0.5 m0 then Al-ZrO2 barrier is 0.3 eV Current (A/cm2) Fresh Samples Weak accumulation condition (0<Vgate<1 V) 10 -8 10 -9 t-1 10 -10 10 -11 10 -12 1 10 100 Time (s) 1000 Transport dominated by charge trapping and de-trapping Stressed Samples: Current-voltage measurements 2 Current (A/cm ) Cumulative Constant Current Stress @ 1nA/cm2 10 -6 10 -7 10 -8 10 -9 10 10 Q inj fresh -10 17.05 mC/cm 2 118.2 mC/cm 2 618.2 mC/cm 2 3020 mC/cm 2 -11 -3 -2 -1 0 Voltage (V) 1 2 Stressed Samples: Time behavior of the defect density extracted from PF conduction 6 5 N(t)/N(0) 4 3 2 t 1/2 1 0 10 5 10 6 10 7 10 8 10 9 10 10 Stress time (s) Scattering Induced Degradation (SID) model Stressed Samples: Current-voltage measurements 2 Current (A/cm ) Cumulative Constant Current Stress @ 1nA/cm2 10 -6 10 -7 10 -8 10 -9 10 10 Q inj fresh -10 17.05 mC/cm 2 118.2 mC/cm 2 618.2 mC/cm 2 3020 mC/cm 2 -11 -3 -2 -1 0 Voltage (V) 1 2 Stressed Samples: Weak accumulation condition (0< Vgate < 1V) • I(t)= q.N.A.(2tb)-1 tunneling front model ** q electron charge N defect density A area b 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). 2 10 19 1.6 10 19 1.2 10 19 8 10 18 4 10 18 -3 Defect density (cm ) Weak accumulation condition (0< Vgate < 1V) t1/2 1 10 100 1000 2 Q (mC/cm ) inj Defect density greater than 1019 cm-3 pins the Fermi level Stressed Samples: Current-voltage measurements 2 Current (A/cm ) Cumulative Constant Current Stress @ 1nA/cm2 10 -6 10 -7 10 -8 10 -9 10 10 Q inj fresh -10 17.05 mC/cm 2 118.2 mC/cm 2 618.2 mC/cm 2 3020 mC/cm 2 -11 -3 -2 -1 0 Voltage (V) 1 2 Strong accumulation condition (Vgate > 1V) Unlike in SiO2 the FN treshold voltage shifts 1.7 2 Voltage @ 1 nA/cm (V) 1.8 1.6 1.5 1.4 1.3 0.1 1 10 100 2 Q (mC/cm ) inj 1000 Capacitance measurements: fresh samples 2 Capacitance (nF/cm ) 1000 3 Hz 200 Hz 100 kHz 800 600 400 er ≈ 22 200 0 -4 -3 -2 -1 0 1 Voltage (V) CFB = 168 nF/cm2 VFB = 450 mV 2 3 Nbulk ≈ 1019 cm-3 Capacitance measurements: low level stress 2 Capacitance (nF/cm ) 800 C 700 600 Q =17 mC/cm 500 inj V 400 2 FB 300 Q =5.53 mC/cm 2 inj Q =1.8mC/cm 2 inj 200 Q =0.576 mC/cm inj 100 0 2 fresh 0 0.5 1 1.5 Voltage (V) 2 Negative shift of VFB due to trapping of negative charge A decrease of capacitance in the strong accumulation region due to variation of the interfacial SiOxZr layer thickness Capacitance measurements: high level stress 2 Capacitance (nF/cm ) 800 700 600 500 2 V 400 Q =1022 mC/cm inj 2 FB Q =618.2 mC/cm inj 300 2 200 Q =118.2 mC/cm 100 Q =17 mC/cm 0 inj 2 inj 0 0.5 1 1.5 Voltage (V) 2 Positive shift of VFB due to trapping of positive charge A tendency to saturation in the strong accumulation region Conclusions Electrical characterization of fresh and stressed ZrO2 films deposited onto native SiO2 on Si by ALCVD Densities of native bulk defects were estimated in the order of 1019 cm-3 The density of bulk defects follows a square root time dependence (Scattering Induced Degradation model) Leakage current of ZrO2 films is lower than that of thicker SiO2 films Deposition technology needs to be improved