Reliability of ZrO2 films grown by atomic layer deposition

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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)

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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

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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

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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 19.10.3 nm thick ALCVD ZrO2
layer
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About the leakage in ZrO2 films ..
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Fresh SamplesCurrent-voltage measurements
Fowler-Nordheim
Poole-Frenkel
?
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Fresh SamplesFOWLER-NORDHEIM regime
If meff 0.5 m0 then Al-ZrO2 barrier is 0.3 eV
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Fresh Samples Weak accumulation condition
(0ltVgatelt1 V)
t-1
Current (A/cm2)
Transport dominated by charge trapping and
de-trapping
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Stressed Samples Current-voltage
measurementsCumulative Constant Current Stress
_at_ 1nA/cm2
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Stressed SamplesTime behavior of the defect
density extracted from PF conduction
Scattering Induced Degradation (SID) model
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Stressed Samples Current-voltage
measurementsCumulative Constant Current Stress
_at_ 1nA/cm2
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Stressed Samples Weak accumulation condition
(0lt Vgate lt 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).

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Weak accumulation condition (0lt Vgate lt 1V)
t1/2
Defect density greater than 1019 cm-3 pins the
Fermi level
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Stressed Samples Current-voltage
measurementsCumulative Constant Current Stress
_at_ 1nA/cm2
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Strong accumulation condition (Vgate gt 1V)

• Unlike in SiO2 the FN treshold voltage shifts

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Capacitance measurements fresh samples
er 22
CFB 168 nF/cm2 VFB 450 mV
Nbulk 1019 cm-3
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Capacitance measurements low level stress

• 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

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Capacitance measurements high level stress

• Positive shift of VFB due to trapping of
  positive charge
• A tendency to saturation in the strong
  accumulation region

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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