Total Dose Response of HfSiON MOS Capacitors

1
Total Dose Response of HfSiON MOS Capacitors

• D. K. Chen, F. E. Mamouni, R. D. Schrimpf, D. M.
  Fleetwood, and K. F. Galloway
• Department of Electrical Engineering and
  Computer Science,
• Vanderbilt University, Nashville, TN

G. Lucovsky and S. Lee Dept of Physics and
Material Science, North Carolina State
University B. Jun and J. D. Cressler Dept of
Electrical Engineering and Computer
Science, Georgia Institute of Technology
2
Introduction

• Quantum mechanical tunneling significant in SiO2
• Charge trapping in HfSiON (Amorphous and
  Crystalline HfO2) G. Lucovsky et al., IEEE
  Trans. Nucl. Sci., vol. 53, pp. 3644-3648, 2006.
• Total dose irradiation and constant voltage
  stress
• TID-induced charge trapping compared to previous
  Hf silicate devices

3
Device Description

• (HfO2)0.4(SiO2)0.4(Si3N4)0.2 Low-Si3N4 K 14.6
  tox 15 nm
• (HfO2)0.3(SiO2)0.3(Si3N4)0.4 High-Si3N4 K 12.7
  tox 13 nm
• Equivalent oxide thickness 4 nm

• The samples were prepared by remote plasma
  enhanced chemical vapor deposition (RPECVD), with
  0.6 nm SiON layer Rapid thermal anneal done at
  900 oC

4
Experimental Details

• 1-MHz C-V measurement at a rate of 0.05 V/s
  Irradiated with 10 keV X-ray at a rate of 31.5
  krad(SiO2)/min
• Interface trapped charge density (Nit) unchanged
  after radiation prerad Nit 3.8 x 1012 cm-2
• Relatively small gate leakage current (lt10 pA _at_
  Vfb 1 V) unaffected by radiation

5
Improvement Relative to Hf Silicate Devices

• TID-induced charge trapping factor of 17 less
  relative to previous Hf silicate devices J. A.
  Felix et al., IEEE Trans. Nucl. Sci., vol. 49,
  pp. 3191-3196, 2002

6
Midgap Voltage Shifts

• Weak bias dependence
• Relative low mobility of holes in HfSiON similar
  to hafnium silicates J. A. Felix et al., IEEE
  Trans. Nucl. Sci., vol. 49, pp. 3191-3196, 2002
• Electron trapping
• TID-induced and substrate injection at large
  positive biases

7
Comparison of Low- and High-Si3N4 Devices

• Crystalline HfO2 in the low-Si3N4 film contains
  grain boundaries
• Negative gate bias show similar charge trapping
  levels

8
Constant Voltage Stress (Substrate Injection)

• O vacancies and interstitials in the Low-Si3N4
  film favorable for electron trapping from
  substrate injection
• Defects located near the interface an important
  source for electron trap sites

9
Constant Voltage Stress (Gate Injection)

• Level of charge trapping much less compared to
  substrate injection
• Saturation occurs much earlier than substrate
  injection case
• Electron injection from the gate and movement of
  charge centroid

10
Comparison to Hf Silicate Devices

• Electron injection balances positive charge
  trapping for positive and negative biases
• ?Not factor of 17 smaller at 1 Mrad(SiO2)
• 1.2 x 1012 cm-2 Hf silicate
• 7.1 x 1010 cm-2 HfSiON
• Reduced charge trapping indicates superior
  HfSiON film qualities (smaller bulk trap density)
  and improvements in processing

11
Conclusions

• HfSiON show drastic improvements in total dose
  irradiation hardness relative to Hf silicate
  devices
• Crystallization creates grain boundary defects
  especially vulnerable to stress-induced electron
  trapping
• Reduced charge trapping indicates smaller bulk
  defect density in HfSiON film relative to Hf
  silicates

12
Acknowledgements

• Dr. Lucovskys group at NC State University for
  providing the samples
• Air Force Office of Scientific Research (AFOSR)
  for supporting this work