Radiation induced charge trapping in ultrathin HfO2 based MOSFETs

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Radiation induced charge trapping in ultra-thin
HfO2 based MOSFETs
S.K. Dixit1, 2, X.J. Zhou3, R.D. Schrimpf3, D.M.
Fleetwood3,4, S.T. Pantelides4, G. Bersuker5, R.
Choi5, and L.C. Feldman1, 2, 4 1Interdisciplinary
Materials Science Program 2Vanderbilt Institute
of Nanoscale Science and Engineering 3Department
of Electrical Engineering Computer
Science 4Department of Physics
Astronomy Vanderbilt University, Nashville, TN -
37235 5SEMATECH, Inc., Austin, Texas 78741, USA
MURI meeting June07
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Objective

• Investigate the radiation induced charge
  trapping in MOSFETs based on HfO2 as the gate
  dielectric
• Study charge trapping as a function of
• - thickness of the dielectric
• - gate bias
• Examine the effect of biased annealing in these
  devices following x-ray irradiations

Literature shows electron and hole trapping in
HfO2 based devices Felix et al, IEEE TNS 49 (6),
pp. 3191, 2002 Kang et al, APL 83 (16), pp. 3407,
2003 Xing et al, IEEE TNS, 52 (6), pp. 2231,
2005 Afanasev et al, JAP 95 (5), pp. 2518, 2004
SiO2 - The trapping varies according to
processing (wet/dry), surface preparation, and
other factors
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Sample fabrication
Fabrication

• State-of-the-art samples, SEMATECH, Inc.
• p-type Si (001), with n and p-well doping
  (pMOS/nMOS)
• HfO2 grown by ALD technique (TEMA Hf O3)
• Standard CMOS flow, 1000?C/10 s dopant
  activation anneal
• Post Deposition Anneal in N2

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HfO2 sample details
Samples
Experimental

• 65 nm technology node
• nMOSFETs with W/L 10?m/0.25?m
• tphys 7.5 nm and 3.0 nm
• (EOT 2 nm and 0.8 nm)
• SiO2 interlayer ( 1 nm - TEM, Sematech)

• 10 keV X-rays, RT irradiation
• Function of dose ( 10 Mrad)
• Function of bias
• Characterization done using
• I-V measurements

In-situ irradiations performed
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CVS and biased irradiation
Hole injection - 2V bias stress
?HfO2/TiN
Hole injection saturates after an hour
SiO2
HfO2
TiN
EC
Ei
p-Si
Ef
EV
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
Accumulation
D. Heh, G. Bersuker et al, APL, 88 (152907),
2006. J.F. Zhang, G. Groeseneken et al, IEEE EDL,
27(10), 2006.
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CVS and biased irradiation
Electron injection 2V bias stress
EC
Electron injection saturates
Ei
SiO2
Ef
p-Si
?HfO2/TiN
HfO2
EV
TiN
Inversion
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
MURI meeting June07
7
Total dose results (0V bias)
Threshold voltage shifts at 0V gate bias
tphys 7.5 nm
tphys 3.0 nm
Contribution from charge injection is negligible
at zero bias Predominant net hole trapping
observed
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
MURI meeting June07
8
HfO2 total dose results

• -2V bias and 0V bias reveal net positive charge
  trapping
• 2V bias indicates a turnaround effect
• 0V bias, all hole trapping radiation induced

tphys 7.5 nm
Threshold voltage shifts influenced by injection
radiation
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
MURI meeting June07
9
HfO2 total dose results

• Similar trend observed for the three bias
  conditions
• No significant difference between the bias
  stress and irradiated samples

tphys 3.0 nm
Thinner dielectric traps significantly less net
charge
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
MURI meeting June07
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Tunneling probability
T 1(E02sinh2kW/4E(E0-E)-1 where k
2m(E0-E)/ h 21/2
We expect increased neutralization from tunneling
of charges in the 3nm thick samples
Jleakage 4-5 orders of magnitude more for 3 nm
as compared to 7.5 nm
S.M. Sze, Physics of Semiconductor devices, Wiley
sons, pp. 97, 1981
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HfO2 results
?HfO2/TiN
SiO2
p-Si
HfO2
TiN
EC
Ei
n-MOSFET cross-section
Ef

• Carrier injection from tunneling
• Si- surface condition dependent
• Both electron and hole trapping observed
• Predominant bulk hole trapping (radiation)
• Zero bias - radiation induced trapping

EV
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Biased irradiations and anneals
Annealing at 300 K (-2V)
Irradiation (-2V)

• Post-irradiation negative 2V bias annealing
  flattens the curve indicating
• no h injection under bias stress
• Further proves that following initial carrier
  injection, ?VT vs dose curves
• exhibit a slope only under exposure to
  radiation dose

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Biased irradiations and anneals
RT Annealing (2V)
Irradiation (-2V)

• Substantial recovery observed with a 2V
  annealing gate bias due to e- injection
• Additional electron trapping highlights the
  problem of switch bias anneal as discussed
  previously by Xing et al.

Xing et al, IEEE TNS, 52 (6), 2005
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Biased irradiations and anneals
Irradiation (-2V)
RT Annealing (0V)

• Partial recovery observed with a 0V annealing
  gate bias
• No additional voltage shifts observed for time
  scales of up to 13 hours indicating the existence
  of residual positive charge in the oxide

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Conclusions

• Electron and hole traps in HfO2 - Constant
  Voltage Stress and Irradiation experiments
• Combined Constant Voltage Stress Irradiation
  is detrimental for the device operation
• The thinner samples (3 nm) show negligible
  shifts relative to the 7.5 nm samples
• - Reduced density hole traps due to net
  reduced volume
• - Increased tunneling induced neutralization
  in thinner samples

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Acknowledgements
Work supported by the Air Force Office of
Scientific Research through the MURI program
We express our sincere thanks to SEMATECH, Inc.
for providing us with the samples for these
experiments
Thank you
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Back-up slides
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Charge injection under stress
Hole injection - 2V bias stress
Electron injection 2V bias stress
Accumulation
Inversion
S. K. Dixit et al., Radiation induced charge
trapping in ultra-thin HfO2 based MOSFETs,
accepted for NSREC 2007
MURI meeting June07
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Introduction Motivation
Image courtesy. Intel website, R. Chau
Wilk G.D. et al, JAP, 89 (10), 2001

• Device scaling -- J (A/cm2) -- replacement of
  SiO2
• Alternate gate dielectrics, higher ?
• Same capacitance, higher tphys -- J (A/cm2)

C k?0A/d
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?NT (Threshold voltage shifts)
?NT (Threshold voltage shifts) ?Not ?Nit
For Si, assuming acceptor level traps below Ei
and donor level traps above Ei are neutral, we
have ?Vot ?Vmg, . Interface traps neutral at
midgap ?Vit ?Vfb - ?Vmg n-type
?Not (cm-2) Cox ?Vot/q.A
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