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Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs. S.K. Dixit 1, 2 , X.J. Zhou 3 , R.D. Schrimpf 3 , D.M. Fleetwood 3,4 , S.T. Pantelides 4 , G. Bersuker 5 , R. Choi 5 , and L.C. Feldman 1, 2, 4 1 Interdisciplinary Materials Science Program - PowerPoint PPT Presentation
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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 Program2Vanderbilt Institute of Nanoscale Science and Engineering3Department of Electrical Engineering & Computer Science
4Department of Physics & AstronomyVanderbilt University, Nashville, TN - 372355SEMATECH, Inc., Austin, Texas 78741, USA
MURI meeting June’07
Radiation induced charge trapping in ultra-thin HfO2
based MOSFETs
2MURI meeting June’07
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
SiO2 - The trapping varies according to processing (wet/dry), surface preparation, and other factors
Literature shows electron and hole trapping in HfO2 based devicesFelix et al, IEEE TNS 49 (6), pp. 3191, 2002Kang et al, APL 83 (16), pp. 3407, 2003Xing et al, IEEE TNS, 52 (6), pp. 2231, 2005Afanas’ev et al, JAP 95 (5), pp. 2518, 2004
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• 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, 1000C/10 s dopant activation
anneal
• Post Deposition Anneal in N2
Fabrication
Sample fabrication
HfO2 based nMOSFET
Gate
p-substrate
p-well
Source Drain Body
n+ n+ p+
TiN HfO2
SiOx
p-Si
MURI meeting June’07
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• 65 nm technology node
• nMOSFETs with W/L = 10m/0.25m
• tphys = 7.5 nm and 3.0 nm
(EOT ~ 2 nm and 0.8 nm)
• SiO2 interlayer (~ 1 nm - TEM, Sematech)
Samples Experimental
• 10 keV X-rays, RT irradiation
• Function of dose (~ 10 Mrad)
• Function of bias
• Characterization done using
I-V measurements
HfO2 sample details
MURI meeting June’07
In-situ irradiations performed
HfO2 based nMOSFET
Gate
p-substrate
p-well
Source Drain Body
n+ n+ p+
5
S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
CVS and biased irradiation
Simultaneous injection + irradiation
is even worse
MURI meeting June’07
SiO2
p-Si
EC
EV
Ef
Ei
HfO2TiN
HfO2/TiN
Hole injection- 2V bias stress
Accumulation
Hole injection saturatesafter an hour
D. Heh, G. Bersuker et al, APL, 88 (152907), 2006.J.F. Zhang, G. Groeseneken et al, IEEE EDL, 27(10), 2006.
6S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
CVS and biased irradiation
MURI meeting June’07
Electron injection saturates
Simultaneous stress + irradiationhole trapping dominates with dose
SiO2
p-Si
EC
EV
Ef
Ei
HfO2
TiN
HfO2/TiN
Electron injection+ 2V bias stress
Inversion
7
tphys = 7.5 nm
S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
Total dose results (0V bias)
MURI meeting June’07
Threshold voltage shifts at 0V gate bias
tphys = 3.0 nm
Contribution from charge injection is negligible at zero bias
Predominant net hole trapping observed
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tphys = 7.5 nm
S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
HfO2 total dose results
MURI meeting June’07
• -2V bias and 0V bias reveal net positive charge trapping
• +2V bias indicates a turnaround effect
• 0V bias, all hole trapping radiation induced
Threshold voltage shifts influenced by injection + radiation
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tphys = 3.0 nm
S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
HfO2 total dose results
MURI meeting June’07
• Similar trend observed for the three bias conditions
• No significant difference between the bias stress and irradiated samples
Thinner dielectric traps significantly less net charge
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Tunneling probability
MURI meeting June’07
T = {1+(E02sinh2kW/4E(E0-
E)}-1
where k = [2m(E0-E)/ h 2]1/2
Jleakage = 4-5 orders of magnitude more for 3 nm as compared to 7.5 nm
We expect increased neutralization from tunneling of charges in the 3nm thick samples
S.M. Sze, Physics of Semiconductor devices, Wiley & sons, pp. 97, 1981
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n-MOSFET cross-section
Carrier injection from tunneling Si- surface condition dependent Both electron and hole trapping observed Predominant bulk hole trapping (radiation) Zero bias - radiation induced trapping
SiO2 p-SiEC
EV
Ef
Ei
HfO2TiN
HfO2/TiN
HfO2 results
MURI meeting June’07
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Biased irradiations and anneals
MURI meeting June’07
Irradiation (-2V) Annealing at 300 K (-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
MURI meeting June’07
Irradiation (-2V) RT Annealing (+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
MURI meeting June’07
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
MURI meeting June’07
• 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
MURI meeting June’07
Work supported by the Air Force Office of Scientific Research
through the MURI program
Thank you
We express our sincere thanks to SEMATECH, Inc. for providing us with the samples for these
experiments
17MURI meeting June’07
Back-up slides
18S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007
SiO2
p-Si
EC
EV
+
+
+
+
Ef
Ei
HfO2TiN
- --
---
-
-
-
++
HfO2/TiN
SiO2
p-Si
EC
EV
+
++
+Ef
Ei
HfO2
TiN- -
----
-
-
-
++
HfO2/TiN
---
Electron injection+ 2V bias stress
Hole injection- 2V bias stress
AccumulationInversion
Charge injection under stress
MURI meeting June’07
19MURI meeting June’07
Introduction & Motivation
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Device scaling -- J (A/cm2) -- replacement of SiO2
Alternate gate dielectrics, higher
Same capacitance, higher tphys -- J (A/cm2)
tphys , Bulk, interface different - radiation damage important
C = k0A/d
Image courtesy.: Intel website, R. Chau
Wilk G.D. et al, JAP, 89 (10), 2001
20MURI meeting June’07
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 midgapVit = Vfb - Vmg …… n-type
Not (cm-2) = Cox Vot/q.A
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