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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,

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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+

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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

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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

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

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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

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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

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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 midgapVit = Vfb - Vmg …… n-type

Not (cm-2) = Cox Vot/q.A