Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs

1. 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 June’07

2. 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 Afanas’ev et al, JAP 95 (5), pp. 2518, 2004 SiO2 - The trapping varies according to processing (wet/dry), surface preparation, and other factors MURI meeting June’07

3. TiN Gate Source Drain Body HfO2 SiOx p+ n+ n+ p-Si p-well p-substrate HfO2 based nMOSFET 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 MURI meeting June’07

4. Gate Source Drain Body p+ n+ n+ p-well p-substrate HfO2 based nMOSFET 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 MURI meeting June’07

5. Simultaneous injection + irradiation is even worse CVS and biased irradiation Hole injection - 2V bias stress HfO2/TiN Hole injection saturates after an hour TiN HfO2 SiO2 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. MURI meeting June’07

6. Simultaneous stress + irradiation hole trapping dominates with dose 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 June’07

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 June’07

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 June’07

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 June’07

10. Tunneling probability T = {1+(E02sinh2kW/4E(E0-E)}-1 where k = [2m(E0-E)/ h 2]1/2 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 MURI meeting June’07

11. HfO2 results HfO2/TiN TiN HfO2 SiO2 p-Si 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 MURI meeting June’07

12. 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 MURI meeting June’07

13. 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 MURI meeting June’07

14. 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 MURI meeting June’07

15. 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 MURI meeting June’07

16. 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 MURI meeting June’07

17. Back-up slides MURI meeting June’07

18. HfO2/TiN EC - - - - TiN HfO2 SiO2 - - + - EC Ei + - - SiO2 + - Ef + + - p-Si HfO2/TiN - + - Ei HfO2 - - - - EV p-Si Ef - + + TiN - + + + - + EV - Charge injection under stress Hole injection - 2V bias stress Electron injection + 2V bias stress Inversion Accumulation S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO2 based MOSFETs”, accepted for NSREC 2007 MURI meeting June’07

19. tphys , Bulk, interface different - radiation damage important 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 MURI meeting June’07

20. 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 MURI meeting June’07