1 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 2 Vanderbilt Institute of Nanoscale Science and Engineering 3 Department of Electrical Engineering & Computer Science 4 Department of Physics & Astronomy Vanderbilt University, Nashville, TN SEMATECH, Inc., Austin, Texas 78741, USA MURI meeting June’07 Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs

2 MURI meeting June’07 Objective  Investigate the radiation induced charge trapping in MOSFETs based on HfO 2 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 SiO 2 - The trapping varies according to processing (wet/dry), surface preparation, and other factors Literature shows electron and hole trapping in HfO 2 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

3 State-of-the-art samples, SEMATECH, Inc. p-type Si (001), with n and p-well doping (pMOS/nMOS) HfO 2 grown by ALD technique (TEMA Hf + O 3 ) Standard CMOS flow, 1000  C/10 s dopant activation anneal Post Deposition Anneal in N 2 Fabrication Sample fabrication HfO 2 based nMOSFET Gate p-substrate p-well SourceDrainBody n+n+ n+n+ p+p+ TiN HfO 2 SiO x p-Si MURI meeting June’07

4 65 nm technology node nMOSFETs with W/L = 10  m/0.25  m t phys = 7.5 nm and 3.0 nm (EOT ~ 2 nm and 0.8 nm) SiO 2 interlayer (~ 1 nm - TEM, Sematech) SamplesExperimental 10 keV X-rays, RT irradiation Function of dose (~ 10 Mrad) Function of bias Characterization done using I-V measurements HfO 2 sample details MURI meeting June’07 In-situ irradiations performed HfO 2 based nMOSFET Gate p-substrate p-well SourceDrainBody n+n+ n+n+ p+p+

5 S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 CVS and biased irradiation Simultaneous injection + irradiation is even worse MURI meeting June’07 SiO 2 p-Si ECEC EVEV EfEf EiEi HfO 2 TiN  HfO2/TiN Hole injection - 2V bias stress Accumulation Hole injection saturates after an hour D. Heh, G. Bersuker et al, APL, 88 (152907), J.F. Zhang, G. Groeseneken et al, IEEE EDL, 27(10), 2006.

6 S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 CVS and biased irradiation MURI meeting June’07 Electron injection saturates Simultaneous stress + irradiation hole trapping dominates with dose SiO 2 p-Si ECEC EVEV EfEf EiEi HfO 2 TiN  HfO2/TiN Electron injection + 2V bias stress Inversion

7 t phys = 7.5 nm S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 Total dose results (0V bias) MURI meeting June’07 Threshold voltage shifts at 0 V gate bias t phys = 3.0 nm Contribution from charge injection is negligible at zero bias Predominant net hole trapping observed

8 t phys = 7.5 nm S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 HfO 2 total dose results MURI meeting June’07 -2V bias and 0 V bias reveal net positive charge trapping +2V bias indicates a turnaround effect 0 V bias, all hole trapping radiation induced Threshold voltage shifts influenced by injection + radiation

9 t phys = 3.0 nm S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 HfO 2 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

10 Tunneling probability MURI meeting June’07 T = {1+(E 0 2 sinh 2 kW/4E(E 0 -E)} -1 where k = [2m(E 0 -E)/ h 2 ] 1/2 J leakage = 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

11 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 SiO 2 p-Si ECEC EVEV EfEf EiEi HfO 2 TiN  HfO2/TiN HfO 2 results MURI meeting June’07

12 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,  V T vs dose curves exhibit a slope only under exposure to radiation dose

13 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

14 Biased irradiations and anneals MURI meeting June’07 Irradiation (-2V)RT Annealing (0V) Partial recovery observed with a 0 V 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

15 Conclusions MURI meeting June’07 Electron and hole traps in HfO 2 - 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

16 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

17 MURI meeting June’07 Back-up slides

18 S. K. Dixit et al., “ Radiation induced charge trapping in ultra-thin HfO 2 based MOSFETs”, accepted for NSREC 2007 SiO 2 p-Si ECEC EVEV EfEf EiEi HfO 2 TiN  HfO2/TiN SiO 2 p-Si ECEC EVEV EfEf EiEi HfO 2 TiN  HfO2/TiN -- - Electron injection + 2V bias stress Hole injection - 2V bias stress AccumulationInversion Charge injection under stress MURI meeting June’07

19 MURI meeting June’07 Introduction & Motivation  Device scaling -- J (A/cm 2 ) -- replacement of SiO 2  Alternate gate dielectrics, higher   Same capacitance, higher t phys -- J (A/cm 2 ) t phys, Bulk, interface different - radiation damage important C = k  0 A/d Image courtesy.: Intel website, R. ChauWilk G.D. et al, JAP, 89 (10), 2001

20 MURI meeting June’07  N T (Threshold voltage shifts)  N T (Threshold voltage shifts) =  N ot +  N it For Si, assuming acceptor level traps below E i and donor level traps above E i are neutral, we have  V ot =  V mg, …. Interface traps neutral at midgap  V it =  V fb -  V mg …… n-type  N ot (cm -2 ) = C ox  V ot /q.A