FELIX,J.A.

FLEETWOOD,D.M.

RIEWE,LEONARD CHARLES

WINOKUR,PETER S.

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sam2L500-cL z74a cEffects of Alternating Bias Irradiation on Defects in MOS DevicesJ. A. Felix and D. M. FleetWood, Vanderbilt University, EECS Dept., PO Box 92, Station B, Nashville, TN 37235 USAL C. Riewe and P. S. Winokur, Sandia National Laboratories, PO Box 5800, Albuquerque, NM 87185-1083 USAA comprehensive model ~of MOS radiationresponse must apply to switched and alternating biasexposure as well as static bias irradiation [1]. Pre-vious work has shown that defect buildup andannealing during alternating and switched biasirradiation is often qualitatively different than atsteady-state bias [2-4]. For example, changing thebias from positive to zero or negative values anddelivering an additional dose of radiation can lead asignificant reduction in net oxide-trap charge due toradiation-induced charge neutralization [4,5]. More-over, enhanced interface trap buildup has beenobserved under some alternating bias conditions [2-4]. These effects have been characterized at a pheno-menological level, but the underlying defect buildupand annealing processes are not well understood.To date, alternating and switched bias irradia-tions have been assessed via standard capacitance-voltage (C-V) and current-voltage (I-V) techniquesthat are sensitive only to the net oxide-trap chargeand interface traps. In this summary, we use ther-mally stimulated current (TSC) for the first time toevaluate defect buildup during positive-to-negativeand positive to O V alternating bias irradiation ofMOS devices. This enables contributions of positiveand negative charge to net oxide-trap charge to beseparated. Surprisingly similar levels of trappedelectrons are observed in the near-interracial oxidefor these two types of AC biases. However, there aredifferent levels of trapped positive charge. Moreinterface traps are found for positive to O V switch-ing than positive to negative switching. Implicationsfor charge trapping and recombination are discussed.The devices used here were 0.004 cm2 n-sub-strate capacitors with 45 nm radiation-hardenedoxides. These devices were chosen because (1) theyare easier to characterize via TSC than thinneroxides [6], (2) their response to static bias irradiationhas been well characterized [7,8], and (3) thedominant defects in these devices are also thedominant defects in high-quality thermal oxides(thicknesses from 6 to 1000 nm) used in previousstudies of MOS radiation response and high-fieldelectrical stress [7,9]. Hence, defect generation andannealing processes in these devices are expected tooccur quite generally in MOS oxides.Figure 1 shows TSC measurements for (a)steady-state and (b) 1 kHz 5V/OV and 5V/-5V ACbias irradiations of capacitors to 2 Mrad(SiOJ with10-keV x rays at a dose rate of 900 rad(SiOJ/s. Theshapes of the TSC curves reflect the energydistributions of the trapped positive charge; the totaltrapped charge density zlNP can be estimated fromthe areas under the curves [7]. The main peak in theTSC curve is evidently associated with holesdetrapping from E’ centers, although a contributionof transporting H+ cannot be ruled out.Net oxide-trap charge densities ANOZ areestimated from fi,dgap voltage shifts, and interface-trap charge densities zlNi, are estimated frommidgap-to-flatband stretchout of 1 MHz C-V curves.(a)2.50.50A-k- Ov-E- 5V-o--wo 100 200 300Temperature (°C)(b)20.50o 100 200 300Temperature (“C)Fig. 1. TSC vs. temperature and bias during irradiation for 0.004cmz n-substrate capacitors irradiated to 2 Mrad(Si02) with 10-keV x rays at -900 rad(Si02)/s. The TSC measurements wereperformed at a bias of –10 V, and a ramp rate of - 0.11°C/s.The difference between the total trapped positivecharge (from TSC) and net oxide-trap charge (fromC-V) allows one to estimate the density of trappedelectrons AN. in the near-interracial Si02 [7]. Chargedensities for the irradiations of Fig. 1, as well as 100Hz AC bias irradiations, are shown in Table 1. Asexpected, more radiation-induced trapped positivecharge is observed for 5 V bias than for O V bias,due to the higher charge yield under positive bias[10]. Negative bias irradiations lead to still lesstrapped positive charge because holes aretransported away from the critical Si/SiOz interface.Alternating bias irradiations in Fig. l(b) showsimilarly shaped TSC curves, but the values of dNPdiffer widely among the various bias conditions. InTable 1 it is seen that the values of ZINP for the5V/OV AC bias cases lie between the correspondingvalues for the 5 V and O V static bias cases, and the5VI-5V AC bias cases lie between the O V and –5 Vstatic bias cases. Results at other frequenciesconfirm these trends, as we will discuss at the SISC.Table 1. Summary of trapped-charge densities after 2Mrad(SiOJ steady state or square-wave AC bias x-rayirradiation, for the measurements of Fig. 1, and for 100 Hz ACbias. AH charge densities are quoted in multiples of 1012cm-2.Bias5 V static5/0: 1 kHz5iO: 100 Hzov static5/-5: 1 kHz5/-5: 100 Hz-5 V staticANP ANO, ANti2.95 1.43 1.132.77 1.02 0.922.96 1.09 0.942.56 0.71 0.912.19 0.38 0.632.16 0.51 0.530.53 0.18 0.23ANe1.521.751.871.841.821.640.36Table 1 also shows that significantly less inter-face trap buildup occurs for 1 kHz 5V/-5V AC biasirradiations than for 1 kHz 5V/OV AC exposures. Ata given frequency, values of ZINPand dNOt follow thesame trend. During the steady-state 5 V and thepositive-bias portions of the AC bias irradiations,radiation-induced holes and other positively chargespecies (e.g., H+) transport toward the Si/Si02interface. That this transport is more efficientlyreversed during the 5V/-5V AC bias irradiationsthan the 5V/OV irradiations accounts for most of theobserved results for AVP, dNOfi and zliVil. However,the values of zIN. are nearly constant for all switchedbias irradiations. We find this result to be surprising,because one might intuitively expect that electronswould be swept out of the oxide more efficientlyduring the low phase of 5VI-5V AC bias irradiationthan 5V/OV irradiation, due to the more negativeelectric field near the interface in the 5V/-5V case.Indeed, preliminary results appear to show areduction in trapped electron density at 5V/-5V biaswhen the frequency is very low (i.e., below 1 Hz).Hence, these types of measurements may allow oneto measure a characteristic detrapping rate for atleast some compensating electrons in the SiOz.That AC bias and O V irradiations show suchsimilar levels of electron trapping strongly suggeststhat most trapped electrons in these devices do nottunnel in from the Si, as commonly assumed inmodels of trapped positive charge compensation[7, 11]. In contrast, these results seem much moreconsistent with the idea that most of thesecompensating electrons originate within the bulk ornear-interracial Si02. This in turn implies that deepelectron trapping likely has been mistaken forrecombination events iri prior work. This conclusionis especially significant because hole and electrontrapping are known to depend significantly ondevice processing [7], while recombination eventsare considered to be process independent [10,12]. Irlcontrast, trapping properties of oxides are typicallyindependent of radiation energy (at least above - 1keV), while recom-bination rates are strongly energydependent [10, 12]. At the SISC, we will show howthese results may help explain some discrepancies inrecombination data in the literature.AcknowledgmentsThis work was supported by the US Department of Energy(DOE], the US Navy, and the Defense Threat Reduction Agency.Sandia is operated for the DOE by Sandia Corporation, a Lockh-eed Martin Company, under Contract DE-AC04-94AL85000.References[1] P, Pavan, et al., JEEE Trans. Nucl. Sci. 41,2619 (1994).[2] P. V. Dressendorfer, et al., JEEE Trans. Nucl. Sci. 2S, 4281 (1981).[3] T. D. Stanley et al., IEEE Trans. Nucl. Sci. 32,3982 (1985).[4] D. M, Fleetwood, et rd., IEEE Trans. Nucl. Sci. 37,1806 (1990).[5] D. M. Pteetwood, J. Appl. Phys. 67,580 (1990).[6] D. M. Fleetwood, et at., MicroeL Reliab. 39, 1323 (1999).[7] D. M. Fteetwood et al., JEEE Trans. Nucl. Sci. 39,2192 (1992).[8] D. M. Fteetwood et at., fEEE Trans. Nucl. Sci. 45,2366 (1998).[9] D. M. Fteetwood aud N. S. Saks, J. Appl. Phys. 79,1583 (1996).[10] M. R. Shaneyfelt et al., LEEE Trans. Nucl. Sci. 38,1187 (1991).[11] A. J. Lelis et al,, IEEE Trans. Nucl. Sci. 36,1808 (1989).[12] C. M. Dozier and D. B. Brown, IEEE Trans. Nucl. Sci. 28,4137(1981); T. R. Oldham and J. M. McGarrity, tEEE Trans. Nuc1.Sci. 30,4377 (1983).(’XT0420000s71DISCLAIMERThis repo~ was prepared as an account of work sponsoredby an agency bf the United States Government. Neitherthe United States Government nor any agency thereof, norany of their employees, make any warran~, express orimplied, or assumes any legal liability or responsibility forthe accuracy, completeness, or usefulness of anyinformation, apparatus, product, or process disclosed, orrepresents that its use would not infringe privately ownedrights. Reference herein to any specific commercialproduct, process, or service by trade name, trademark,manufacturer, or otherwise does not necessarily constituteor imply its endorsement, recommendation, or favoring bythe United States Government or any agency thereof. Theviews and opinions of authors expressed herein do notnecessarily state or reflect those of the United StatesGovernment or any agency thereof.DISCLAIMERPortions of this documentin electronic imageproduced from thedocument.may be illegibleproducts. Images arebest avaiiable original

Effects of Alternating Bias Irradiation on Defects in MOS Devices

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Effects of Alternating Bias Irradiation on Defects in MOS Devices

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