IEEE Transactions on Nuclear Science, Vol. NS-34, No. 6, December 1987

SINGLE-EVENT, ENHANCED SINGLE-EVENT AND DOSE-RATE EFFECTSWITH PULSED PROTON BEAMS*

M.A. Xapsos11 L.W. Massenqill3, W.J. Stapor1, P. Shapiro2,A.B. Campbell , S.E. Kerns , K.W. Fernald and A.R. Knudson'

ABSTRACT

Pulsed proton beams can create various upseteffects in memory circuits. The response of theIDT 6116RH static RAM to these effects has beeninvestigated over a range of flux extending from asingle-event dominated region to a dose-ratedominated region. A surprisingly largeintermediate region has been observed for thefirst time in which nuclear reactions add tobackground ionization to produce synergistic upseteffects. From the viewpoint of memory reliability,the threshold of this synergistic region appearsto be more important than the dose-rate upsetthreshold. Comparisons to gamma/electron dose-rateand single-event upset measurements are presented,as well as analysis and modelling of the results.

INTRODUCTION

Single-event and dose-rate upset experimentshave traditionally been performed and modelled asindependent phenomena. Single-event upsets arecharacterized by the interaction of a singleionizing event at a single sensitive circuit node.By contrast, dose-rate radiation, due to itshighly uniform nature, simultaneously affects manynodes of a circuit. While both types of radiationcan affect the same sensitive volumes, thestatistical nature of single-events compared tothe uniform nature of the dose-rate effects leadsto very different transient upset behavior. Forthis reason, detailed studies of synergism betweenthese effects are lacking in current literature.The focus of this work is to provide such a studyfor a particular static RAM by examining itsresponse to proton beams over broad ranges ofdose-rate and pulse lengths. The resultingtransient upset responses are treated in a moregeneral and cohesive way than has been done in thepast.

The following points concerning proton upsetmechanisms in the IDr static RAM are addressed inthis work:

1) Evaluation of single-event upset (SEU)effects, dose-rate effects, and synergistic orcombined effects.

2) Analysis of the main factors contributingto dose-rate upset over a broad range of pulselengths.

1 Naval Research Laboratory, Washington, DC20375-5000

2 Sachs/Freeman Associates, Inc., Landover, MD20785-5396

3 North Carolina State University, Raleigh, NC27695-7911

* This work is partially supported by the NRL/ONTSpacecraft Survivability/Vulnerability Program,the NRL/SDI LTH-4 .2 Program, and the NR/SDIPassive Survivability Program.

3) Whether or not the proton andgamma/electron dose-rate upset thresholds areequivalent.

4) Evaluation of the possibility thatmultiple-event upsets occur.

EXPEPIMNAL PROCEDJRES

The facilities and corresponding beamparameters used for the static RAM upsetmeasurements are listed in Table 1. The pulsedproton and electron beans were obtained at theBrookhaven National Laboratory (BNL) RadiationEffects Facility and the Naval ResearchLaboratory (NRL) LINAC Facility, respectively.SEU measurements were made both at the HarvardUniversity Cyclotron Facility and theUniversity of California, Davis Crocker NuclearLaboratory Cyclotron.

TABLE 1. Experimental Facilities and BeamInformation for SRAM Upset Measurements.

Facility: Particle: Dose-Rate Pulse Length(Rad(Si)/s): (us):

BNL LIIAC 175 MeV p 3x104-3x107 6-440

NRL INAC 25 MeV e lx108-5x108 .044-1.4

Harvard 149 MeV pCyclotron

U.C. Davis 63 MeV pCyclotron

200 200

200 continuous

Figure 1 shows the experimental set-up atthe BNL facility. Use of a high atomic numberscatterer at the beam exit is critical toensure beam uniformity across the device undertest (DUT). Used in this manner, the scattereralso reduces the beam intensity to a desiredlevel with the proper choice of distance to theDUT. The beam was collimated with a 2-inchdiameter aperture after which a PIN diode waspositioned for dosimetry. The PIN diode and theDUT were laser-aligned with the diode directlyin front of the [UT. The RAMs were exercisedfrom an adjacent experimenter's room with acomputer-controlled tester which has beenpreviously described.1 The PIN diodemeasurements are described in the followingsection on dosimetry. Typically, upsetmeasurements began at dose-rates which wereclearly in the single-event dominated regime.Measurements were then performed atsuccessively higher dose-rates by focussing thebeam spot at the beam exit with the magneticquadrupole shown in Figure 1. This procedure

0018-9499/87/1200-1419$01.00 c 1987 IEEE

1419

was followed until beam intensities were

reached that were clearly in a dose-rate

dominated regime. This succession of

measurements would all be performed at a fixedpulse length. Care was taken to see that the

RAMs, which are moderately radiation hard, did

not receive sufficient exposure to cause

measurable damage. Each RAM received less than

4 kRad(Si) for all combined LINAC testing.

Printer

Transient

DigitizerL L.

SRAM

XPINI .............. DjQOd

Test System

CPU/Disk Drives

Terminal Printer

Experimental Setup

2' Collimator

- .200 M.V p

0.25' Ta ScattererShielding

Focussing

Quadrupole

Figure 1. Experimental set-up for pulsed protonexperinents- at BNL. The transient digitizer and

the test system are set up in an experimenter'sroom.

For the electron LINAC testing at NRL, a

sinilar experimental procedure was used, exceptthat the dose-rate for each exposure was

determined by the distance the RAM and thediode, aligned on a moveable table, were fromthe scattering material. Scattering materialfor this testing is also useful for avoidingbeam non-uniformity effects, although thechoice of a low atomic number material is

adequate due to the relative ease at whichelectrons scatter.

SEU measurements with protons were

performed at the Harvard and Davis cyclotronfacilities. The procedures for these

measurements have been described elsewhere. 1

All upset measurements were made with the

same batch of Integrated Device Technology6116RH 16k static RAMs. These devices are

arranged 2kx8 with NMOS/resistive-load memorycells ard C4OS peripherals.

against accelerator employed dosimetry (TLDs,

activation foils, etc) by averaging a number of

calibration measurements. This minimizesstatistical fluctuations associated with basingeach upset measurement upon an individual TLDor activation foil. The approach also avoidserrors resulting from the variation of TiLDsensitivities from batch to batch, as can occur

as a direct result of their irradiation.2 Andfinally, use of a single group of accuratelycalibrated diodes avoids uncertainties involvedin a comparison of the independent methods ofdosimetry employed at each LINAC, each of whoseabsolute uncertainty may be 10 to 20%.Additionally, the fast rise time (a fewnanoseconds) of the diodes was exploited tomonitor the pulsed beam shape and accuratelydetermine the pulse length. They were used in a

resistor-sampling photocurrent circuit, withthe output observed on either a 200 MHz or a

400 MHz computer-controlled transientdigitizer. Figure 2 shows a PIN diodephotocurrent response to the indicated dose-rates of both 175 MeV protons obtained at BNLand approximately 25 MeV electrons obtained atNRL. The proton measurements were performed bycalibrating the diode against 12C activation

foils. The electron measurements were performedby calibrating the same diode against TLDs. The

excellent agreement demonstrates that thecarrier generation rate per Rad(Si) is the same

for both lightly ionizing radiations to a highdegree of accuracy. More importantly, it showsthat this is an excellent dosimetry techniquefor precisely comparing the memory response atthe two facilities. Care was taken, by re-

checking diode calibrations at the end ofexposure periods, to ensure that effects suchas displacement damage did not shift the diodecalibrations. For the SEU measurements atHarvard and Davis, cross-calibrations with theBNL and NRL facilities were performed using

activation foils and TLDs.

PIN Diode Calibration1000.0

* 25 MeV electrons

175 MeV protons

E40c

0a-J-

0c0

DOSIMEIRY

Because of the importance of performingaccurate inter-facility comparisons of

measurements, careful consideration was givento available dosimetry techniques. PIN diodes,which are known to respond linearly to ionizingradiation over several orders of magnitude of

dose-rate, were chosen for the inter-facilitypulsed beam dosimetry comparisons. The diodes

can be calibrated to a high degree of accuracy

104

110.0 -

0.1 -. In 10.00.-0.1101QID .

Dose Rate (MRad(Si)lsec)

Figure 2. Typical PIN diode calibration. The

diode response was calibrated against 12C

activation foils for protons and TlDs for

electrons.

1420

i

Ae n

Q.1

1421

RESULTS AND MODELLING

Figure 3 is a schematic of the IDT 6116RAM cell in a static state. The large loadresistors (130 Gohms) , which maintain a stablestate under the influence of leakage currents,give this cell an extremely long recovery time.An estimate of the time constant for re-supplyof charge through the load resistors, using asimulated nodal capacitance, is about 8milliseconds. Therefore, milliseconds areneeded to restabilize a perturbed cell. Themost significant photocurrents are shown in theFigure. They are those generated at the drain,p-well junctions (Pl and P2) and at the p-well,substrate junction (PW).

Schematic of 6116 RAM Cell

rate. The upset cross section level shown forthe Davis facility was obtained byextrapolating the measured cross section for 63MeV protons to the value predicted at 175 MeVby the model of Bendel and Petersen.3 Themeasured cross section level displayed for theHarvard facility is for 149 MeV protons and isexpected to be close to that of 175 MeVprotons.3 The upset levels for the Davis andHarvard facilities are each for a dose-rate of200 Rad(Si)/s, and are not associated with thex-axis of the Figure. Since these SEUmeasurements correspond closely to those lowdose-rate cross sections (at each pulse length)measured at BNL over the full range of pulselengths, it is clear that the low dose-rateexposures at BNL are in the single-eventdominated regime.

Upset Cross Sections vs. Proton Dose Rate

~~c Cot t o~~~~~~~~~PW

Pi|( NI "22T P2

vssFigure 3. Schematic of the IDT 6116 RAM cell ina static state. The n-channel FETs are denotedby NI and N2, while the corresponding loadresistors and simulated nodal capacitances arealso shown. P1 and P2 represent photocurrentsgenerated at the drain, p-well junctions, andPW represents the photocurrent at the p-well,substrate junction.

Synergistic Upset Effects

Static RAMs have displayed upsetsensitivity to both a single-event proton and atraditional dose-rate (gamma-dot) environment.Using high-intensity, pulsed proton beams, botheffects are present simultaneously. Synergybetween the two effects can be evaluated byanalysis of the transition region between theSEU dominated regime (low flux) and the dose-rate dominated regime (high flux).Specifically, the synergy referred to is anenhanced SEU vulnerability due to thesimultaneous presence of a backgroundionization rate.

Measurements performed at Harvard andDavis at a very low dose-rate were used toestablish a reference upset level that isconsistent with a purely SEU environment.Comparisons to measurements made at BNL for thelongest and shortest pulse lengths availableare shown in Figure 4. Plotted is the IDT6116RH upset cross section (number of upsetaddresses per unit proton fluence) vs. dose-

10E4 10E8Proton Dose Rate (Rad(Si)/sec)

Figure 4. Measured upset cross sections as afunction of proton dose-rate for 6 and 440microsecond pulses obtained at BNL. The twoindicated constant levels were obtained fromSEIJ measurements made at Harvard and UC Davis,and have no association with the x-axis. IheDavis level is an extrapolated one based on themodel of reference 3.

Figure 5 displays a plot of the number ofupset addresses per proton pulse vs. dose-ratefor 6 microsecond pulse lengths obtained atBNL. Measurements performed at the lowest dose-rates show numbers of upsets that areconsistent with a totally single-eventscenario. If all upsets in the Figure were duestrictly to single events, their increase withdose-rate would be linear since each protonwould interact independently of all others.This clearly is not the case. At the otherextreme, the rapidly rising number of upsetswith only a very small increase in dose-rate(note the two upper-right data points) isconsistent with a dose-rate upsetinterpretation. However, the complete curve cannot be interpreted as a totally dose-rateeffect because the increase in the number ofupsets is too gradual. It is thereforeplausible that between the upper limit of theSEU dominated regime (3x106 Rad(Si)/s) and thelower limit of the dose-rate dominated regime(3x107 Rad(Si)/s) exists a region in which a

combination of these two effects results inintermediate upset levels.

Comparison of No. of Upsets to Qef,1000 0.14

0.12

- 0.10100

0.08

0.06

10 - Upsets 0.04

0.02

1 0.001OE5 10E6 10E7 10E8

0

t

Dose Rate (Rad/sec)

Figure 5. Comparison of number of upsets perproton pulse to Qefff both as a function ofdose-rate, for 6 microsecond pulses. Qeff wasobtained using a TRIGSPICE simulation.

Consider an effective critical charge,Qeff, defined as:

Qeff = Qc - Q(P)

where Qc is the conventional critical chargefor upset and Q (p) is the dose-rate dependentshift produced by background ionization. Theeffective critical charge can be interpreted asthat which must be deposited in the sensitivevolume by nuclear reaction products to cause amemory upset. For low dose-rates, Q(p) = 0, andQeff is simply the charge required to produce aconventional SEU. With increasing dose-rate,background ionization can begin to affectcharge stored at information nodes withoutitself changing the state of the memory cell.Then, 0 < Q(P) < Qc, and the effective criticalcharge falls below Qc. It follows that anuclear reaction whose products alone do notdeposit enough charge for upset can combinewith the proton-induced photocurrents toproduce an upset. The result is an enhanced SEUvulnerability due to the presence of backgroundionization. As the dose-rate is furtherincreased to the point where Q (P) = Qc, theeffective critical charge falls to zero, andupsets are produced by photocurrent alone.

Displayed on the right ordinate axis ofFigure 5 is the simulated effective criticalcharge as a function of dose-rate for the same6 microsecond pulse length. The simulationswere performed with TRIGSPICE, a SPICE-likecircuit simulator developed for the study ofdose-rate effects. They account only for the"local" photocurrents generated at the drain,p-well junctions. As can be seen, the curve isflat at low dose-rate, and its value at the

3X106 Rad(Si)/s measurements is close to theconventional critical charge of 0.14 pC. Thecurve intercepts the abscissa at 3.5x107Rad(Si)/s, corresponding to an effectivecritical charge of zero. At this point, upsetsare dominated by photocurrents, and thecorrelation with the experimental measurementof 3x107 Rad(Si)/s is good. Between these twoextremes, the proton-induced photocurrent atthe "high" drain node partially depletes thestored charge at the node. The result is anincreased susceptibility of the cell to single-event effects, manifested in a circuit contextas a reduction in the effective criticalcharge. Since not all nuclear reactions causeupsets due to the range of energy the productsmay deposit in the sensitive volume, areduction in Qeff implies a higher fraction ofthe reactions cause upsets. The region wherethese effects are observed to take placeexperimentally (approximately between 3xlO and3x107 Rad(Si)/s) corresponds well with thesimulated degradation of Qeff in Figure 5.

Figure 6 is a comparison of the number ofupsets vs. dose-rate for the same 6 microsecondproton pulses and 1.4 microsecond electronpulses. These are the two closest pulse lengthsavailable at the two LINAC facilities at thetime of this work. Synergistic upset effectsare not seen for the electrons due to the lackof single-event nuclear reactions of largeenough cross section.4 In this case, there is astep-like increase in the number of upsets,which is characteristic of a purely dose-ratephenomenon.

Number of Upsets vs. Dose Rate

U,

U)OL0~

0

nEz

Dose Rate (Rad/sec)

Figure 6. Number of upsets per pulse vs dose-rate for 6 microsecond proton pulses and 1.4microsecond electron pulses. The extrapolationof the line which the proton data approachesasymptotically indicates a dose-rate upsetthreshold of 3x107 Rad(Si)/s.

1422

S

a)

0.0

Ez

10E9

The Dose-Rate and Enhanced SEU Thresholds

In previous modelling efforts, it has beenshown that two important factors contributingto dose-rate upset are local photocurrent andrail span collapse.5,6 Local photocurrents arethose which directly affect the internal memorycell charge stored at information nodes, asoccurs with photocurrents P1 and P2 in Figure3. An upset can be caused by the photocurrentat the side of the cell with the high state bydepleting enough charge to cause the storedinformation to become unreliable. Rail spancollapse is associated with photocurrent flowaway from individual RAM cells into the VDD andVSS power supply runs, which results in areduction of available supply voltage to thecells. This occurs for all photocurrents inFigure 3, but is dominated by that denoted byPW because of its comparatively largemagnitude.

associated with individual cells. For shorterpulses (beginning with the 1.4 microsecondpulse lengths) upsets are grouped at one edgeof the memory array. Furthermore, as the dose-rate is increased (at fixed pulse lengths) thearea of upsets progresses across the array asis consistent with a rail-span collapsemechanism.7

1-1

0

5)(U)

cocc

a)a(U)

(a

CMa:

Evaluation of the dose-rate upsetthreshold for protons is not as clear cut asfor the case of electrons. Note, however, thatin Figure 6, for the high proton dose-rates thenumber of upsets approaches an asymptote whichparallels the behavior of the electron data.The asymptote is displayed in the Figure. Ifthis asymptote is extrapolated back to itsintersection with the x-axis, it is plausiblethat this value of the x-axis represents thedose-rate at which one upset occurs due tophotocurrents alone. This is, therefore, areasonable procedure to determine the dose-rateupset threshold for protons. In the Figure,this value turns out to be 3x107 Rad(Si)/s.

Figure 7 shows the dose-rate upsetthresholds plotted vs. pulse length for allLINAC testing. The dose-rate upset thresholdsfor protons were determined as described in thepreceding paragraph. The Figure shows twodistinct regions of behavior, the transitionoccurring at a pulse length slightly greaterthan one microsecond. This difference resultsfrom the relative importance of two competingfactors, each of which can control the dose-rate upset threshold in its own regime. Forlong pulse lengths and low dose-rates, the longcharge re-supply time through the large loadresistors allows charge on the "high" node tobe depleted by the drain photocurrent withoutappreciable recovery during the radiationpulse. The dose-rate upset threshold thereforedepends on the total dose delivered by theradiation pulse. This can be seen in Figure 7,as the upset thresholds for long pulses fall ona "constant dose" line. The divergence of theexperimental data from the "constant dose" lineat short pulse lengths is consistent with theon-set of rail-span collapse. The recovery timeof the circuit from photocurrent effects in theVSS supply runs does not depend on re-supply ofcharge through the large load resistors. Sincethe memory cell can recover much more quicklyfrom this, these upset effects do not show upuntil comparatively high dose-rates arereached. This interpretation is stronglysupported by analysis of the distribution ofupset addresses in the memory array. For longpulses; the upsets are random and evenlydistributed, indicating the effect is

Dose Rate Upset Threshold vs. Pulse Length1OE9 -

* M~......- simulated upset threshold

10E8 v Xv(drain photocurrent only)

10E8-electron pulses

1OE7

N,10E6 proton pulses

10OE50.01 0.10 1.00 10.00 100.00 1000.00

Pulse Length (10E-6 sec)

Figure 7. Dose-rate upset threshold vs. pulselength for all LINAC data. The dose-rate upsetthresholds for protons were determined by theextrapolation procedure shown in Figure 6 anddescribed in the text. The simulated dose-ratethreshold was performed with TRIGSPICE andaccounts only for photocurrent generated at thedrain, p-well junctions.

This interpretation is furthersubstantiated by consideration of the simulateddose-rate upset threshold shown in Figure 7.The simulation, performed with TRIGSPICE,accounts for drain photocurrent only. The dose-rate threshold is taken to be the point atwhich the high node voltage drops to 0.8 V, thethreshold voltage for the n-channel FET. Atthis point, both transistors are off, and thestored information becomes unreliable. As canbe seen, agreement at long pulse lengths isexcellent, but the simulation diverges from thedata at short pulse lengths, indicating the on-set of some other factor. Complete powerbussing distributions were not available, butpreliminary rail-span collapse simulations wereconsistent with the above interpretation. Thetwo distinctive regions in Figure 7, then,result from photocurrents originating atdifferent circuit junctions which results inthe COmpetition of a local (drain) photocurrentupset mechanism and a rail-span collapse upsetmechanism.

Of more siOnificance than the proton dose-rate upset threshold is a threshold which can

be used to define the enhanced SEU orsynergistic region. This threshold can bedefined as the dose-rate at which the number ofupsets begins to increase non-linearly withdose-rate. In terms of the effective critical

1423

charge defined in the previous section, theenhanced SEU threshold corresponds to the pointat which Qeff first falls below theconventional critical charge. It signifies thepoint at which photocurrents begin tomeasurably degrade the circuit reliability.From the viewpoint of memory reliability, thelower threshold is more important than thedose-rate threshold, since the lower one iswhere significant numbers of upsets begin tooccur. These two experimentally determinedthresholds, shown in Figure 8, separate theplot of dose-rate vs. pulse length into threecharacteristic regions. The lower left regionis where SEU effects dominate, and the numberof upsets per beam pulse is proportional to thebeam flux (to within statistical errors). Thecentral area is characterized by a non-linearincrease in upsets vs. flux, resulting fromnuclear reactions combining with the loweringof Qeff by background ionization. The upper

right area is where dose-rate effects dominate.Here, photocurrents alone are sufficient tocause upset, so nuclear reactions which occur

are uninportant.

TRANSIENT UPSET MECHANISMS

However, close examination of Figure 7 stronglysuggests that the p-dot and gamma-dot upsetthresholds would be equal at a given pulselength. This results from the considerationthat a straight line extrapolation of theproton data connects smoothly to the electrondata at the 1.4 microsecond pulse length. Ittherefore appears that there is no significantdifference between the two ionization-induceddose-rate thresholds.

Evaluation of the Possibility of Multiple-EventUpsets

The concept of a multiple-event upset(MEU) is related to the statistical nature ofrandomly arriving particles at a microscopiclevel. Suppose that due to statisticalfluctuations, a discrete number of particlesthat significantly exceeds the average numberarrive at a sensitive node within the cellrecovery time. If the charge deposited bydirect ionization equals or exceeds thatrequired for upset, this is called a multiple-event upset. The discrete and statisticalnature of this mechanism differentiates it fromphotocurrent effects, which result from(nearly) uniform irradiation of the entirecircuit.

10E8 -

111 ~~~~~DOSE-RATE10< ^ threshold

w 10E6 - * ENHANCED SEU

cc enhanced SEU

u, 10E5 lhreshold SEU

0

10 100

PULSE LENGTH (JASEC)

Figure 8. Transient proton upset mechanisms forthe IDT 6116 static RAM as a function of dose-rate and pulse length. The two threshold linesseParate the figure into three regions - onepurely SEUt, one purely dose-rate, and theintermediate synergistic region of enhancedSEU.

Comparison of p-dot and Gaa-dot UpsetThresholds

The behavior difference of long and shortpulses shown in Figure 7 has little, ifanything, to do with differences betweenprotons and electrons. The divergence from thedose-dependent behavior at shorter pulselengths has been explained as a circuit effect.Unfortunately, the two LINAC facilities did nothave overlapping pulse lengths at the time ofthis work, so a direct comparison of the dose-rate upset thresholds was not possible.

Suppose N is defined as the number ofparticle strikes at a particular sensitivevolume within the recovery time of the memorycell. Assuming that the probabilitydistribution of N follows Poisson statistics,the pertinent parameter for MEEU is the standarddeviation, 41N, which indicates the magnitude ofthe statistical fluctuations. For changes indose-rate which also affect the upsetthreshold, the coefficient of variation, 4T/N,indicates the importance of such fluctuations.This leads to the normalized distributions ofFigure 9, which depicts the probability of Nparticle strikes at a sensitive volume vs. Nfor average N of 10, 100 and 1000. In thepresent work, about 1600 175-MeV proton strikesare required at the sensitive drain node tocause a direct ionization bit-flip from the "Ollto the "1" state. This number was calculatedfrom the measured proton fluence at the upsetthreshold (see Figure 7) and knowing thesensitive junction area. It ixrplicitly accounts

for charge collected by both drift anddiffusion, the latter of which may be importantbecause of the long recovery time of thismemory cell. Since the coefficient of variationis small for this number of proton strikes, MEUis an improbable upset mechanism in this work.The data can be well accounted for usingtraditional dose-rate concepts, so an MEUinterpretation is unnecessary. As is indicatedin Figure 9, for large N, the irradiation of

the array of memory cells becomes uniform, andthe multiple-event concept approaches that of a

photocurrent.

1424

1425

Probabilities of N hits in a cell vs. Nfor N.,. = 10, 100, and 1000

14

12

10 ' N.,.= 1000

Z 8xz 6,

4 ",*- / N. = 100

2 ,.. N.,, 10

0 F

0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

N/N.,

Figure 9. Probabilities of N particle strikesin a cell vs. N for average N values of 10, 100and 1000, assuming Poisson statistics.

CONCLUSIONS

The response of the IDr 6116RH static RAMto proton beams has been investigated over wideranges of dose-rate and pulse length.Synergistic upsets have been observed for thefirst time in which nuclear reactions add tobackground ionization to produce upsets whichwould not be observed without either one of thetwo components. For a given pulse length, theupset behavior shows a smooth transition withincreasing flux through three characteristicregions. One is the low flux regime in whichSEU dominates. At the other extreme is the highflux region in which dose-rate effectsdominate. The third region is the surprisinglylarge intermediate one in which combinations ofeffects occur. In a circuit context, thisintermediate region results from backgroundionization reducing the circuit's effectivecritical charge for upset, resulting in aregion of enhanced SEU vulnerability. Thethreshold for enhanced SEU vulnerability islikely to be more important than the dose-ratethreshold because it is at this lower thresholdthat significant numbers of memory upsets beginto occur.

Based on the present results and oncurrent modelling of CMOS static wAqs8, whichboth show degradation of circuitcharacteristics at dose-rates significantlybelow the dose-rate upset threshold, it isanticipated that a region of enhanced SEUvulnerability exists for a wide variety ofstatic RAMs, although the degree ofvulnerability is likely to be less for memorieswith shorter recovery tims. Furthermore, thisresult will become more significant as memoriesare designed for lower power consumption andscaled to smaller dimensions.

Comparisons of dose-rate upset thresholdsfor protons to those due to electrons over abroad range of pulse lengths indicate little orno difference would result if the two werecompared at the same pulse length. In theparticular circuit chosen for this work, thelong recovery time of a memory cell allowed aclean separation of two competing mechanisms indose-rate upset. The first, dominant at longpulse lengths, was the result of the (local)drain photocurrent directly depleting storedcharge at the "high" information node. Thesecond, dominant at shorter pulse lengths, wasprimarily the result of photocurrent generatedat the p-well, substrate junction, and wasconsistent with a rail span collapse effect.

ACKNOWLEDGEMENTS

We would like to thank Drs. Ed Petersen ofNRL and Al Wolicki of Wolicki Associates forseveral valuable technical discussions of thiswork. We are also grateful to Dr. Tom Ward ofBNL, Dr. Ken Murray of M Sciences and Mr. KenGage of NRL for their assistance in performingsome of the dosimtry. Further thanks are dueto Ms. Suzanne Swickert for assistance in datataking.

REFERENCES

1. A.B. Campbell & W.J. Stapor, "The Total DoseDependence of the Single Event UpsetSensitivity of IDr Static RAMs," IEEE Trans.Nucl. Sci. NS-31, 1175, December 1984.

2. S.W.S. McKeever, Thermoluminescence ofSolids, Chpt. 6 (Cambridge Univ. Press,Cambridge; 1985).

3. W.L. Bendel & E.L. Petersen, "Proton Upsetsin Orbit," IEEE Trans. Nucl. Sci. NS-30, 4481,December 1983.

4. A.B. Campbell and E.A. Wolicki, "Soft Upsetsin 16k Dynamic RAMs Induced by Single HighEnergy Photons," IEEE Trans. Nucl. Sci. NS-27,1509, December 1980.

5. J.L. Wirth and S.C. Rogers, "The TransientResponse of Transistors and Diodes to IonizingRadiation," IEEE Trans. Nucl. Sci. NS-11, 24,November 1964.

6. L.W. Massengill and S.E. Kerns, "TransientRadiation Upset Simulations of CMOS MemoryCircuits," IEEE Trans. Nucl. Sci. NS-31, 1337,December 1984.

7. L.W. Massengill, et al, "Dose-Rate UpsetPatterns in a 16k CMOS SRAM," IEEE Trans. Nucl.Sci. NS-33, 1541, December 1986.

8. M.R. Ackermann et al, "Factors Contributingto CMOS Static RAM Upset," IEEE Trans. Nucl.Sci. NS-33, 1524, December 1986.