IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 6, DECEMBER 2014 3055

Experimental Characterization and Simulation ofElectron-Induced SEU in 45-nm CMOS TechnologyA. Samaras, P. Pourrouquet, N. Sukhaseum, L. Gouyet, B. Vandevelde, N. Chatry, R. Ecoffet, F. Bezerra, and

E. Lorfevre

Abstract—This paper presents the single-event upset character-ization of a commercial field programmable gate array (FPGA)using electron radiation. FPGA radiation test results under highenergy electrons are described and the dependence between elec-tron energy and SEU cross section is highlighted. A technologicalcross section is performed to evaluate the back end of line (BEOL)layers composition and thickness. These values are used to per-form Monte Carlo simulations of the commercial FPGA exposedto 20-MeV primary electron beam. Calculation results show thatelectrons are able to generate SEU on the FPGA embedded RAMand confirmed experimental data. SEU rates induced by Jovianelectrons are estimated using two different tools: Monte Carlo inGEANT4 and the OMERE Software.

Index Terms—45-nm FPGA, embedded RAM, high-energyelectron, Monte Carlo simulation, radiation test, SEU rate,single-event upset.

I. INTRODUCTION

S INGLE-event upset sensitivity is a major constraint forspace application design. This effect is usually character-

ized under heavy ions and high energy protons to evaluate theSEE sensitivity for near earth missions. However, missions likeJUICE (JUpiter ICy moons Explorer), which will reach Jupiterand its icy moons, are subject to other kinds of harsh radia-tion environment. Compared to other planetary environments,the main difference comes from the electron energy spectrumwhich is higher in terms of flux and energy. High energy elec-trons (up to 1 GeV) can easily reach the electronic devices insidethe satellite.A previous study using Monte-Carlo radiation transport sim-

ulations [1] has shown that an ion passing through material in-duces single electrons, also named –rays, which can depositenough energy in a sensitive volume to cause upset in highlyscaled SOI SRAMs. Consequently, –rays will have an impacton the cross section. This publication proves that secondaryelectrons generated by heavy ions are able to induce a single-event upset (SEU) in SRAM devices.

Manuscript received July 11, 2014; revised September 25, 2014; acceptedNovember 02, 2014. Date of publication November 25, 2014; date of currentversion December 11, 2014. This work was supported by the Centre Nationald’Etudes Spatiales (CNES).A. Samaras, P. Pourrouquet, N. Sukhaseum, L. Gouyet, B. Vandevelde, and

N. Chatry are with TRAD Tests & Radiation, 31670 Labège, France (e-mail:anne.samaras@trad.fr).R. Ecoffet, F. Bezerra, and E. Lorfevre are with the Centre National d’Etudes

Spatiales, 31401 Toulouse, France.Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TNS.2014.2367544

TABLE ISTUDIED DEVICE DESCRIPTION

The reference [2] presents the evidence of single electron-in-duced SEU in 28- and 45-nmCMOSSRAMs. Ballistic electronsare generated by exposure of the SRAMs to an X-ray sourcewith a 1-mm aluminum attenuator placed above the device. Sim-ulations of a 45-nm SRAM, performed with the MRED code,support these conclusions [2]. This publication proves that sec-ondary electrons generated by X-rays are also able to induce anSEU in SRAM devices.Therefore, a primary electron with a sufficient energy could

have the same impact on those devices thanks to an electromag-netic shower. The high energy electron interacts with matter andcreates a succession of electromagnetic particles, including sec-ondary electrons that may reach the sensitive volume. Conse-quently, the energetic electrons could have an impact on the de-vice sensitivity whereas they are not taken into account duringthe radiation qualification process.The aim of the present study is to evaluate the SEU sensi-

tivity of electronic devices under high energy electrons. Radia-tion test results using several electron energies and theoreticalresults, provided byMonte Carlo simulation, are presented. Twomethods for SEU rate calculation are compared using the Jo-vian environment. This work is supported by the Centre Na-tional d’Etudes Spatiales (CNES).

II. DEVICE UNDER TEST

A 45 nm CMOS copper process technology FPGA has beenselected for the experiments, as it is sufficiently integrated to bepotentially sensitive to SEE induced by electrons and is com-mercially available. (Table I)Technological cross sections were performed on the FPGA.The device was cut perpendicularly to the die surface. The re-

sulted cut was analyzed using Scattering Electron Microscopy.An X-Ray analysis, performed at INSA Toulouse [3], providedthe composition and thickness of the back end of line (BEOL)layers.

0018-9499 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

3056 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 6, DECEMBER 2014

TABLE IICOMPOSITION AND THICKNESS OF BEOL LAYERS CONSIDERED FOR

SIMULATION

Fig. 1. FPGA technological cross section.

Fig. 2. FPGA BEOL layers description.

Figures hereunder show the results obtained on this deviceand Table II shows oxide and metallization thicknesses con-tributing to the back end of line thickness.

III. IRRADIATION FACILITY

Radiation tests were performed at the National Physical Lab-oratory (NPL), Teddington, UK [4] using Clinical Elekta Linacequipment. This facility is a standard clinical electron beamcommonly used for external radiotherapy. Parameters are thenexpressed in total ionizing dose level and have to be convertedfor SEE testing.This equipment is able to produce electrons with energies

from 4MeV up to 20MeV. Available fluxes are from 0.5Gy/minto 5Gy/min and the maximum irradiation area is cm .The beam is pulsed at 200 pulses per second and the pulse

width is approximately s.The Dose Rate Peak is evaluated using these values at

rad/s. The calculated Dose Rate Peak is small enoughto perform SEE tests without inducing Flash events.As shown in Fig. 3, the samples were horizontally disposed

on a table in the irradiation zone.

Fig. 3. Device under NPL irradiation test facility.

Fig. 4. Test Bench description.

IV. TEST BENCH DESCRIPTION

The test hardware is designed in order to perform an SEUcharacterization of the FPGA internal embedded RAM. Radia-tion tests were performed on delidded FPGA.The test setup was developed by TRAD. (Fig. 4)The test hardware is separated into several active boards.The test board, supporting the device under test (DUT), has

been exclusively designed with passive devices. The othercomponents of the test hardware (power supply, oscilloscope,Graphical Universal Autorange Delatcher (GUARD–SEL de-tection) system, laptop and FPGA test board) can be located upto ten meters away from the test board. With this configuration,it is possible to perform irradiations at any energy without anyincidence on the test system.This test bench is able to detect single-event upset (SEU),

single hard error (SHE), single-event latch-up (SEL), and someparticular single-event functional interrupt (SEFI) events.SEU are detected using a reference SRAM on the FPGA test

board. A specific pattern–A/5: “0” and “1” alternation–is writtenin the embedded RAM of the DUT and in the reference SRAM.During the irradiation, the embedded RAM DUT is continu-ously read and compared address by address to the referenceSRAM.When a difference is detected, the internal RAM data isread again. If the corrupted data is still observed, then an SEUis counted and the data is rewritten with the expected value.The write operation is checked and if the embedded RAM datacannot be rewritten, then a SHE is counted and the referenceSRAM is updated with the corrupted DUT value.The test bed also includes a Graphical Universal Autorange

Delatcher (GUARD) System to detect SEL, measure their cross

SAMARAS et al.: EXPERIMENTAL CHARACTERIZATION AND SIMULATION OF ELECTRON-INDUCED SEU IN 45-NM CMOS TECHNOLOGY 3057

TABLE IIISEU TEST RESULTS ON FPGA 45 NM EMBEDDED RAM UNDER HIGH ENERGY

ELECTRONS

section and monitor their characteristics with an oscilloscope.The SEL current threshold was fixed at 300 mA.The SEFI occurrence is managed by the test system. A pre-

liminary characterization under showed a recurring SEFIpattern consisting in the detection of IO data corruption (samedata values at several consecutive addresses while different areexpected). For this test, a SEFI is detected whenever 20 consec-utive addresses present the same value on one IO. In this case,the SEFI count is incremented and the DUT is reconfigured.After SEFI and SEL detection, a power OFF/ON of the DUT

is performed. Then the FPGA initialization routine is reloadedfrom the PROM located on the FPGA test board and the em-bedded RAM is rewritten identically to the SRAM reference.During the radiation test, no SEU were detected at nominal

supply voltage up to e/cm. To increase the SEU sen-sitivity, the FPGA internal power supply was reduced as sug-gested in [1]. The core value was decreased gradually up to0.672 V in order to observe SEU. As this value was determinedto be high enough to ensure the DUT functionality, this one wasused up to the end of this study This value is lower than theminimum data retention voltage specified in the datasheet but ishigh enough to keep the device functional during the test.

V. RADIATION TEST RESULTS

Neither SEFI nor SEL were detected during high energy elec-tron irradiation.A device similar to the selected FPGA was tested under

proton beam in [5]. In this paper, no change in post irradiationsupply current was observed up a total dose of 5.4 krad(Si).These are the only available test results, so TID sensitivityabove 5.4 krad(Si) is unknown.To ensure the DUT functionality, tests were performed step

by step (10 krad(Si)/step) up to 40 krad(Si). No loss of function-ality and SEU susceptibility dependence was observed up to thefinal TID level.Tests were performed using three different fluxes:

e/cm/s; e/cm/s; e/cm/s. No impact onSEU susceptibility was detected.Table III shows SEU test results obtained on the 45 nm em-

bedded RAM of FPGA under high energy electrons.Fig. 5 presents the resulting SEU cross section curve for the

embedded RAM under electron beam. Dots represented with anarrow pointing down indicate that no event was observed at thecorresponding energy.As shown in Fig. 5 and in Table III, an SEU susceptibility is

observed depending on the electron energy. However, the cross

Fig. 5. SEU Cross section curve on FPGA 45 nm embedded RAM under highenergy electrons.

section saturation hasn’t been reached due to the maximum en-ergy available on this facility.Another test campaign has been performed at ALTO IPN

(ORSAY, France) facility [5]. The ALTO electron linac facilityis able to produce electrons from 15 MeV up to 50 MeV with amaximum flux of electrons/cm s. During this test 90-nmSRAM have been irradiated using primary electrons with anenergy up to 50 MeV. Events were detected on the testedSRAM. However, these errors were identified as Flash Eventsinduced by the high Dose Rate Peak of this electron beam.Indeed High dose rate may induce upset of a great numberof cells. All cells receive simultaneously ionizing radiationson all the active devices. Photocurrents are induced in all theactive and parasitic junctions. As a confirmation, Monte Carlosimulations were performed. The electron beam energy wasset to 45 MeV. No single-event effect was observed for thesimulation up to electrons/cm , which corresponds to thetotal fluence during the radiation test.Further characterizations with higher electron energy are con-

ceivable if an appropriate electron facility is found. Future in-vestigations will be performed to identify a suitable electronbeam with energy higher than 20 MeV. Then the cross sectionsaturation could be evaluated from these complementary results.

VI. MONTE-CARLO SIMULATION

A device model was performed considering the preliminarytechnical cross section results. This 3D model was used inMonte Carlo simulation to study the physical interaction be-tween a mono-energetic electron beam and a sensitive device,and to compare the response of the FPGA component underreal and simulated beam.

A. Device Model Description

The layout of the FPGA circuit nodes is not known. However,the technological cross section obtained during the device study(Fig. 2) allows to create a detailed model in Geant4 [7]. Thismodel was made of successive layers.As shown in Fig. 6, an additional silicon layer was set at the

bottom to describe the sensitive volume. The thickness and thearea of this layer are part of the calculation parameters, whichare unique to each device and technology. These simulation pa-rameters also include the critical charge, i.e., the minimal charge

3058 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 6, DECEMBER 2014

Fig. 6. FPGA BEOL layers Geant4 model and electron trajectories.

that can induce a single-event effect. This critical charge de-pends on the type of SEE considered.During their transport through the different layers, electrons

are subjected to lateral deviation. The area of the differentlayers, excluding the sensitive volume, was therefore widenedto m . Moreover primary electrons are tracked one by oneduring the simulation.

B. Simulation Description

Under an electron beam radiation, charge deposition is dueto the ionizing energy loss of primary electrons, of secondaryelectrons created by ionization through the different layers andof recoil atoms created by electron-nucleus collisions.Due to the small thicknesses of the different layers, including

the sensitive volume, dedicated physical processes were usedduring the Monte Carlo simulation to create secondary parti-cles. Low-energy physics PENELOPE code [8] allows creatingand tracking electrons down to low energy and therefore witha high LET. A single Coulombic scattering process (G4eSin-gleCoulombScatteringModel) was added to create recoil atomsdown to 1 keV.As primary electrons are tracked one by one, detected events

are actually due to a single primary particle. Preliminary simu-lation results show that the main contributors to the energy de-position leading to SEU occurrence are the ionizing energy lossby the primary and secondary electrons. Due to the low LET ofthese particles and the small size of the sensitive volume, thecritical charge is exceeded thanks to the combined energy de-position of several electrons.A temporal condition has been taken into account for the col-

lection of charges from several secondary electrons generatedin a same time lapse. Following [9], this condition was set to10 ps, which corresponds to the typical inverter delays for 45 nmCMOS technology. During Monte Carlo simulation, the longesttime between the entrance and the exit from the sensitive area

TABLE IVITERATION PROCESS PARAMETERS AND COMPARISONS TO TEST RESULTS

of all particles generated by a single primary electron was lowerthan 10 ps. As the time between the first entry, or creation, ofan electron inside the sensitive volume and the exit, or the totalenergy loss, of the last electron is lower than the delay for thebit transition in 45 nm technology, then all the detected upsetsoccur during the temporal condition. Consequently all SEUs ob-served during the simulation have been retained.

C. Comparison Between Monte Carlo Simulation andRadiation Test

Device parameters such as sensitive cell sizes and criticalcharge values are not available for the FPGA device. However,since only the internal RAM implemented in the FPGA is testedunder high energy electron beam, these parameters can be setaccording to the ITRS roadmap which provides these values for45 nm SRAMs [10]. Hypotheses have been taken for the MonteCarlo simulation considering different sensitive cell sizes andcritical charge values available in the literature [1]–[12]. Thesimulated electron energy beam was set to 20 MeV corre-sponding to the maximum SEU cross section obtained duringtest, i.e., cm dev. All simulations were performedusing fluences between and electron/cm Tosum up the simulation setup, it represents a 20 MeV electronpoint beam on a 3D model made of successive layers (Fig. 6).The particles are shot normally to the surface hitting first thefluoropolymer down to the silicon layer including the sensitivelayer.Table IV summarizes the results obtained for different Monte

Carlo simulation cases representing variations of three param-eters: the critical charge, the sensitive area and thickness. Thediscrepancies between the SEU cross section results obtainedfor each case show the impact each of them has on the SEErate. The goal of this calculation sequence is to determine theconfiguration allowing to match the SEU rate obtained duringthe irradiation. Therefore the results are compared to the max-imum experimental cross section obtained at 20 MeV.The best simulation/test cross section ratio, i.e., the closest

to 1, is obtained for the following parameters (case 4): criticalcharge of 0.4 fC, sensitive area of m and thickness of

m. These parameters are equivalent to parameters usedin [11] except for the critical charge which is fixed at 0.4 fCinstead of 0.5 fC. This value was fixed taking into account thatduring the radiation test the internal power supply was reduced(to increase SEU sensitivity).Case 4 parameters are considered as representative of the de-

vice and test condition (low bias voltage) used during the radi-ation test campaign at NPL facility.

SAMARAS et al.: EXPERIMENTAL CHARACTERIZATION AND SIMULATION OF ELECTRON-INDUCED SEU IN 45-NM CMOS TECHNOLOGY 3059

Fig. 7. Electron flux comparison between geostationary and Juice missions.

Fig. 8. Schematic representation of Aluminium shielding and FPGA BEOLlayers under isotropic electron flux for Geant4 SEU rate estimation.

VII. SEU RATE CALCULATION FOR JUICE MISSION

The two Van Allen earth radiation belts contain energeticelectrons with energies between 10 keV and 7 MeV. From thetest results, it appears that electrons on earth orbit are not ableto induce SEE in this integrated device. However, as shown in(Fig. 7), the JUICE environment is composed of high energyelectrons (up to 1 GeV) [13].Monte Carlo simulations have been performed using FPGA

sensitive cell dimensions previously determined, BEOL layerscomposition and thickness. The critical charge has been raisedto 0.5 fC which corresponds to the 45-nm SRAM technologyin standard condition according to the ITRS roadmap [10] asdescribed in [11] (case 6). Moreover, a 15 mm aluminum platehas been added to the model to take into account the shieldingprovided by the spacecraft (Fig. 8).Considering these hypothesis, it is possible to evaluate if the

Jovian environment is able to induce SEU on commercial FPGAused in standard supply condition on board of a spacecraft.Two isotropic electron spectra have been considered (Fig. 7):

TABLE VSEE RATES FOR JUICE ELECTRON ENVIRONMNENT USING MONTE CARLO

SIMULATION

Fig. 9. SEU cross section curves considered for SEU rate calculation with theOMERE Software.

— worst case flux, corresponding to the worst case missionlocation (i.e., the closest approach to Jupiter);

— average flux over the scientific phase of the JUICE mis-sion.

The results are shown in theTable V.Rate calculations were also performed based on the radiation

test results by using the OMERE Software [14]. The angulardistribution of secondary electrons is assumed to be isotropic,as for recoil atoms induced by protons. In a first approximation,the geometric effects of sensitive volumes are not taken intoaccount and SEU electron rate can be determined using the samemethod as for proton rate calculation. The following equation isapplied for the SEU electron rate calculation

(1)

where is the electron flux, is the cross section andis the energy threshold.However, as shown in Fig. 5, the maximum energy used

during the radiation test was 20 MeV, since it is the maximumenergy available at the NPL facility. As it can be seen in Fig. 5,the saturation cross section is probably not reached during theSEU test. Three different hypotheses on the saturation crosssection have been considered for the SEU rate calculationwith OMERE. Case 1 considers that saturation cross sectionis reached during the test. Case 2 and case 3 take into consid-eration that saturation cross section are probably not reachedduring the test and that a factor of 10 and 50, respectively,can be observed between the saturation cross section and thehigher cross section observed during the test. The three casesare represented by the three curves in Fig. 9.

3060 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 6, DECEMBER 2014

TABLE VISEE RATES FOR JUICE ELECTRON ENVIRONMNENT FOR THE STUDIED CASES

USING OMERE SOFTWARE

Table VI presents the SEU rate obtained with the OMEREsoftware depending on the Weibull curve case.

VIII. CONCLUSION

Radiation test results presented in this paper show thatsecondary electrons generated by primary energetic electrons,through an electromagnetic shower, are capable of inducingSEUs on the embedded RAM of 45-nm Commercial FPGAunder low power supply voltage conditions. These results alsoshow that the cross section depends on the incident electronenergy. Monte Carlo simulations support this result and showthat SEU are induced by one primary electron and secondaryelectrons.However, due to the limited energy available at the NPL fa-

cility, the saturation cross section is probably not reached. Nev-ertheless SEU rates were estimated for the Jovian mission. FirstMonte Carlo simulations were performed using parameters es-timated based on the obtained test results. Then the OMEREsoftware was used with test results up to 20 MeV and three sce-narios concerning the electron saturation cross section.A complementary study needs to be performed to evaluate

if high energy electrons have to be included in the radiationanalysis process and how to characterize devices for this effect.In order to obtain all the necessary information for the SEU ratecalculation:— Test results have to be completed at higher energies. A newirradiation facility must be found with energies higher than20 MeV and with flux compatible with SEU detection.

— Or, Monte Carlo simulations have to be performed usingaccurate device description parameters. A collaborationwith circuit designers should be set up.

Both these solutions are difficult to adopt. An alternative so-lution would be, first, to use radiation test results up to 20 MeVto define component parameters for the Monte Carlo simulationby iteration process (the strategy adopted in this paper). Thendetermine the saturation cross section using these parameters inMonte Carlo simulations with higher energy electrons. In thisway, the complete cross section curve can be determined up to20 MeV with test results and at higher energies with the simu-lation results. Finally the SEU rates can be evaluated using theoutput cross section curve.

ACKNOWLEDGMENT

The authors would like to thank J. Manning (NPL) and M.Chabot (IPN) for their availability to adapt the experiments totheir high energy electron beam facilities and J. L. Gauffier forhis technical involvement during the X-ray analysis.

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