domain: spi phys

text:
G Hubert

L Artola

Experimental evidence of ground albedo neutron impact on Soft Error Rate for nanoscale devices

Keywords: Ground albedo neutron, Soft Error Rate (SER), spectrometry, MUSCA SEP3, SEE tests

This work demonstrates the experimental evidence of ground albedo neutron impact on Soft Error Rate (SER) for nanoscale devices. The SER of a 45 nm technology was measured according to an experimental protocol using a californium sealed source and several scenes based on material blocks. High density polyethylene and concrete materials were considered to investigate the intrinsic role in albedo neutron productions and their effect on devices. Results show the impact in the spectrum concern mainly energies below 5 MeV. Devices are characterized by a sensitivity which varies according to the presence or not of thethe material block. Simulations using GEANT4 and MUSCA SEP3 tool were performed to extend analyses. A final part is devoted to investigate the impact of ground albedo neutrons on SER by considering realistic terrestrial neutron field.

I. INTRODUCTION

INGLE Event Effects (SEE) risks are a growing concern for avionic and terrestrial applications [START_REF] Hubert | Continuous high-altitude measurements of cosmic ray neutron and SEU/MCU at various locations: correlation and analyses basedon MUSCA-SEP3[END_REF] [START_REF] Hubert | Impact of scaling on the soft error sensitivity of bulk, FDSOI and FinFET technologies due to atmospheric radiation[END_REF]. Thus, the atmospheric radiation characterization is essential to assess their effects on electronic systems.

Primary galactic and solar cosmic-rays (GCR and SCR, respectively) travel toward the Earth at a rate which varies according to the solar activity cycle (11-year) and solar events which can considerably increase particle fluxes over durations of few hours (extreme event as Ground Level enhancement). The shielding of primary particles, provided by the geomagnetic field, is described by the cutoff rigidity and is defined as the minimum momentum per unit charge that an incident particle can have and still reach a given location above the earth. The cutoff rigidity is greatest at the geomagnetic equator and smallest at high latitudes. In the atmosphere, primary Cosmic Rays (CR) interact with the nuclei of atmospheric gases, creating cascades of secondary particles including neutrons, protons, pions, muons and electrons. These secondary particles are named secondary Cosmic Rays.

Secondary CRs are also attenuated as they interact with nuclei in the Earth's atmosphere, and the number of interactions and the intensity of cosmic-ray particles decrease with increasing atmospheric depth [START_REF] Simpson | Neutrons Produced in the Atmosphere by the Cosmic Radiation[END_REF] [START_REF] Zreda | Land-surface hydrology with cosmic-ray neutrons: principles and applications[END_REF]. High-energy neutrons (typically > 10 MeV) colliding with nuclei can produce fast evaporation neutrons, i.e. on the order of 1 MeV. Evaporation neutrons are reduced to epithermal and thermal neutrons as they travel and interact with the elements of the air, vegetation, and soil [START_REF] Zreda | COSMOS: the COsmic-ray Soil Moisture Observing System[END_REF] [START_REF] Köhli | Footprint characteristics revised for field-scale soil moisture monitoring with cosmic-ray neutrons[END_REF]. Hydrogen is by far the most important element for the moderation (loss of energy) of evaporation and epithermal neutrons as both the cross-section and the energy loss per collision is relatively high. Indeed, the intensity of neutrons produced by CR is sensitive to soil moisture in the upper decimeters of the ground. The interactions of neutrons with the soil induce re-emission of particles named albedo. This contribution is therefore additional to neutrons from the cosmic shower.

Previous works [START_REF] Hubert | Ground Albedo Neutron impacts to Seasonal Variations of Cosmic-Ray-induced Neutron in medium Geomagnetic Latitude and Antarctica: Impacts on Soft Error Rate[END_REF][8] investigated the impact of ground albedo neutrons in spectrum, particularly in the thermal, epithermal and evaporation domains. The objective was to interpret the neutron fluxes dynamics recorded in high altitude stations (in the Pic-du-Midi in French Pyrenees and the Concordia polar station) and to correlate these variations according to the hydrogen content mainly induced by precipitations and snowfalls. Transport simulation results were compared with measured spectra (Bonner sphere system neutron spectrometer) in the Concordia (Antarctic) and the Pic-du-Midi (France) stations. Thus, the evaporation domain, which implies neutrons with energy between 0.2 eV to 20 MeV, is a field of interest since these energies are likely to induce SEEs in electronics, particularly for advanced technologies. Indeed, most recent SEE tests conducted in ground facilities show threshold neutron energies in the order of MeV. First investigations devoted to impacts of groundalbedo neutron on SER assessment were presented in [START_REF] Hubert | Ground Albedo Neutron impacts to Seasonal Variations of Cosmic-Ray-induced Neutron in medium Geomagnetic Latitude and Antarctica: Impacts on Soft Error Rate[END_REF]. Contributions of ground albedo neutrons on the SER risk were investigated using Monte-Carlo simulations and MUSCA SEP3 [START_REF] Hubert | Operational SER calculations on the SAC-C orbit using the Multi SCAles Single Event Phenomena Predictive Platform (MUSCA SEP3)[END_REF]. Results have showed that evaporation neutrons contribute significantly to SER, around 20 to 40 % and up to 50 % considering typical critical charges and low critical charges, respectively. This work proposes to demonstrate experimentally the impact of ground albedo neutrons on the SER considering a commercial FPGA (Feld Programmable Gate Array) SRAM based (Static Random Access Memory) designed in 45-nm. Thus, the SER of SPARTAN6 FPGA was measured according to an experimental protocol using a californium source and material blocks to simulate the soil impact. Two materials are selected, i.e. high density polyethylene (HDPE) and concrete. SER are compared with SER issued from reference configuration without soil. The final part is devoted to investigate the impact of ground albedo neutrons on SER by considering a realistic terrestrial neutron field.

II. EXPERIMENTAL AND MODELING PLATFORMS

A. Experimental platform based on SEE tester and neutron spectrometer 1) Global approach

To evaluate the impact of albedo neutrons on the SEE rate, neutron irradiations were performed using a 252 Cf source in the ONERA's facility, called CIRIL. A HDPE block (chemical formula: (C 2 H 4 ) n , density = 0.941 g/m 3 ) and a concrete (density = 2.3 g/m 3 ) block were used.

The concrete materials are a mixture of elements with relative proportions that vary according to types and specificities. Thus, the classical cement is mainly composed by oxides as lime (CaO), silica (SiO 2 ), alumina (Al 2 O 3 ), and iron ( Fe2O3 ). Depending on the properties, there are also magnesia, alkalizes and sulfuric anhydrite. Moreover, the sand mainly contains silica. In this study, the classical concrete material is considered [START_REF] Bashter | Calculation of radiation attenuation coefficients for shielding concretes[END_REF], and contains the chemical elements with the following proportions (in %): H (0.94), C (0.09), O (53.66), Na (0.46), Mg (0.12), Al (1.32), Si (36.74), S (0.08), K (0.31), Ca (5.65) and Fe (0.63). In order to conduct coupled experimental and simulation investigations, three complementary approaches are proposed:

1) Radiation field measurements based on a neutron spectrometer to record the neutron spectra as function of the scene characteristics, 2) SER measurements based on SEE test board to correlate the scene characteristics and the observed variations in failure rate, 3) SEE modelling using spectra measurements and device description (topology and SEU occurrence) to correlate both experimental approaches.

Fig. 1(a) details the experimental facility. The neutron facility is based on 252 Cf sealed source which characterized by spontaneous fission yielding an average of 3.75 fast neutrons per fission. The source produces 2.3.10 9 neutron.s -1 .mg -1 and the associated yield is thus 0.116 neutron s -1 .Bq -1 . The energy spectrum of 252 Cf source, being a fission spectrum, is well characterized and can be described by Maxwellian distribution. The material block was located behind the measuring system which can be the neutron spectrometer (b) or the SEE test board (c). In both cases, the presence or absence of the block will make it possible to evaluate their effect, i.e. the contribution of albedo neutrons can be deduced by differentiating measurements. Moreover, it is possible to compare the impact of the material properties in comparing HDPE and concrete results.

The source to FPGA distance is 5 mm while the distance between the blocks and the FPGA board is 1 cm. The objective was to maximize the albedo neutron contribution to spectra and SEE occurrence. The HDPE and concrete blocks are composed by elementary bricks and their total thicknesses are 30 cm (this thickness being limited by the facility dimensions.

2) Neutron spectrometer

The neutron spectrometer used in this work [1][11] is similar to instruments operated in the Pic-du-Midi observatory and in the Concordia station (Antarctica) [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF], and devoted to characterize the high-altitude neutron spectra. This instrument was therefore presented in previous papers [START_REF] Hubert | Continuous high-altitude measurements of cosmic ray neutron and SEU/MCU at various locations: correlation and analyses basedon MUSCA-SEP3[END_REF] [START_REF] Cheminet | Characterization of the IRSN neutron multisphere spectrometer (HERMEIS) at European standard calibration fields[END_REF], and its description is limited to their main characteristics. It is based on multisphere system using ten high density polyethylene homogeneous spheres with increasing diameters (from 3" to 12"). Two additionnal spheres integrate inner tungsten and lead shells to increase their response to fast neutrons (typically higher than 20 MeV, i.e. cascade neutrons). Considering the typical energies induced by the californium source, metallic spheres are not required in measurements. The neutron detection is based on 3 He spherical proportional counter placed in the center of each HDPE sphere, and allows the thermal neutron detection thanks to the 3 He(n,p) 3 H reaction. The thickness of the spheres aims to thermalize the energetic neutron by elastic collisions with the hydrogen nuclei. This detection principle is named "Bonner sphere". When spheres are exposed to a neutron field characterized by a spectral fluence φ E (E) (in cm -2 .MeV -1 ), each detector integrates a count number or a spectrum depending on whether counter chains or analyzer multi-channels are used, respectively. Thus, a set of readings M i is collected for each integration time, generally chosen in the range of a few minutes when the atmospheric environment is studied, but in the order of a few seconds when an artificial field is recorded. To deduce the neutron spectrum from the count rates, the fluence response functions of each detector R i (E) were calculated using nuclear transport code (MCNPx, GEANT4) and by solving the system of equations. There are several referent tools that can solve this system of equation, among which the GRAVEL unfolding code [START_REF] Matzke | Unfolding procedures[END_REF]. Since the number of detections (issued from 10 spheres) is insufficient in comparison to the neutron spectrum range, it is necessary to define a priori information and it requires an estimation of the shape of the neutron spectrum at the measurement location [START_REF] Matzke | Unfolding procedures[END_REF] [START_REF] Matzke | Unfolding of Pulse Height Spectra: The HEPRO Program System[END_REF].

In the case of atmospheric neutron field, measurements are performed simultaneously on each sphere, under the pretext that the radiation field is negligibly affected by the positioning of detectors on the allocated area (including interaction between adjacent spheres). In this study devoted to characterize the field undergone by the DUT (Device Under Test), measurements are performed using each detector alternately. This approach is justified because the neutron field is stable.

3) SEE test board and irradiated device

ONERA developed a SEE test board, called BIBURAD, which allows for testing SRAM memories ship and embedded in FPGA. As mentioned, in this work, a SPARTAN 6 FPGA designed in 45-nm technology by Xilinx Series was tested. The reference is XC6SLX100.

The test board is composed by a mother board driven by a FPGA as shown in Fig. 1(c). This main board controls the functional behavior of the DUT located on a daughter board. For reliability reasons, no active component is on the daughter board excluding the DUT. During the irradiation runs, the test board reads the Bacl-RAM (BRAM) and the Configuration RAM (CRAM) blocks of the studied FPGA to detect potential SEUs. If an upset is detected, the test board reads again the memory block in order to confirm the occurrence of a SEU or to identify a Single Event Transient. Moreover, in order to prevent and detect Single Event Latchup, the power supply current is monitored. If the current increase over a determined threshold current, a SEL is considered and a off/on cycle is done on the DUT board.

B. Test campaign details

As presented in Fig. 2, measurements are conducted from May 2017 to June 2018. The irradiation steps require a significant duration to obtain a statistically relevant number of SEU. Preliminarily measurements were conducted until August 2017 to quantify the intrinsic daily SEU rate (without neutron source), including contributions induced by noises, Cosmic Rays or α-SER. This rate is systematically taken into account in the analysis.

Thus, there are four steps for SEE measurements, including references (without material block) and configurations using HDPE or concrete blocks. Steps devoted to neutron field characterizations are time-consuming because each detector is alternately used. The cumulative irradiation time required for this study was in the order of 7 months.

C. SEE simulations using MUSCA SEP3

The SEE modeling is based on MUSCA SEP3 methodology presented in previous works [START_REF] Hubert | Continuous high-altitude measurements of cosmic ray neutron and SEU/MCU at various locations: correlation and analyses basedon MUSCA-SEP3[END_REF][2] [START_REF] Hubert | Operational SER calculations on the SAC-C orbit using the Multi SCAles Single Event Phenomena Predictive Platform (MUSCA SEP3)[END_REF]. The Monte Carlo method consists in considering a large number of random configurations (Monte-Carlo) for which a sequential modeling of each physical layer will be conducted, i.e. the interaction mechanisms, the transport in semiconductor and the electrical disturbance of the circuit. Each of these physical layers is based on reduced physical models or databases. This approach allows for considering the device description (layout, thickness details), its local and global environment (overlayer, structures) and the radiation field environment (nature, direction, spectrum and dynamics).

The 252 Cf source was modeled with the isotropic emission and energetic neutron spectra deduced from measurements (see Fig. 1). The interaction transport and the carrier deposition in active Silicon regions are based on databases containing secondary details induced by neutron silicon nuclear interactions for neutron energy range from low to 20 MeV. Coulombian databases allow to quantify the energy loss of the secondary ions in matter and their 3D morphology deposits in the active areas [START_REF] Raine | Implementing realistic heavy ion track in a SEE prediction tool : Comparison between different approaches[END_REF]. This refinement is necessary given the level of integration of the studied technology. The transport/collection models and the occurrence criteria make it possible to calculate the collected charge and the impact of this charge level on the elementary memory cell.

III. RESULTS AND ANALYSES

A. Measured and simulated Neutron spectra analyses

Fig. 3 presents the neutron spectra measured without material block, using HDPE block and using concrete block placed behind the spectrometer. Enrichment of neutron populations below 5 MeV is observed when the material block is placed behind the sphere. However, the HDPE induces a much larger enrichment in the evaporation domain. These differences are due to several factors, primarily the chemical composition, the density and the hydrogen content.

In order to quantify these variations, Fig. 4 presents the ratio between spectra deduced from experiment and calculations. Calculations are based on GEANT4 simulations reproducing the scene, i.e. geometries, the 252 Cf source and the HDPE block. Results show a satisfactory correlation between measurements and calculations. Deviations observed for low energies can be explained by the impact of the Bonner spheres, composed in HDPE, and not taken into account in the simulations. Indeed, this matter can induce additional albedo contributions. This hypothesis is reinforced by the fact that nuclear interaction models are quite validated (including response functions used for the deconvolution process). Results based on neutron spectrometry show an impact in the neutron spectrum for energies below to 5 MeV. It has been shown that nanoscale technologies can be sensitive to SEE for these energy ranges [START_REF] Lambert | Single Event Upsets Induced by a few MeV Neutrons in SRAMs and FPGAs[END_REF]. This suggests that technologies should have a sensitivity which varies according to the albedo neutron quantities, i.e. to the material and their properties.

B. Impact of albedo neutrons on SEE rates

First analyses demonstrate the significant impact of material block. To evaluate the impact on SER, the DUT was irradiated alternately using and not using a material block to simulate the soil. Fig. 5 presents the daily SER obtained considering HDPE block, including error bars defined by the cumulative events. At first the 252 Cf sealed source is placed in front of the DUT without material (behind).The test campaign is performed over a period which is long enough to reduce uncertainties, i.e. to impact of material block. Significant number of SEUs was detected after 45 days of irradiation then the HDPE block is introduced (behind the FPGA) and the SEU measurement was reseted. As shown in Fig. 5, an increase in the SER of 30% was observed for the configuration using HDPE. In term of average SER, the presence of material block induced an increase of 2.8 SEU per day. This trend is quite relevant with the observations devoted to spectrum analyses. A second campaign using a concrete block was conducted. Reference measurements obtained without block are quite reproducible. Thus, after 40 days, the average SER issued from the first and the second campaign are of 9.19 ± 0.22 and 9.15 ± 0.23 SEU day -1 , respectively. Fig. 6 presents the average daily SER obtained using and not using the concrete block. The presence of the concrete block induces an increase of the SER in the order of 16%. In term of average SER, the presence of material block induced an increase of 1.46 SEU per day.

Table I summarizes the average SER measured during both campaigns , allowing to cross-compare the impact of material blocks. Time integrations are 40 and 30 days for configurations without and with block, respectively. Results indicate that the SER is dependent on the material, and this can be linked to the neutron spectrum variations. The magnitude of the soil influence depends on the technology through the failure threshold.

Simulations were performed using MUSCA SEP3. The material environment (facility, block …), the neutron source and the SRAM were considered. The neutron source was described as a point source, isotropic and its spectrum is issued from measurements ( 252 Cf source). The scene was described alternatively with and without material block. The technological critical charge value is not precisely known. A Fig. 7 shows the SER increase in percentage obtained by simulations, the neutron spectra corresponds to the measured neutron spectra presented in Fig. 3. Results show that albedo neutrons contribute significantly to SER, around 25 to 35 % considering the typical critical charge value. The perimeter added in Fig. 7 corresponds to the relevant area for which both representative of typical critical charge values for the 45-nm technological node [2][17][19], and secondly orders of quantities deduced from SER measurements. Simulations are consistent with this perimeter, which makes it possible to correlate physically observed SER variations with albedos induced by the HDPE block.

There is no difference between the SER obtained with and without HDPE block for critical charge values up to 5 fC. Indeed, secondary particles or recoils do not generate enough carriers or charge in the sensitive zones. The event multiplicity can be easily extracted from simulations, and no event was observed. For energies typically less than few MeV, the predominant mechanism is the elastic scattering inducing lowenergy silicon recoils. Thus, ranges are very short and LET values are conversely high (i.e. Bragg Peak), which is not favorable from the point of view of the multiple event occurrence. Results show that albedo neutrons are mainly characterized by energies below 5 MeV, i.e. energy range of evaporation neutrons in the atmospheric fields. Hydrogen content in the soil (wet or liquid water thickness) will impact the evaporation ground spectra and given the SEU sensitivity of nanoscale devices, it is necessary to extend spectrum specifications for SER calculations. It also implies the need to describe soil properties (i.e. composition, moisture etc.).

IV. REALISTIC TERRESTRIAL ENVIRONMENT

The energy range of the 252 Cf source is suitable for studying the impact of neutron albedo. However, the purpose is also to take into account this impact in the SEU risk assessment for realistic terrestrial spectra. Previous works [START_REF] Hubert | Modeling of ground albedo neutrons to investigate seasonal cosmic rayinduced neutron variations measured at high-altitude stations[END_REF] considering terrestrial environment had shown that soil properties impact the ground-albedo neutron production and SERs (only based on simulations). Roles of the porosity and the water content were investigated.

Analyzes consist in discriminating the contributions of the neutron spectrum resulting from the atmospheric shower and the albedo neutrons induced by the soil. The calculations were obtained by considering the Paris city location (i.e. 48°51′12″ N, 2°20′55″ E and +42m) and were based on ATMORAD [START_REF] Hubert | Radiation effects investigations based on atmospheric radiation model (ATMORAD) considering GEANT4 simulations of extensive air showers and solar modulation potential[END_REF]- [START_REF] Hubert | Analyses of Cosmic Ray induced-Neutron based on spectrometers operated simultaneously at mid-latitude and Antarctica high-altitude stations during quiet solar activity[END_REF]. The Solar activity corresponds to the June 2018 period, i.e. a quiet activity characterized by a solar modulation of 600 MV. Validations of ATMORAD [START_REF] Hubert | Radiation effects investigations based on atmospheric radiation model (ATMORAD) considering GEANT4 simulations of extensive air showers and solar modulation potential[END_REF] were performed using ground and high altitude measurements [START_REF] Goldhagen | Measurements of the energy spectrum of cosmic ray induced neutrons aboard an ER-2 high-altitude airplane[END_REF]- [START_REF] Ghelfi | Neutron monitors and muon detectors for solar modulation studies: 2. φ time series[END_REF]. In order to be consistent with the part using the californium source, the ground was alternatively considered as HDPE and concrete.

The geometry of the simulated scene in GEANT4 consists in a series of rectangular shapes nested in one another, including the particle source, the material block definition and the atmosphere defined as the air. As shown in Fig. 8, the volume "World" consists of vacuum is a cube whose edge is 200 m. A smaller cube of 180×180×180 m 3 is the volume of the atmosphere filled with air. The material block is defined by a parallelepiped with a square face of 170×170 m 2 and 80 m deep. The scorer for the albedo neutron is located 1 m above the material block. ATMORAD and GEANT4 simulations assess the shower spectra and the albedo neutron spectrum, respectively. The particle generation disc is also visible at the top of the atmosphere (dimensions are in meter). Air shower neutron, proton and muon spectra are issued from ATMORAD calculations.

Fig. 9 presents the neutron spectra representative to Paris city obtained for three configurations: 1) only shower, 2) shower and HDPE soil, 3) shower and concrete soil. Fig. 9 Neutron spectra in Paris (48°51′12″ N, 2°20′55″ E and +42 m) calculated using ATMORAD, considering configurations defined by: 1) only shower, 2) shower + HDPE soil and 3) shower + concrete soil Cascade neutrons are weakly impacted by the soil modeling. This property is commonly used in cosmic ray monitoring and analyses. Moreover, evaporation neutrons are reinforced by the albedo contribution in the energy range from 0.1 to 5 MeV, i.e. a field of interest of SEU in nanoscale devices. At the same time, there is a significant impact in the thermal and epithermal domains. Indeed, a decrease in the epithermal neutron domain is visible, whereas the thermal domain is considerably reinforced. This point needsto be studied in detail because of the Boron 10, although eliminated from the point of view of the BPSG layers, which remains a chemical element present in the device for doping. Fig. 10 presents the SER increases (in %) as function of the technological critical charge. Two ratios are considered, firstly between the shower + HDPE and shower only, and secondly between the shower + concrete and shower only. Moreover, a perimeter is drawn to designate the typical critical charge values for the 45-nm technology. The impact of albedo neutrons is of the order of 10 to 20 %. With continued advancements in the technological integration, the SEE sensitivity is expected to increase and experimental studies have demonstrated the occurrence due to direct ionization of protons [START_REF] Ghelfi | Neutron monitors and muon detectors for solar modulation studies: 2. φ time series[END_REF]- [START_REF] Hubert | MUSCA SEP3 contributions to investigate the direct ionization proton upset in 65 nm technology for space and atmospheric applications[END_REF] and muons [START_REF] Sierawski | Effect of scaling on Muon-Induced Soft Errors[END_REF] for nanoscale devices [START_REF] Aguiar | Evaluation of Radiation-Induced Soft Error in Majority Voters Designed in 7nm FinFET Technology[END_REF]. Moreover, it is essential to take into account the alpha emission contribution when considering terrestrial environments. The emission rate depends on the quantity and the purification grade of alpha-emitting impurities naturally present in packaging materials and materials used in the chip fabrication process. It is useful to compare the neutron albedo contribution with other contributions induced by protons, muons and alpha emitters. This work is performed by simulations using MUSCA SEP3.The α-emitter contamination is considered as a uniform alpha-emissivity rate of 1×10 -4 α /cm²/hr (hyper-low-alpha, HLA [START_REF] Heald | How Cosmic Rays Cause Computer Downtime[END_REF]- [START_REF] Baumann | Call for improved ultra-low background alpha-particle emission metrology for the semiconductor industry[END_REF]). Based on the same method as for neutron spectra, the muon and proton spectra are issued from ATMORAD (Paris location).

Fig. 11 presents the stacked SER contributions induced by neutron (shower and albedo), protons, muons and alpha emitters. Results are presented for three critical charge values from 0.1 to 0.4 fC.

It is complex to interpret results because threshold phenomena intervene on the SEU contributions induced by each particle type (i.e. sensitivity to protons, muons and low energy neutrons). Moreover, it is undeniable that the technology, the process and the using conditions have a significant impact on results. Concerning the 45-nm technology we investigate in this work, α-SER is the main contribution, whatever the SEU criteria (i.e. critical charge). As the critical charge value decreases, the contributions of Recent publications [33][34] were devoted to assess the soft error rate on advanced technologies in the field of terrestrial and/or aeronautical environments. In a study focusing on the impact of alphas emitters in 7 nm FinFET SRAMs, Fang et al. [START_REF] Fang | Soft Errors in 7nm FinFET SRAMs with Integrated Fan-out Packaging[END_REF] show that alpha and neutron SER trends are similar when the voltage is low enough. The interpretation of results is based on the comparison of critical charges and the low LET alpha particle, this argument that can be extended to muons. These results seem contradictory with Fig. 11 since the trends between neutron and alpha contributions are opposite. However, impacts of technological details, integration or usage parameters (as power supply) affect drastically the SEU sensitivity. This is mainly due to threshold phenomena that can change trends over several decades. The work proposed by Trippe et al. [START_REF] Trippe | Predicting Muon-Induced SEU Rates for a 28-nm SRAM Using Protons and Heavy Ions to Calibrate the Sensitive Volume Model[END_REF] shows that muon contribution to the total SEU rate is considerably lower than the neutron contribution in 28-nm SRAMs. This trend is validated whatever the supply voltage value. This result is compatible with Fig. 11 for critical charge levels greater than 0.25 fC, for which the muon contribution is negligible. These works reveal the complexity of generalizing trends and analyzes from a given technology.

V. CONCLUSION

This work investigated impacts of ground albedo neutrons on the SER on a 45-nm technology, using a 252 Cf facility whose neutron energy range is particularly suitable for this study. Albedo neutrons are due to the soil inducing backscattered neutrons, which implies the importance of the soil properties (chemical composition, density, hydrogen content).

Thus, SER increase was recorded using a 252 Cf source by planning several scenarios. The first scenario considers the DUT in an environment composed of air. The two following scenarios consist in placing a material behind the DUT whose composition is alternately HDPE and concrete. Results show the impact in the neutron spectrum concerns mainly energies below to 5 MeV.

The average SER measured during the campaigns using material blocks demonstrate SER increases in comparison to the reference SER (air environment). Moreover, crosscomparisons show the importance of the material properties in the SER increases. In the case of realistic terrestrial environment, i.e. considering a neutron spectrum, the impact of albedo neutrons is of the order of 10 to 20 % for HDPE and concrete material. This order of magnitude is significant and it may depend on the technological sensitivity.

This problem can be extended to the specification of radiation fields for terrestrial applications. Indeed, the impacted energy ranges, of the order of a few tens of MeV to a few MeV, correspond to the sensitivity thresholds of nanoscale technologies. Moreover, the integration involves improving the radiation field assessment, especially in the evaporation energy domain. This also concerns thermal neutrons, although the Boron 10 was eliminated from the point of view of the BPSG layers. An important perspective would be to plan measurement campaigns in an adequate environment (high altitude platform) ,using advanced technologies and investigating the soil property impacts.

Although this work concerns the natural environment, it would be interesting to extend this analysis to accelerated neuron testing. Thus, future work will be devoted to consider various neutron facilities (LANL, RCNP, TRIUMF, MIRAGE, GENEPI etc.) and to model characteristics (fluxes and spectrum) of albedo neutron in their device test chambers.

S

  0018-9499 (c) 2018 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.This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2018.2877263, IEEE Transactions on Nuclear Science

Fig. 1

 1 Fig. 1 Overview of the global approach, (a) the description of the experimental facility (b) a pictures of the neutron spectrometer (c) and the SEE test board.

Fig. 2

 2 Fig. 2 Test campaign chronology including SEE rate measurements and neutron spectrometry characterization with and without material block. It is possible to distinguish the both test campaigns i.e. HDPE (#1) and concrete (#2), respectively.

Fig. 3

 3 Fig. 3 Neutron spectra measured without material block, with a HDPE block and with a concrete block.

Fig. 4

 4 Fig. 4 Ratio HDPE/no HDPE as function of neutron energy (measurements and GEANT4 simulations).

Fig. 5

 5 Fig. 5 Daily SER for each irradiation runs, (a) without and (b) with HDPE block behind the FPGA device. Error bars are calculated with cumulative events.

1 )

 1 Error bars defined by cumulative event Time (day) no HDPE (a) (b) HDPE 0018-9499 (c) 2018 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. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TNS.2018.2877263, IEEE Transactions on Nuclear Science range of critical charge has been considered in order to get brief roadmap overview of albedo neutron contributions on the CMOS (Complementary Metal Oxide Semiconductor) integration scale. The impact of albedo is represented by the increase in the SER values obtained with material block.

Fig. 6

 6 Fig. 6 Daily SER for each irradiation runs, (a) without and (b) with Concrete block behind the FPGA device. Error bars are calculated with cumulative events.

Fig. 7

 7 Fig. 7 SER increase between HDPE and no HDPE configurations as function of the critical charge.

  Fig.8Description of the modeled scene from GEANT4, the elementary volumes representing the atmosphere and the material block are represented. The particle generation disc is also visible at the top of the atmosphere (dimensions are in meter). Air shower neutron, proton and muon spectra are issued from ATMORAD calculations.

Fig. 10

 10 Fig. 10 SER increases (in %) as function of the technological critical charge. Ratios between 1) shower + HDPE and shower and 2) shower + concrete and shower are presented. Perimeter designates a relevant area of typical critical charge values for the 45-nm technological node.

  PEHD or concrete soil 170 × 170 × 80 Air: 180 × 180 × 180 Source: ∅ = 160 -neutron World: 200 × 200 × 200 / shower shower + concrete / showerother particles increase. Neutron albedo contributions are not preponderant, they remain quite significant and it justifies their being taken into account in the terrestrial radiation field specifications.

Fig. 11

 11 Fig. 11 Stacked contributions to the total SER as function of the SEU criteria (critical charge in fC).

TABLE I .

 I

	WITHOUT MATERIAL BLOCK
	Without material	With material
	Time integration (days) 40	30
	Campaign 1 (HDPE) Campaign 2 (Concrete) 9.15 ± 0.23 9.19 ± 0.22	11.92 ± 0.28 10.61 ± 0.31

AVERAGE SER (IN SEU PER DAY) MEASURED DURING THE BOTH CAMPAIGN (USING HDPE AND CONCRETE, RESPECTIVELY), USING AND


Jean-Charles Matéo-Vélez

Thierry Paulmier

Angélica Sicard

Bernard Dirassen

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Experimental investigation and modelling of surface potentials of materials under electron spectra representative of isotropic GEO worst-case environments

Keywords: Space vehicle reliability, Surface charging, Environmental radiation effects, Testing Specialty spacecraft charging, space environments, flight data, ground testing

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stimating worst-case surface charging remains a challenge because it depends on the complex interaction between the spacecraft and its environment. One of the key aspects is to perform ground testing of covering materials under space-like conditions. This is a long-standing challenge because of the numerous physical processes that are involved in surface charging. Test conditions must also adapt to worst-case environments extracted from new flight data.

Flight data show that hundreds to thousands of volts of negative potentials are observed on spacecraft at geosynchronous orbit when the spacecraft is in the sector especially when it is in Earth eclipse [START_REF] Choi | Analysis of GEO spacecraft anomalies: Space weather relationships[END_REF]- [START_REF] Thomsen | Statistical properties of the surface-charging environment at geosynchronous orbit[END_REF]- [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF]. High differential voltages of covering dielectrics with respect to the structure were measured in the past [START_REF] Mizera | Charging Results from the Satellite Surface Potential Monitor[END_REF] but too few spacecraft are equipped with dedicated instrumentation to allow extrapolating to other situations. The space industry performs testing campaign using small size coupons to predict the response of materials sensitive to charging hazards under flight This work was supported by CNES R&D program 2017. J.-C. Matéo-Vélez, Th. Paulmier, A. Sicard and B. Dirassen are with ONERA / DPHY, Université de Toulouse, F-31055 Toulouse -France (e-mail: mateo@onera.fr).

Denis Payan is with CNES, FR-31401, Toulouse (e-mail: denis.payan@onera.fr) representative conditions. These data can be used as is and/or serve as inputs of larger scale simulations using numerical tools [START_REF] Roussel | SPIS multi time scale and multi physics capabilities: development and application to GEO charging and flashover modelling[END_REF]- [START_REF] Thiebault | SPIS 5.1, an innovative approach for spacecraft plasma modelling[END_REF]- [START_REF] Mandell | Nascap-2k Spacecraft Charging Code Overview[END_REF]- [START_REF] Muranaka | Development of multi-utility spacecraft charging tool (MUSCAT)[END_REF] that combine the three-dimensional geometrical description of a given spacecraft, the physical mechanisms ruling the behavior of its covering materials and, finally, the energy distribution of worst-case environments [START_REF]ECSS-E-ST-10-04C, Space environment[END_REF]- [START_REF]NASA-HDBK-4002A[END_REF].

The objective of this paper is to present the results of an experimental campaign dealing with surface charging under electron spectra that are representative of severe environments measured on GEO spacecraft. Section II presents specifications obtained from the analysis of recent GEO data, including LANL and Van Allen Probes (RBSP-A and RBSP-B). We will in particular focus on the ratio between high and low energy electron fluxes. Section III describes how the ONERA/SIRENE facility that is equipped with electron accelerator and guns has been upgraded to allow representing electron spectra below 400 keV, in addition to the classical kp>5 energy differential flux in use at ONERA/CNES for more than a decade. Section IV presents surface charging test results obtained with new specifications and compare them to a numerical model. Section V discusses how the radiation induced conductivity resulting from the penetration of high energy electrons can strongly differ from one specification to another and result in very different behaviors.

II. SPACE ENVIRONMENTS AT GEO

A. Van Allen Probes Data

A series of severe or worst-case electron environments for surface charging was previously compared in [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF]. The data came from NASA and ECSS guidelines, that use the SCATHA event on April 24th, 1979. Extra environments were extracted from 15 years of LANL data using fluxes averaged over 86 seconds and 15 minutes, that is, representative of differential charging time scales.

In this section, we present our analysis of 56 months of Van Allen probes data from Jan 2013 to Aug 2017 (version 3 of data available from https://www.rbsp-ect.lanl.gov/data_pub). The RBSP A and RBSP B potentials are estimated with the ion peak method of [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF] also used in [START_REF] Thomsen | Statistical properties of the surface-charging environment at geosynchronous orbit[END_REF], from the data extracted from HOPE instrument. HOPE measures electron and proton fluxes Experimental investigation and modelling of surface potentials of materials under electron spectra representative of isotropic GEO worst-case environments Fig. 1. Proton energy distribution during the charging event on Feb 8th, 2013, the same as reported in [START_REF] Sarno-Smith | Spacecraft surface charging within geosynchronous orbit observed by the[END_REF]. The black curve represents the ion peak related to the Van Allen Probe potential.

Fig. 1 presents the proton energy distribution measured by RBSP-A/HOPE on Feb 08, 2013 from 0 PM to 4 PM. The black curve represents the spacecraft potential. A few hundreds of volts negative is observed around 2 PM. This perfectly matches previous results [START_REF] Sarno-Smith | Spacecraft surface charging within geosynchronous orbit observed by the[END_REF]. The histograms of charging events measured on Van Allen Probes over 56 months are presented in Fig. 2 considering the occurrence of potentials, MLT position, month and Kp index. The events can be splitted in two series : below and above -500 V. [START_REF] Sarno-Smith | Spacecraft surface charging within geosynchronous orbit observed by the[END_REF] showed that some hundreds of volts of negative potential have been reached at Sun even though the satellites were fully conductive. This is confirmed by Fig. 3 that represents the number of events associated with satellite potentials between -100 and -500 V as a function of the L-shell radiation belt parameter (L*) and magnetic local time (MLT). Tens of events have been detected with a spacecraft fully at Sun. Our analysis, similar to what was performed in [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF], shows that the worst potentials (thousands of volts negative) have been obtained during equinoxe season around midnight MLT around GEO orbit (L*~5.5-6), suggesting the satellites were in eclipse during these events, see Fig. 4. Such event combines enhance electron flux and the absence of photoemisison, which is very efficient at reducing the impact of the ambient plasma. One point worth to be noticed is the relatively moderate values of Kp index during the charging events, from values as low as 3 and no more than 7. This is consistent with previous findings ([1]-[2]- [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF]). Charging events do not necessarily require high values of Kp to occur.

B. Severe Electron Fluxes above 10 keV

The same methodology as in [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF] has been used to compute the most severe environments in terms of electron flux above 10 keV (referred as FE10k). The measurements performed on RBSP and LANL spacecraft qualitatively agree with each other at GEO. A quantitative analysis indicates that the FE10k 23 seconds averaged RBSP data slightly exceed the FE10k 86 seconds averaged LANL data above 20 keV, at GEO. Inversely, the latter significantly exceed the former between 1 and 10 keV. We suggest that RBSP FE10k specification could be worst than FE10k LANL because of reduced generation of secondary electrons. Extra efforts should be conducted in the future to compare numerically the impact of such an envrionment on a realistic 3D spacecraft.

III. ADAPTATION OF GROUND FACILITIES

A. SIRENE Facility

SIRENE is a sophisticated and unique ONERA/CNES test facility especially designed for the study of surface and internal charging of space materials and satellite components under extreme environment (usually geostationary orbit radiation conditions). This facility is able to reproduce the geostationary electron spectrum in the energy range [0-400 keV]. It allows realistic assessment of potential built up in geostationary orbit. Its flexibility allows the simulation of extreme environments different from the geostationary one, e.g., Middle Earth Orbit (MEO) or other planetary radiation environments. This facility is widely used as well for the characterization of radiation induced conductivities of space materials in vacuum. Using the high energy electron beam and a high incoming flux on the sample (up to 10 nA.cm -2 ), we are able as well to perform radiative ageing on the material. Space materials, such as polymers, adhesive or coverglasses can then be aged in SIRENE with equivalent flight durations equal to several months or years. The electron spectrum simulation was initially achieved by the use of two monoenergetic electron beams (20 and 400 keV), these two beams being diffused in energy and angle to produce a space-like electron flow with a good flux homogeneity in a diameter equal to 20 cm on the sample holder. The nominal integrated fluxes used for the 20 keV monoenergetic beam and the distributed 0-400 keV one are respectively equal to 250 pA.cm -2 and 50 pA.cm -2 but these different fluxes can be changed independently. Electron impact the surface with a quasi-normal incidence angle. The temperature of the sample holder can be controlled in the range [-180 °C, +250 °C] allowing to reproduce the temperature variations of materials on flight. A pumping system allows experiments at vacuum of around 10 -6 mbar. It is instrumented with a contact-less electrostatic probe, current measurement systems and non-contact pulsed electro-acoustic device (PEA).

In the frame of the current study, SIRENE has been amended to improve its flexibility and get close to the different spectra met in space, especially worst-case ones. For this purpose, a third 100 keV electron gun has been installed on this facility and the beam diffusion system has been improved. We can then enhance the flux homogeneity on the sample holder and get a good fit with the required spectra on the overall energy range. With the amended irradiation system, we can then come to a dispersion on the flux of the 20 keV, 100 keV and 400 keV beams of only 10 % along the 20 cm length of the sample holder (versus 20-30 % for the previous system). , with q the elementary charge, k the Boltzmann constant and m e the electron mass. Our new irradiation system allows producing worst case spectra met in space using a quasi-unidirectional electron flux, as we can see in Fig. 6 for the FE10k worst case spectra measured on LANL [START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF]. The SIRENE spectrum is of course tunable in regard of the spectrum required by the user, as we can see in Fig. 7 where the FE10k-15min from LANL spectrum is compared to the 'Kp > 5' spectrum.

B. Reproduction of WC Electron Spectra

IV. MATERIAL SURFACE CHARGING

A. Experimental Procedure

Three different space used polymers (100 µm thick PTFE, 100 µm thick PEEK, and 127 µm thick Kapton®) have been irradiated to measure their charging behavior under two electron spectra : 'Kp>5' and 'FE10K-15min from LANL' (referred by FE10k only in the following). The experiments have been performed at room temperature. The electric surface potential has been measured along the different samples using a moveable Kelvin probe. It is then possible to draw the evolution of the maximum surface potential as a function of irradiation time.

B. Experimental Results

We can observe from Fig. 8 and Fig. 9 strong differences on the charging behavior of the different materials between both spectra. Charging kinetics is much higher for the FE10K spectrum with potentials reaching around -3000 V after 15 minutes irradiation. For the Kp>5 spectrum, the charging behavior is smoothed by the effect of high energy electron flux that significantly enhances radiation induced conductivity. For the FE10K spectrum, the flux of high energy electrons is much lower leading to a lower electric bulk conductivity for the three materials and therefore to a higher charging kinetics that could lead to stronger discharge hazards. We have to be careful however about the fact that radiation history (before the worst-case spectrum occurrence) can strongly affect and smooth the charging behavior of the irradiated polymers.

C. Numerical Modelling

The second part dealing with charging analysis aimed at performing numerical simulation to assess the charging behaviour of Kapton® under other specific electron environment, such as RBSP spectrum. The first task was to compare the charging behaviour assessed experimentally in the irradiation facility and numerically with the dedicated charge transport model described in [START_REF] Paulmier | Analysis of Charge Transport and Ionization Effect in Space-Used Polymers Under High-Energy Electron Irradiation[END_REF]. From Fig. 10 and Fig. 11, we can observe that the numerical model allows a good prediction of the charging kinetics of dielectrics in different irradiation conditions (here FE10K electron spectrum and Kp>5 electron spectrum). This model has then been applied to assess the charging behaviour of 127 µm thick Kapton® under RBSP FE10K electron spectrum for L between 5 and 5.5. A comparison on the charging kinetics between LANL and RBSP FE10K conditions is presented in Fig. 12. We can notice that the RBSP electron spectrum induces slightly higher charging kinetics than the LANL electron spectrum. This can be explained by the higher charging current measured on RBSP spectrum (185 pA.cm -2 ) than the one measured on LANL FE10K (100 pA.cm -2 ). At the start of the irradiation process, radiation induced conductivity (which should be higher for RBSP due to higher ionizing current) does not affect significantly the charging process and smooth only at low level the charging kinetics. V. DISCUSSION Pending on the selected spectrum and material, charging at a few hundreds or thousands of volts can occur at GEO within a few minutes. The study of worst-case environments is important to define adequate experimental conditions used to test space material charging under electron irradiation. On one hand, environments with large low energy electron fluxes induce the largest negative differential voltages of insulators, as seen in this paper. They also promote high level of inverted potential gradients, especially on solar arrays, because electrons below 10 keV produce secondary electron emission. On the other hand, surface charging can be significantly reduced by penetrating electrons of energy above 100 keV typically for thin surface coatings. Finally, the radiation history of materials implies ageing whose effects are very difficult to predict. Ageing materials with a dose equivalent to middle and end of life provides valuable information on material charging. A more detailed analysis presented in another companion paper, even though it refers to internal charging, shows that recent history also impacts charging at a given times [START_REF] Sarrailh | Simulation of the Effect of Time-Dependent Electron Environment on the Internal Charging Dynamic using SPIS-IC[END_REF].

The recent update of the SIRENE facility allows tailored sequences of material testing under representative space conditions, including the main effects of the local and historical environment: radiation dose and dose rate, charge deposition and transport across the sample thickness. For new missions, we recommend to assess the worst-case environments in terms of surface charging, looking at the electron fluxes provided by flight instruments if available on that precise orbit, or by default by international guidelines. For GEO, we recommend to perform and compare testing under kp>5 and under an envelope of GEO worst-cases environments. It would thus provide margins on the ESD risk estimation obtained with the classical Kp>5 environment currently in use at ONERA. SIRENE facility is flexible enough to do that, after some more upgrades to get the full energy range < 400 keV covered.

The numerical charging model developed in a previous activity allows predicting charging levels under GEO-like environments with a very limited number of input material properties. However, it is worth noticing that these properties must be measured following a robust and precise procedure in order to be reproducible [START_REF] Paulmier | Radiation-Induced Conductivity of Space Used Polymers Under High Energy Electron Irradiation[END_REF], though versatile enough to adapt to changing environmental conditions (temperature, ageing effects, etc) in order to be representative of space environment conditions. The physical model describing dielectric charging under electron spectra has been successfully compared to ground measurements using electron fluxes representative of kp > 5 and of LANL data at GEO. The extrapolation to RBSP flight data suggests an enhanced risk on orbits located between MEO and GEO than at GEO.

J

  .-C. Matéo-Vélez, Th. Paulmier, A. Sicard, B. Dirassen, eV up to 50 keV. HOPE data are interpolated with data from MAGEIS instruments (electron and proton fluxes above 30 keV). Each measurement corresponds to a flux averaged over 23 seconds. The measured electron flux is corrected with the probe potential to get the undisturbed ambient flux following [3].

Fig. 2 .

 2 Fig. 2. Histograms of Van Allen Probes charging events between Jan 2013 and Aug 2017 as for the occurrence of a) negative potentials (given in absolute value), b) MLT position, c) month and, d) Kp index.

  Fig. 5 presents the electron energy distribution function for different L* as compared to previous data obtained at GEO on LANL spacecraft.

Fig. 3 .

 3 Fig. 3. Map of charging events with a potential in the range [-500 V; -100 V] as a function of L and MLT.

Fig. 4 .

 4 Fig. 4. Map of charging events with a potential in the range [-5000 V; -1000 V] as a function of L and MLT.

Fig. 5 .

 5 Fig. 5. Electron energy distribution of RBSP FE10k worst events as a function of L*, compared to LANL worst cases at GEO

Fig. 6 and

 6 Fig.7present integral values of surface current density of various electron environments. It is the integral over 2π steradian and above a given energy of the electron current j e . For a Maxwellian electron spectrum of total density N e and of temperature T e , j e would be equal to -2 ⁄

Fig. 6 .Fig. 7 .

 67 Fig.6. LANL FE10K 15min and 86 worst-case spectra[START_REF] Matéo-Vélez | Spacecraft surface charging induced by severe environments at geosynchronous orbit[END_REF] reproduced in the SIRENE facility (red curve)

Fig. 8 .Fig. 9 .

 89 Fig. 8. Evolution of the surface potential as a function of irradiation time for the three different space-used polymers under 'Kp>5' electron spectrum

Fig. 10 .Fig. 11 .Fig. 12 .

 101112 Fig. 10. Comparison on the surface potential assessed experimentally and numerically (RC model) as a function of irradiation time for 127 µm thick Kapton® under FE10K electron spectrum

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