Radiation performance of new semiconductor power devices for the LHC experiment upgrades

1. Radiation performance of new semiconductor power devices for the LHC experiment upgrades C. Abbate, M. Alderighi, S. Baccaro, G. Busatto, M. Citterio, P. Cova, N. Delmonte, V. De Luca, S. Fiore, S. Gerardin, E. Ghisolfi, F. Giuliani, F. Iannuzzo, A. Lanza, S. Latorre, M. Lazzaroni, G. Meneghesso, A. Paccagnella, F. Rampazzo, M. Riva, A. Sanseverino, R. Silvestri, G. Spiazzi, F. Velardi, E. Zanoni APOLLO Project INFN Milano, Padova, Pavia, Roma1, ENEA, INAF, FN S.p.A. and Universities of Cassino, Milano, Padova, Parma

2. Outline • A brief introduction to APOLLO • Radiation sensitivity of SiC devices • Devices and technology • Experimental setup for Single Event Effects (SEE) • Heavy ions irradiation • Radiation sensitivity of GaN devices • Devices and technology • Heavy ions irradiation • Low-energy protons irradiation • Conclusions

3. APOLLO • Alimentatori di POtenza per aLtiLivelli di radiaziOne • Collaboration among several INFN sections • Milano • Padova • Pavia • Roma1 • Goal: improvement of the radiation tolerance of power supplies for the LHC phase 2 upgrade • High radiation levels during phase 2 • Total dose: up to 10kGy(Si) • Protons: 2x1013 protons/cm2 • Neutrons: 7.7x1013neutrons/cm2

4. SiC Power MOSFETs: Devices • Devices • Planar SiC power MOSFETs • Manufactured by CREE and STMicroelectronics • Blocking voltage 1200 V • Maximum IDS 31,6 A continuous, 60 A pulsed (25°C) • SiC Benefits • SiC like GaN are wide bandgap materials • SiC junction can block high voltage in volumes much smaller than Si counterparts • SiC is hard to displacement effects. Minimum energy to displace SiC atoms is larger than in Si and GaAs • SEE Sensitive Volumes smaller than Si power MOSFET

5. SiC Power MOSFETs: Experiments • Performed/Ongoing Irradiations • High-energy protons (final paper) • g rays (final paper) • Neutrons (final paper) • Heavy ions (this presentation) • Irradiations at INFN LaboratoriNazionali di Legnaro

6. Source Gate Body _ + N P + P _ N + N Drain SiC: Single Event Effects • An ionizing particle releases charge in the active volume, i.e. the region where high electric field develops in the blocking condition • The charge interacts with the device and may cause a strong increase in the local current, destroying its structure (Single Event Burnout, SEB) or may cause a large electric field across the gate oxide capable of destroying it (Single Event Gate Rupture, SEGR) • SEB and SEGR can be recognized by increases in the leakage current at the drain and gate, respectively

7. Vgs Vds 1 MW 1 MW Cd 50 W DUT Cg Impacting Ion 50 W SiC: SEE Setup • Bias is applied at the gate and drain • Gate and drain leakage currents are monitored during and after the irradiation • The drain current pulses associated with the impacts of the particles are logged during the irradiation. The integral of these current pulses supplies the collected charge • A post irradiation statistical analysis allows us to find the mean value of the collected charge The current pulses

8. ST SiC: Heavy-Ion Results • The charge collected is larger than the one deposited by the impacting particles (charge amplification) • It is lower than the one measured in equivalent Si power devices (MOSFETs and IGBTs), confirming the smaller SEB sensitive volume 266-MeV Iodine, LET > 60 MeV∙mg-1∙cm2

9. ST SiC: Safe Operating Area • Operating area is limited by SEGR. SEB is less of a concern • The lower sensitivity to SEB, at the LET used for the irradiation, is paid off with a larger sensitivity to SEGR • The gate oxide, apart from the interface, is similar to that used in standard Si MOSFETs 266-MeV Iodine, LET > 60 MeV∙mg-1∙cm2

10. CREE SiC: Energy Deposition • SRIM simulation of Bromine @ 20 - 550MeV • The thickness of the surface structure (metal + polysilicon + SiO2) is about 5 mm • The thickness of the epitaxial layer is about 10 mm

11. CREE SiC: Collected Charge • Measured mean value of the collected charge vs. drain voltage used to bias the device during the irradiation • Each point corresponds to the mean value of the collected charge over a population of 1500 events • Charge amplification is very strong for 79Br at 240MeV for which damages can be observed in the vertical device structure

12. SiC: SEGR and SEB after 20MeV and 60MeV Br IDSS and IGSS after each 20-MeV Br irradiation step IDSS and IGSS after each 60-MeV Br irradiation step • No damage was found during irradiation with 79Br @ 20MeV • SEGR (IGSS>1mA) occurred during irradiation at VDS=750V with 79Br @ 60MeV

13. SiC: SEGR and SEB after 240MeV and 550MeV Br IDSS and IGSS after each 240 MeV Br irradiation step IDSS and IGSS after each 550 MeV Br irradiation step • A SEGR (IGSS>1mA) is detected in the irradiation with 79Br @ 240MeV and 550 MeV at VDS=150V and VDS=100V • The results confirm the weakness of the gate oxide of SiC power MOSFETs

14. GaN HEMTs: Devices • GaN High Electron Mobility Transistors • Enhancement-mode GaN transistors • Manufactured by Efficient Power Conversion (EPC) • Blocking voltage 40/200 V • Maximum IDSis 12/33 A continuous, 60/150 A pulsed From EPC website

15. GaN HEMTs: Potential for Radiation Hardness • Material • GaN is quite hard to displacement effects. Minimum energy to displace GaN atoms is larger than in Si and GaAs, close to SiC • Heterostructure • Channel is formed through band engineering • In principle, no dielectric layers are used underneath the gate, so tolerance to total dose is expected to be excellent From EPC website

16. GaN HEMTs: Potential for Radiation Hardness (2) • Questions marks • No information are provided as to how enhancement-mode has been achieved • Some additional layers have been probably introduced to engineer the positive threshold voltage • What’s the impact on radiation hardness? From EPC website

17. GaN HEMTs: Experiments • Performed/Ongoing Irradiations • g rays (final paper) • Heavy ions (this presentation) • High-energy protons (this presentation) • Low-energy 1.8-MeV protons (this presentation)

18. GaN HEMTs: Experiments (3) • Heavy-ion irradiation at LaboratoriNazionali del Sud • Maximum LET of about 50 MeV∙mg-1∙cm2 • High-energy proton irradiation at LaboratoriNazionali del Sud • 60 MeV protons • Fluence up to 2∙1013 p/cm2 • Low-energy proton irradiation at LaboratoriNazionali di Legnaro • 1.8 MeV protons • Fluence up to 4∙1014 p/cm2 • Energy is too low, to give rise to secondary particles  no indirect SEE • Ionization and displacement damage

19. GaN HEMTs: Heavy Ions irradiation • 100V devices • Used ions: 266-MeV Iodine and 240-MeV Bromine • No SEEs • Large charge amplification  evidence of a regenerative effect • 266MeV I • LET ~ 50 MeV∙mg-1∙cm2

20. GaN HEMTs: 60MeV Proton Irradiation • 100V devices • No SEE • We experienced some problems in the testing, mainly due to the intrinsically different structure and behavior of the devices under test  no waveforms are available

21. GaN HEMTs: Low-energy Protons Effects • DC parameters have been measured before and after 1.8 MeV proton irradiation • Observed effects include: • Increase in gate current • Threshold voltage reduction • Transconductance drop • We investigated damage dependence on • proton fluence • blocking voltage

22. GaN HEMTs: Post-rad Gate Current Device exposed to 1014 p/cm2 in unbiased conditions • Increase in gate current at all voltages, up to one order of magnitude • More pronounced for negative voltage • Some room temperature annealing

23. GaN HEMTs: Post-rad Drain Current Device exposed to 1014 p/cm2 in unbiased conditions • Unexpected decrease in threshold voltage (1V) • Degraded substreshold slope • Modest room-temperature annealing

24. GaN HEMTs: Post-rad Drain Current Device exposed to 1014 p/cm2 in unbiased conditions • Peak transconductance drop, more than 30% • Drain current almost unchanged: threshold voltage reduction offset transconductance degradation

25. GaN HEMTs: Literature Data on Depletion-mode GaN From Karmakar et al., IEEE-TNS 2004 • Drain current decrease is typically reported on depletion-mode devices in the literature, in contrast with the behavior of EPC samples • Difference due to unknown process steps or introduction of dielectric layers?

26. GaN HEMTs: Drain Current Reduction From Weaver et al., IEEE-TNS 2012 • Compilation of several test data over the course of ten years for GaN HEMTs • All devices show decreases in drain

27. GaN HEMTs: Discussion • Small dependence of degradation on blocking voltage: 40V vs 200V • Small dependence of degradation on proton fluence • Sample-to-sample variability • Possible superimposition of displacement effects and ionization effects • Unbiased conditions may not be the worst-case, if ionization effects are present • Total ionizing dose data needed to clarify this • Degradation only at very high fluence of low-energy protons (worst-case condition for ionization and displacement)

28. Conclusions • SiC and GaN power devices are being extensively tested under different types of radiation, in the framework of the APOLLO R&D collaboration, aiming to use these new technologies for designing power supplies for the future LHC experiments upgrades • SiC power MOSFETs are expected to show very good performances in terms of total ionizing dose, but exhibited some issues with SEGR • Enhancement-mode GaN transistors displayed considerable hardness to low energy protons, and practical immunity from SEEs • Further measurements and irradiations are ongoing, to provide a complete picture of radiation effects in these devices