Total Dose Effects and Modeling Approaches for Devices and ICS

1. Total Dose Effects and Modeling Approaches for Devices and ICS H. J. Barnaby School of Electrical, Computer, and Energy Engineering Ira A. Fulton Schools of Engineering Arizona State University, Tempe, AZ

2. The Total Ionizing Dose Problem • Degradation in integrated circuits due to ionizing radiation exposure can deteriorate the circuit characteristics, potentially leading to system failure • Most space electronics, implantable medical devices, and radiation/accelerator facility instrumentation therefore require hardness to total ionizing dose (TID)

3. Outline • Ionizing Radiation Environment andDamage Processes • Total Ionizing Dose Effects in CMOS • Effects in Bulk CMOS • Effects in SOI CMOS • Total Ionizing Dose Effects in Bipolar Technologies • Effects in Bipolar Junction Transistors • Enhanced Low Dose Rate Sensitivity (ELDRS) • Modeling Approaches

4. Outline • Ionizing Radiation Environment andDamage Processes • Total Ionizing Dose Effects in CMOS • Effects in Bulk CMOS • Effects in SOI CMOS • Total Ionizing Dose Effects in Bipolar Technologies • Effects in Bipolar Junction Transistors • Enhanced Low Dose Rate Sensitivity (ELDRS) • Modeling Approaches

5. Electronic Stopping Power • Electronic Stopping or Linear Energy Transfer (LET)measures energy depositedinto a material by ionizingradiation • LET is a strong functionof energy and radiationsource • Total ionizing dose is ameasure of energy depositedthrough ionization A unit of TID is a rad or a gray(100 rad = 1 gray) (after Paillet et al., IEEE TNS 2002)

6. Converting Fluence to Rad(SiO2) Rad (SiO2) = K X fp X LET Units of Rad (Si) = (radg/MeV)(cm-2)(MeVcm2/g) K = (106ev/MeV)(1.6x10-12 erg/eV)(1radg/100erg) = 1.6x10-8 radg/MeV Example: 100MeV protons in SiO2 - The LET for a 100 MeV protons in SiO2 is 6.13 MeVcm2/g(from SRIM code)- To get total dose [rad(SiO2)] from a 100 MeV proton fluence (fp), multiply fp by (6.13)x(1.6x10-8) Ans. If the total fluence of 100 MeV protons for the mission is 1012 protons/cm2, the part will receive ~ 100 krad(SiO2) from these particles Note: to compute the dose across the full proton energy spectrum, we must take into account all LETs as well as the energy attenuation of lower energy particles

7. Overview of Total Ionizing Dose Damage Processes in SiO2 • Processes • EHP Generation • EHP Recombination • Hole and H+ transport • Defect formation: • Fixed oxide-trapped-charge (Not) • Interface traps (Nit) (after McLean and Oldham, HDL-TR-2129 1987)

8. EHP Generation - Number of electron-hole pairs (ehp)generated in SiO2 is computed asfollows: EC ionizing radiation Eg~ 9 eV Ep SiO2 density Ev fluence + [MeVcm2/g]•[cm-2] •[#ehps/MeV] •[g/cm3] Ep for SiO2 is 17 eV/ehp (after McLean and Oldham, HDL-TR-2129 1987) (after Srour, DNA-TR-82-20 August 1982)

9. EHP Recombination Fraction of holes survivingprompt recombination (fy) is afunction of type and energyof radiation as well aselectric field. (after Schwank, NSREC SC 1994) 9

10. Hole Trapping Processes - surviving hole (p) + - hole trap (NT) - trapped hole (Not) + fp - hole flux area = s(e)

11. PDE model for Not(with simplifying assumptions) (steady state) (fp > 0 for all x) fot D (No saturation or annealingand traps at interface) (After Rashkeev et al. TNS 2002)

12. Simple closed form model for DNot (after Fleetwood et al. TNS 1994) Model Parameters D - total dose [rad] kg - 8.1 x 1012 [ehp/radcm3] fy - field dependent hole yield [hole/ehp] fot - trapping efficiency [trapped hole/hole] tox - oxide thickness [cm] ε - local electric field [V/cm]

13. Interface trap buildup processes - hydrogen defect (NDH) - protons H+ - Si-H (NSiH) H - dangling bond (Nit) - proton flux fH area = sit

14. PDE model for Nit(with simplifying assumptions) (steady state) (fH > 0 for all x) fDH D fit (assumes no saturation or annealingand traps at interface) (after Rashkeev et al. TNS 2002)

15. Simple closed form model for DNit (after Rashkeev et al. TNS 2002) Model Parameters D - total dose [rad] kg - 8.1 x 1012 [ehp/radcm3] fy - field dependent hole yield [hole/ehp] fDH - hole, D’H reaction efficiency [H+/hole] fit - H+, SiH de-passivation efficiency [interface trap/H+] tox - oxide thickness [cm]

16. H H H H H H H H Impact of molecular hydrogen H2 molecules Empty D centers Rad-inducedholes H2 transportinto material H DH centers Molecular hydrogen reacts with empty D centers to generate more DH centers

17. 1D model fit to data DNit [cm-2] H2 [cm-3] (after Chen et al. TNS 2007)

18. Outline • Ionizing Radiation Environment andDamage Processes • Total Ionizing Dose Effects in CMOS • Effects in Bulk CMOS • Effects in SOI CMOS • Total Ionizing Dose Effects in Bipolar Technologies • Effects in Bipolar Junction Transistors • Enhanced Low Dose Rate Sensitivity (ELDRS) • Modeling Approaches

19. TID effects in bulk CMOS • TID defect buildupin CMOS devices can • degrade “as drawn”device characteristics • create edge leakageparasitics in parallelwith “as-drawn” device • create inter-deviceleakage paths in bulkintegrated circuits Gate Gate Dielectric(e.g., SiO2) defects +++++++ Source Source Drain Drain n n n n p p Body Body

20. Not effects on threshold voltage DNot Oxide charge shifts flatband voltage

21. Not effects on MOSFET I-V Trapped holes in contribute net positive charge in the oxide, leading to a parallel, negative shift in MOSFET I-V characteristics (Not)

22. Not Effects on 1/f noise Trapped holes near interface can act as slow “border” traps that exchange charge with semiconductor and increase 1/f noise (after Meisenheimer and Fleetwood, IEEE TNS 1990)

23. Nit effects on surface potential(p-channel example) Interface traps reducesurface potentialsensitivity to gatebias between flatbandand threshold

24. Nit effects on MOSFET I-V Traps cause increase in subthreshold swing, threshold voltage shifts, and reduced drive current via mobility degradation

25. Dependence on dielectric thickness tox qDNit qDNot tox DVT = - eox eox DNota tox X Dose DVT/tox2a Dose TID threat in modern bulkCMOS is defect buildupin thicker field oxides (After Lacoe, IEEE REDW 2001)

26. Gate oxide n+ drain n+ source STI STI halo implants p - body Primary TID Threat insub-micron CMOS TID defect build-up in “thick” isolation oxides (LOCOS or STI)create edge and inter-device leakage parasitics in bulk ICs Trapped chargebuildup in STI < 3 nm > 300 nm

27. 4 NMOS Drain-to-Source 1 NMOS D/S to NMOS S/D 2 3 NMOS D/S to NWELL 3 2 1 NWELL to NWELL(assume separate bias) 4 CMOS inverters Leakage Paths

28. Drain-to-Source Edge Leakage Increasingtotal dose

29. Inter-device Leakage n+ D/S to n-well n+ D/S to n+ D/S Charge build-upin STI base

30. Degradation in SRAM Measurements in SRAMshow increased supplycurrent after radiationexposure Cause: Positive charge buildupin trench isolation causingleakage currents betweenn-type regions. Drain-source (after Clark et al., IEEE TNS 2007) 30

31. Outline • Ionizing Radiation Environment andDamage Processes • Total Ionizing Dose Effects in CMOS • Effects in Bulk CMOS • Effects in SOI CMOS • Total Ionizing Dose Effects in Bipolar Technologies • Effects in Bipolar Junction Transistors • Enhanced Low Dose Rate Sensitivity (ELDRS) • Modeling Approaches

32. Silicon on Insulator (SOI) MOS • Key Advantages: • Reduced junctioncapacitance • VT control viadual gate operation Front Gate Gox tox Drain Source n+ n+ tSi p Buried Oxide (Box) tbox tSUB P-sub Back Gate

33. TID Effects in SOI Negligible defectbuildup in thin Gox Much of SOI TIDsusceptibility due todefect buildup in thickburied oxide (Box) tox Drain Source n+ n+ tSi p +++++++ tox < 5 nm tbox > 80 nm tbox Box Defect buildup tSUB P-sub Back Gate

34. TID Effects in SOI 140nm • Radiation damage to Boxcan cause • Reduced frontgate Vtcaused by gate coupling • GIDL enhanced back-channel leakage • “Latch effect” due to non-uniform charge build-upand impact ionization(not discussed here) 80nm Negative Vtshift with Boxthickness 380nm 410nm after Flament et al., IEEE TNS 2003

35. Coupling Effect (Data) Fully depleted SOI devices with body contact can exhibit front-gate threshold voltage shift due to electrostatic coupling from back gate Front gateVt shift after Paillet et al., IEEE TNS 2005

36. Coupling Effect (Model)

37. GIDL Enhanced Leakage(Back-Channel) • High current at high total dose • Drain current increase withnegative gate bias via gate induced drain leakage (GIDL) enhancement after Schwank et al., IEEE TNS 2000

38. Mechanism for GIDL:band-to-band tunneling (BBT) • Local band-bending in high fielddrain-body region generatesfree carriers via electron tunneling After J-H Chen, IEEE TED 2001

39. + - GIDL Enhancement Mechanism • Holes generated by BBTtransport to source, forward biasing the source-body junction Gate Drain Source n+ n+ ++++++++++ Box P-sub Back Gate after Adell et al., IEEE NSREC 2007

40. + - GIDL Enhancement Mechanism • Holes generated by BBTtransport to source, forward biasing the source-body junction • Electrons back-injected into body increase electron concentration along back gate, enhancing back channel leakage Gate Drain Source n+ n+ ++++++++++ Box P-sub Back Gate after Adell et al., IEEE NSREC 2007

41. Prior to radiation exposure and without BBT, back sideinterface is weakly depleted • Trapped charge increases back-side surface potential, back channel concentration and current • GIDL current increases electron Fermi level further raising back channel density and current GIDL Enhancement Mechanism (Band Effects)

42. TID Effects in SOI Technologies:The BAD News • Charge buildup in the buried oxide continues to be a significant total ionizing dose threat in SOI technologies • The threats include: • Front gate threshold voltage reduction due to electrostatic coupling form the back gate. • Drain-to-source leakage caused by back-side inversion enhanced by GIDL and/or impact ionization (latch) • Traditional radiation-hardening-by-design techniques do notaddress the effects caused by damage to the Box

43. TID Effects in SOI Technologies:The Good News • Commercial manufacturers typically increase doping along the back channel to reduce static power in CMOS circuits. This may mitigate the impact of charge buildup in the Box • The use of body ties not only improves SEE effects in SOI parts but there is strong evidence that they also suppress latching and GIDL enhancement • Some commercial manufacturers reduced body lifetime thereby reducing diffusion lengths, which suppresses bipolar action, a principle mechanism in latching and GIDL enhancement

44. Outline • Ionizing Radiation Environment andDamage Processes • Total Ionizing Dose Effects in CMOS • Effects in Bulk CMOS • Effects in SOI CMOS • Total Ionizing Dose Effects in Bipolar Technologies • Effects in Bipolar Junction Transistors • Enhanced Low Dose Rate Sensitivity (ELDRS) • Modeling Approaches

45. PN Junctions The PN junction is a fundamental structure for BJTs

46. PN Junctions When forward biased (VAC > 0V), recombination (R) is maximized within the depletion region x R x Depl. region

47. Interface Trap Effects Interface traps increase the recombination rate x X X X X X X X X X X X R x Depl. region

48. Fixed Oxide Charge Effects Fixed positive charge inthe oxide enhances recombination byincreasing depl. region x + + + + + + + + + + + + X X X X X X X X X X X R x Depl. region For lighter doped p-region recombination increases super-linearly with dose

49. Reverse Bias Generation Trap defects will also increase generation (G) in reversed biased junctions (VAC < 0V) x + + + + + + + + + + + + X X X X X X X X X X X G x Depl. region

50. Bipolar Junction Transistors emitter base collector Collectorcurrent Current N Base current P-substrate Vbe