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Total Dose Effects on Devices and Circuits - Principles and Limits of Ground Evaluation-

Total Dose Effects on Devices and Circuits - Principles and Limits of Ground Evaluation-. Outline. Sensitive structures and degradation processes Rules for effective device selection Limits of total dose evaluations. Problematic of total dose ground evaluation.

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Total Dose Effects on Devices and Circuits - Principles and Limits of Ground Evaluation-

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  1. Total Dose Effectson Devices and Circuits -Principles and Limits of Ground Evaluation-

  2. Outline • Sensitive structures and degradation processes • Rules for effective device selection • Limits of total dose evaluations

  3. Problematic of total dose ground evaluation • Impossible to reproduce in-orbit device environment • radiative environment complexity • operating conditions(bias, temperature) • Necessary to understand the physics to establish rules to extrapolate from ground to space • Need of realistic data for non-hardened devices (COTS)

  4. One answer: reasonable conservativity • Conservative conditions to assure that: • a satisfying behaviour during device evaluation implies • a satisfying in-orbit behaviour • “Reasonable” for “not too much” • These conditions must be defined regarding each experimental parameter modifying the device degradation • Irradiation nature • Dose rate/experiment duration • Device bias and temperature

  5. nMOS transistor NPN bipolar transistor Surface passivation oxide Gate oxide Interface Interface n+ n+ n+ emitter p-type Si substrate p-type base n-type collector Sensitive structures (Si technologies)

  6. - - +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- +- + + +- +- +- E +- +- +- +- +- +- +- + +- +- + +- + + + +- + + +- + +- + +- +- +- +- +- - - - - - MOS structure degradation mechanism Gate (Vg) Oxide Interface Silicon Energy

  7. Drain Source Ideal nMOS transistor Gate (Vgs) Vth Gate oxide p-type Si substrate

  8. Drain Source + + + + + + + + + + + + + + + + + + + + - - - - - Ideal nMOS transistor: total dose D- Oxide trapped charge-induced degradation - Gate (Vgs) With h, fractional yield

  9. Fractional yield dependencies Worst case regarding two parameters: 1- Nature of ionising source: Electrons or Co60 2- Electric field in sensitive oxide: Maximum value After Ma T.P., Dressendorfer P.V. (1983)

  10. Gate (Vgs) Drain Source - - - - - - - Ideal nMOS transistor: stable state- Effect of oxide trapped charge annealing - - Theoretical condition: infinite post irradiation time - Practical conditions: 168 hours at 100°C is a compromise for large oxide charge annealing and slight interface traps annealing

  11. -Qit 0 Parameter variation DVth tir tpi -Qot End of ground irradiation state Possible in-orbit states Stable state Interface states growth nMOS ideal case: time-dependant effect (TDE)

  12. x: Measured values : Specified limits x x x Parameter value x x x tpi D Selection of MOS circuits PASS A device is selected if all the measurements are in the specified domain

  13. x: Measured values : Specified limits x x x x x Parameter value x tpi D Selection of MOS circuits FAIL Failure due to oxide trapped charge

  14. x: Measured values : Specified limits x x x x x Parameter value x tpi D Selection of MOS circuits FAIL Failure due to interface traps

  15. Ideal nMOS sensitive parameters (1) • Device level • Threshold voltage (~ linear) • Drive current • Carrier mobility (second order) • Circuit level • Logic levels • Propagation delays • High speed performances

  16. 1 10-2 10-4 10-6 Ids (A) 10-8 Initial 100 Gy(Si) 10-10 200 Gy(Si) 10-12 168h at 100°C 0 1 2 3 4 5 Vgs (V) Ideal nMOS sensitive parameters (2) • Device level • Leakage current (superlinear) • Circuit level • Supply current (superlinear) • Design-dependant parametric degradation Leakage current

  17. Surface passivation oxide - - + + + - - - - + + + + + + + + + Emitter Base Bipolar transistors degradation (1) Recombination rate in the emitter-base junction is modified: 1- In Si: surface potential shift induces change in the carrier densities 2- At the SiO2/Si interface: by the interface traps density increase and change in carrier densities The global resulting degradation strongly depends the transistor structure (design and type) and of the experimental conditions

  18. Bipolar transistors degradation (2) The recombination fraction of the base-emitter current do not participate to the current amplification: - The current gain (IC/IB) decreases - The current gain degradation depends on VBE (non-linear effect) - Device level: Gain degradation has important impact in linear circuits - Circuit level: Leakage currents are induced in all circuit types

  19. Enhanced Low Dose Rate Sensitivity (ELDRS)- True dose rate effect - • Specific to bipolar technologies • Fractional yield dependence to dose rate (# from TDE) • No satisfying experimental method to bound its magnitude After Johnston et al. IEEE TNS (1994)

  20. x: Measured values : Specified limits x x No signification: May be omitted x Parameter value x x x tpi D Selection of bipolar devices PASS A device is selected if all the measurements are in the specified domain Design margins are recommended High dose rate at room temperature prohibited

  21. Calculated current density at the silicon surface Drain Gate Source Z Z Drain Gate Source X X Main I.C. degradation mechanism- Leakage currents in isolating structures -

  22. Standards for ground evaluation: Irradiation conditions - Worst-case conditions to test oxide charge-related failures - -Maximum electric field in sensitive zones (fractional yield) -Avoid chip heating (thermal annealing) Higher fractional yield Compromise between: - benefit of TDE (annealing during irradiation) - cost of time consuming experiments

  23. Standards for ground evaluation: Post-irradiation conditions - Worst-case conditions to test interface traps-related failures - Something simple ! SCC: time for Nit to reach maximum MIL: time less then irradiation time to anneal in the intended use One bias board only MIL: +50% of design margin (compensates possible Nit annealing)

  24. Displacements/ionisation cumulative effect - Dark current in Active Pixel Sensor - Protons create ionisation (in SiO2) and displacements (in Si) Both interaction type can induce dark current 1010 protons/cm²

  25. Displacements/ionisation combined effects:Bipolar circuits After Rax et al. IEEE TNS (1998) A really extreme example of proton-induced fa ilure, but - a smaller effect can reduce bipolar technologies hardness, - RH means “PTDH” (Pure Total Dose Hard)

  26. Summary • Ground evaluation of total dose effects are well defined and can assure devices hardness for most of • device technologies • device types • mission profiles • Some specific devices or applications need particular attention • Necessary to study effect of device scaling

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