Numerical Simulation to Assess Risk of Single Event Burnout in Power Schottky Diodes

Transcription

1 Numerical Simulation to Assess Risk of Single Event Burnout in Power Schottky Diodes Jesse Theiss* Robert M. Moision** Brendan Foran* Brent A. Morgan*** *Electronics and Photonics Lab / Microelectronics Technology Department **Space Materials Laboratory / Surface Science and Engineering Department ***MILSATCOM Division / Cross Program Engineering Operations - Systems Effectiveness The Aerospace Corporation 2015

2 Introduction Computational simulations have been performed to better understand the fundamental physics of diode Single Event Burnout (SEB) Coupled with direct observations of failure, this effort gives a more complete understanding of physics and appropriate risk reduction methodologies Single Event Burnout of Schottky diodes was verified by test in 2013 Tests of DC-DC converters built with non-space qualified diodes resulted in unexpected failures whose root cause was traced to diode burnout Subsequent tests of diode piece parts verified this previously unrecognized failure mode across additional Schottky diode part types Prior to these results, diode SEB had not been considered a risk for space programs, despite previously observed failures in terrestrial power PN diodes Diode SEB has an uncertain impact on reliability Lack of test data and poor understanding of the physics of failure Contractors diode derating guidelines and practices may already be sufficient to mitigate this failure mode, but with how much margin? Not clear if SEB concern extends to all diodes or depends on the design and/or implementation of the diode technology 2

3 Terrestrial power diode single event burnout (SEB) Cosmic radiation identified as the culprit in Still a current concern. Catastrophic failure of terrestrial PN power diodes due to cosmic radiation first reported in Failures are accepted to be the result of recoil/spallation charge multiplication triggered by hot neutrons from the cosmic ray shower followed by localized thermal runaway Observed failures are random in location A number of factors influence the SEB frequency: Altitude (proxy for particle environment) Voltage (lower is better) Temperature (inverse relationship) Eighteen diodes in parallel 1 (4000 V, 65 mm) Failure threshold at STP as a fraction of rated voltage for ABB power diodes 2 1. Kabza, H., Schulze, H. J., Gerstenmaier, Y., Voss, P., Schmid, J. W. W., Pfirsch, F., & Platzoder, K. (1994). Cosmic radiation as a cause for power device failure and possible countermeasures. In Power Semiconductor Devices and ICs, ISPSD'94., Proceedings of the 6th International Symposium on (pp. 9-12). IEEE. (replotted source data) 2. Nando Kaminski & Arnost Kopta, Failure rates of HiPak modules due to cosmic rays I Application Note 5SYA , ver 04, ABB Semiconductor. 3

4 SEB reported in Schottky diodes in 2012, 2013 Burnout observed after test in DCDC converters intended for space1 SEB location often located near guard ring1,2 A bead of silicone added to mask the guard ring region of a Schottky diode caused cross sections drop by almost two orders of magnitude2 1. Robert Gigliuto and Megan Casey, "Observed Diode Failures in DC-DC Converters", Presented by Robert Gigliuto at the NASA Electronic Parts and Packaging Program (NEPP) Electronics Technology Workshop (ETW), NASA Goddard Space Flight Center in Greenbelt, MD, June 11-13, Published on nepp.nasa.gov 2. J.S. George, R. Koga, R. M. Moision, and A. Arroyo, Single Event Burnout Observed in Schottky Diodes, 2013 IEEE Nuclear and Space Radiation Effects Conference (NSREC) Radiation Effects Data Workshop Silicone bead applied at perimeter of diode 4

5 Schottky diodes: SEB images reveal thermal event SEB signature is of a high temperature event at edge of Schottky contact. Al/Si eutectic forms at 877 K. TEM cross section of SEB failure Active Schottky region P+ guard ring J. S. George, R. Koga, R. M. Moision, and A. Arroyo, Single Event Burnout Observed in Schottky Diodes, 2013 IEEE Nuclear and Space Radiation Effects Conference (NSREC) Radiation Effects Data Workshop High temperature Si/Al melt Passivation at edge of Schottky contact 5

6 Schottky diode preliminary results Initial models employed an ionizing radiation strike consisting of electron-hole pairs/cm 3 delivered in 1-3 ps in a 0.2 µm radius, equivalent to an LET of 1.94 MeV-cm 2 /mg. Ion strikes within the guard ring vicinity resulted in remote heating at the edge of the Schottky contact, but the temperature increase was quite small. Ionization track J. S. George, R. Koga, R. M. Moision, and A. Arroyo, Single Event Burnout Observed in Schottky Diodes, 2013 IEEE Nuclear and Space Radiation Effects Conference (NSREC) Radiation Effects Data Workshop That heating is in the wrong area and far too low a temperature for damage! 6

7 Changes to simulations improved results Finer simulation meshing was necessary around the ion strike in both the x and z-dimensions to more accurately model the heat flow and temperature increase from Joule heating Gradient meshing used to reduce simulation size but maintain accuracy around the strike region Change of strike profile and intensity to simulate the LBNL SEB tests with 58 MeV-cm 2 /mg Ag ions (0.601 pc/µm, Gaussian radial distribution with radius of 20 nm) Diode model was modified based on experimental analyses Scanning and transmission electron microscope (SEM/TEM) images of the physical device dimensions Spreading resistance profile data of the dopant concentrations Electron beam induced current (EBIC) mapping of the physical extent of the doped p+ guard ring region Revisions improved fidelity and accuracy of simulation with minimal computational cost 7

8 Schottky diode temperature after ion strike Temperature (K) Strike #3 Strike #2 Strike #1 Cross-section with mesh and guard ring doping profile Time (s) 8

9 Schottky diode temperature after ion strike Strike Location Anode Oxide Cathode Cross-section with mesh and guard ring doping profile Cross-section showing temperature profile at ~100 ps after ion strike centered at x = 70µm xy-cross section of strike yz-cross section of strike 9

10 Modifications to modeled Schottky diode Simulation structure revised / improved based on SEM/TEM imaging 10

11 SEM-EBIC color overlay of a failed Schottky diode These images have been vertically stretched (x1.27) to correct the aspect ratio for the 52 cross-sectional surface tilt relative to the electron beam p+ guard ring Schottky contact Electron beam induced current (EBIC) signal is overlaid on a scanning electron microscope (SEM) image, mapping both the Schottky barrier of the device and the P+ doped guard ring under the field oxide. EBIC signal contours highlight a sharper peak intensity distributed beneath the active Schottky Diode (right side of image) and more diffuse at edge of p+ guardring moving left beneath the field oxide (yellow overlay). 11

12 Temperature and current density versus time Temperature 1 µm Current density 10 µm Strike location nm from the anode edge in the guard ring 12

13 Mechanisms for thermal increase and runaway Without guard ring With guard ring Ion strike-induced charges disturb the intended flat potential at the anode edge, creating a high electric field intensity in the silicon. As charge diffuses away from the initial strike location and reaches this high field point, impact ionization generates additional charge carriers locally about this point. Additional charge multiplication (avalanche) can occur as the electrons travel to the cathode at the bottom of the device. 13

14 Behavior with and without the guard ring Guard ring dramatically improves ion strike resilience 14

15 Current and temperature transients Different strike locations considered 500 nm spacing Al/Si eutectic temperature is 877 K Strike nearest the field oxide edge is the one most likely to damage the diode. 15

16 Temperature transient Different strike locations considered 250 nm spacing Simulated structure closely adheres to actual diode structure Strikes nearest the field oxide edge are the ones most likely to damage diode. 16

17 Strike simulations at derated reverse biases 75%, 55%, 40% of rated voltage assessed Locations Voltage derating reduces sensitivity and sensitive volume 17

18 LET dependence of experimental diode failures George, Jeffrey S., Rocky Koga, Robert M. Moision, and Arturo Arroyo Single Event Burnout Observed in Schottky Diodes. In Radiation Effects Data Workshop (REDW), 2013 IEEE, 1 8. IEEE. 18

19 LET dependence of simulated ion strike Strike 5 A strike of 48 MeV-cm 2 /mg at location 7, 250 nm left of the anode/oxide edge, induces a maximum temperature ~200 K below that of a 58 MeV-cm 2 /mg energy strike and below the designated threshold of material failure (1000K). Strike 7 The lower LET strike produces less electron-hole pairs, a lower current density, and thus less Joule heating than the higher energy strike, effectively reducing the sensitive volume of the device that can lead to ion-strike induced failure. 19

20 Conclusion Simulation of high energy ion strikes on power Schottky diodes has been successful Simulation has same failure location as experimental failures Highly localized temperature increase suggests mechanism for material breakdown or alloying Highest temperatures occur for ions striking near the guard ring at the field oxide edge Guard ring mitigation of high field effects is temporarily deactivated by the temporary high concentrations of electron-hole pairs near the anode edge High electric field intensity and reverse bias results in impact ionization of the ion-induced charges, further increasing the current density and Joule heating BUT: Guard ring is still a net benefit to reliability! Voltage derating efficacy confirmed Reduces the sensitive volume and maximum temperature reached Ion energy dependence confirmed Lower LET strikes result in lower temperatures and a reduced sensitive volume 20

21 Acknowledgements This work was supported by The Aerospace Corporation s Independent Research and Development program. 21

22 Thank you Questions? 22

23 Supplementary Material 23

24 Schottky diode: Structural schematic cross-section Schottky junction Metal p+ guard ring n-si n+ Si High field gradients near Schottky contact edge can limit reverse breakdown voltage p+ doped guard ring under the edge of the contact minimizes edge effects Geometry of metal contact edge can also minimize edge effects Reverse bias breakdown occurs via avalanche breakdown mechanism Electron-hole pair multiplication due to impact ionization 24

25 Simulation of SEB in power PN diodes Power diodes may undergo destructive failures when struck by high-energy particles at high reverse bias Simulation results showed that catastrophic failures resulted from local heating caused by avalanche multiplication of ion-generated carriers (for 17 MeV carbon ion strikes on a diode operating at >= 2700 V reverse bias) A. M. Albadri, R.D. Schrimpf, D. G. Walker, and S. V. Mahajan, Coupled Electro-Thermal Simulations of Single Event Burnout in Power Diodes, IEEE Trans. Nucl. Sci., vol. 52, no. 6, pp , Dec

26 Schottky diode simulation 3-D Schottky diode models are built using the ATLAS simulation framework (Silvaco, Inc.) 2-D slices taken from a 3-D Schottky model are shown to the right Building and testing devices via simulation allows many device properties to be readily modified Geometry Dopant levels Ionizing strike conditions Modeling allows fundamental physics underlying the failure to be understood and will give us the ability to better evaluate reliability. Anode Si Location of bullethole failure site SiO 2 p + doped guard ring 26

27 Effects of stress have yet to be incorporated into model 27

28 Angle dependence simulation issues Cannot define mesh uniformly along an angled ion strike track. Grid can only be defined in Cartesian coordinates. Silvaco has added new selective cylindrical meshing to Victory Process to accommodate this problem. Must move from Devedit/Atlas to Victory framework to use these features (as well as simulate stress in diode). 28

29 Still possible problems with meshing 29

30 Simulation of PN power diode under ion strike p+ Simulation uses Silvaco TCAD 20 µm p-doping of cm µm n-base of cm µm n-doping of cm V reverse bias 17 MeV carbon ion strike (simulated equivalent LET = 4 MeV-cm 2 /mg) n p + n n + n+ Diode cross-section 30

31 Silvaco simulation models used Impact ionization Selberherr model (variation of Chynoweth) lattice temperature dependent, based on E-field, commonly used for reverse-biased avalanche simulation (IMPACT SELB) Mobility - Klaassen models (doping, temperature, and carrier dependence) Auger & SRH recombination - Klaassen models (concentration and temperature dependence) Velocity saturation - Field mobility combined with KLA Heat flow at high currents - GIGA enabled (LAT.TEMP) Band gap narrowing (BGN) - due to high carrier concentration Single Event Upset Electron/hole pairs generated along a track with given radial, length, and time dependence Estimation of Linear Charge Deposition (LCD) value from Linear Energy Transfer (LET) value MeV-cm 2 /mg -> pc/µm 31

32 Lattice temperature at 600 ps after ion strike 2700 V reverse bias PN diode, 17 MeV C p-doping cm -3 Highest increase in lattice temperature localized at the pn-junction n-doping cm -3 n-doping cm-3 32

33 Strike-induced transients in a PN diode 2700 V reverse bias PN diode, 17 MeV C Time of strike Current (A) Temperature (K) Transient time (s) Transient time (s) 33

34 Strike-induced transients in a PN diode using 17 MeV C Reverse biased PN diode (-500, -2700, and V) We were never able to simulate thermal runaway using 17 MeV C ions, even at the highest voltages. Results from Albadri showed thermal runaway even at 2700 V (below). Temperature (K) Time of strike Current (A) A. M. Albadri, R.D. Schrimpf, D. G. Walker, and S. V. Mahajan, Coupled Electro-Thermal Simulations of Single Event Burnout in Power Diodes, IEEE Trans. Nucl. Sci., vol. 52, no. 6, pp , Dec Transient time (s) 34

35 Strike 5 & 7 Ion strike energy dependence 35