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© The Aerospace Corporation 2015

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

[email protected]/MTD

• 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

Introduction


Terrestrial power diode single event burnout (SEB) Cosmic radiation identified as the culprit in 1994. Still a current concern.

• Catastrophic failure of terrestrial PN power diodes due to cosmic radiation first reported in 19941

• 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)

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, 1994. 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 2042-04, ver 04, ABB Semiconductor.

Failure threshold at STP as a fraction of rated voltage for ABB power diodes2

Eighteen diodes in parallel1(4000 V, 65 mm)


SEB reported in Schottky diodes in 2012, 2013• Burnout observed after test in DC-

DC 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

Silicone bead applied at perimeter of diode

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, 2012. 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


SEB signature is of a high temperature event at edge of Schottky contact. Al/Si eutectic forms at 877 K.

Schottky diodes: SEB images reveal thermal event

High temperature Si/Al melt

TEM cross section of SEB failure

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

Passivation at edge of Schottky contact

Active Schottky region P+ guard ring


• Initial models employed an ionizing radiation strike consisting of 1016-1018

electron-hole pairs/cm3 delivered in 1-3 ps in a 0.2 µm radius, equivalent to an LET of 1.94 MeV-cm2/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.

Schottky diode preliminary results

That heating is in the wrong area and far too low a temperature for damage!

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


• 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-cm2/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

Changes to simulations improved results

Revisions improved fidelity and accuracy of simulation with minimal computational cost


Stri

ke #

1

Stri

ke #

2

Stri

ke #

3Schottky diode temperature after ion strike

Time (s)

Tem

pera

ture

(K)

Cross-section with mesh and guard ring doping profile


Schottky diode temperature after ion strike

Cross-section with mesh and guard ring doping profile Cross-section showing temperature profile at ~100 ps after ion strike centered at x = 70µm

Anode

Cathode

Oxide

Strike Location

xy-cross section of

strike

yz-cross section of strike


Modifications to modeled Schottky diode

Simulation structure revised / improved based on SEM/TEM imaging


SEM-EBIC color overlay of a failed Schottky diodeThese images have been vertically stretched (x1.27) to correct the aspect ratio for the 52 ° cross-sectional surface tilt relative to the electron beam

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+ guard-ring moving left beneath the field oxide (yellow overlay).

p+ guard ringSchottky contact


Temperature and current density versus timeTemperature

Current density

Strike location 6 – 500 nm from the anode edge in the guard ring

1 µm

10 µm


Mechanisms for thermal increase and runaway

Without guard ring With guard ring

Ion strike-induced charges disturb the intendedflat potential at the anode edge, creating a highelectric field intensity in the silicon. As chargediffuses away from the initial strike location andreaches this high field point, impact ionizationgenerates additional charge carriers locally aboutthis point. Additional charge multiplication(avalanche) can occur as the electrons travel tothe cathode at the bottom of the device.


Behavior with and without the guard ring

Guard ring dramatically improves ion strike resilience


Current and temperature transientsDifferent strike locations considered

Strike nearest the field oxide edge is the one most likely to damage the diode.

500 nm spacing

Al/Si eutectic temperature is 877 K


Temperature transientDifferent strike locations considered

Strikes nearest the field oxide edge are the ones most likely to damage diode.

250 nm spacing

Simulated structure closely adheres to actual diode structure


Strike simulations at derated reverse biases75%, 55%, 40% of rated voltage assessed

Voltage derating reduces sensitivity and sensitive volume

Locations


LET dependence of experimental diode failures

George, Jeffrey S., Rocky Koga, Robert M. Moision, and Arturo Arroyo. 2013. “Single Event Burnout Observed in Schottky Diodes.” In Radiation Effects Data Workshop (REDW), 2013 IEEE, 1–8. IEEE.


LET dependence of simulated ion strike

A strike of 48 MeV-cm2/mg at location 7, 250 nm left of the anode/oxide edge, induces a maximum temperature ~200 K below that of a 58 MeV-cm2/mg energy strike and below the designated threshold of material failure (1000K).

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.

Strike 5

Strike 7


• 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

Conclusion


Acknowledgements

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


Thank you

Questions?


Supplementary Material


• 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

Schottky diode: Structural schematic cross-section

Metal

n+ Si

p+ guard ringSchottkyjunction

n-Si


• 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)

Simulation of SEB in power PN diodes

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. 2194–2199, Dec. 2005.


• 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.

Schottky diode simulation

p+ doped guard ring

Location of bullet-hole failure site

SiO2Anode

Si


Effects of stress have yet to be incorporated into model


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).


Still possible problems with meshing


Simulation of PN power diode under ion strike

• Simulation uses Silvaco TCAD • 20 µm p-doping of 1017 cm-3

• 400 µm n-base of 3.1 × 1013 cm-3

• 20 µm n-doping of 1019 cm-3

• 2700 V reverse bias• 17 MeV carbon ion strike (simulated

equivalent LET = 4 MeV-cm2/mg)

p+

n

n+

Diode cross-section

np+ n+


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-cm2/mg -> pC/µm


Lattice temperature at 600 ps after ion strike2700 V reverse bias PN diode, 17 MeV C

Highest increase in lattice temperature localized at the

pn-junction

p-doping 1018 cm-3

n-doping 3×1019 cm-3

n-doping 3.1×1013 cm-3


Strike-induced transients in a PN diode2700 V reverse bias PN diode, 17 MeV C

Time of strike

Transient time (s)

Cur

rent

(A)

Transient time (s)Te

mpe

ratu

re (K

)


Strike-induced transients in a PN diode using 17 MeV CReverse biased PN diode (-500, -2700, and -3500 V)

Time of strike

Transient time (s)

Cur

rent

(A)

Tem

pera

ture

(K)We were never able to simulate thermal

runaway using 17 MeV C ions, even atthe highest voltages.Results from Albadri showed thermalrunaway even at 2700 V (below).

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. 2194–2199, Dec. 2005.


Strike 5 & 7 – Ion strike energy dependence

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