Reliability of a Semiconductor Power Switch in a Power ... · • Current methodology used to...
Transcript of Reliability of a Semiconductor Power Switch in a Power ... · • Current methodology used to...
Reliability of a Semiconductor
Power Switch in a Power
Electronics Converter
Krishna Shenai, PhD
Senior Fellow Computation Institute, The University of Chicago, Chicago, IL (USA)
Adjunct Professor EECS Department, Northwestern University, Evanston, IL (USA)
July 25, 2018
IEEE PELS DL @ SFBAC
Today’s Topics for Discussion
• Current methodology used to design power converters
• Current approach used to assess semiconductor
power switch reliability in a power converter
• Case study: Lessons learned from extensive field-
reliability investigation of high-density power supplies
• Silicon vs. Wide Bandgap (WBG) power devices –
Future of WBG power devices
• Moving forward: How to design power converters with
“built-in” field-reliability
What is Field-Reliability?
Current reliability assessment methods are only good to
evaluate infant mortality
“Field-reliability” of a power
converter is among the least
understood topics today!
We are not able to design
power converters with
“built-in” field-reliability!
Information vs. Power Processor
MTBF > 100,000 hrs.
“End-of-Life” = 17 years
MTBF > 1,000,000 hrs.
“End-of-Life” = 125 years
Information Age Energy Age
Increased switching
speed at reduced
power consumption
Cost Increased energy
efficiency with smaller
profile
Cost
What is Needed?
Power
Converter
L
O
A
D
VSUP
iL(t) • Increased current
density, higher
switching frequency,
and higher Tjmax
• Higher system integration
• Increased power density
• Increased cooling density
Trends in Power Conversion
Smaller converter, lower cost and higher efficiency
Today’s Power Converter
Design Approach
K. Shenai et al, Proceedings of the IEEE, vol. 102, no. 1, pp. 35-52, Jan. 2014
Power Chip in an OEM – Tjmax to Ta
AMBIENT (Ta)
OEM
Power Converter
Power Module
Chip Tjmax
Miniaturization demands high-frequency
power conversion and system integration
Today’s Semiconductor Power Switch
Reliability Assessment Approach
• High-Temperature Reverse Bias (HTRB) Test
• High-Temperature Gate Bias (HTGB) Test
• Temperature Humidity Bias (THB) Test
• Thermal Cycling (-40°C to 125°C)
• Power Supply Operating Life Test
The HTRB Test
• Test performed to accelerate
failure mechanisms
• Typical stress conditions are:
- Ta = 125°C to 150°C
- Vdc ≥ VBR
• Test duration ~ 1000 hours
• Failure rate (λ) is estimated by
considering the dependence on
temperature (T), relative humidity (RH)
and electric field (E)
𝜆 = 𝐴 𝑒𝜙
𝑘𝑇 𝑒𝐵
𝑅𝐻 𝑒𝐶𝐸
EPC eGaN® FET Reliability - Example
R. Strittmatter et al, EPC eGaN® FETs Reliability Testing – Phase Six, www.epc-co.com, 2014
Reliability of High-End
Computer Server
Power Supplies –
a Case Study
Failures of Computer Supplies • IBM eServer 900
• 30% volume taken by PS
• 10% volume by cooling
Co
st
(¢/W
)
50
8
4
Frequency (kHz)
100 150
Eff
icie
ncy (
%)
12 90
85
80
Si
MT
BF
(a
.u.)
1
10
1
Power Density (a.u.)
2 3
Co
oli
ng
Co
st
(a.u
.) 100 4
2
1
Power loss in components and packaging!
ROAD BLOCK !
P. Singh et.al., IBM J. Res. & Dev., Nov. 2002
10 kW Power Supply
50 kHz to 75 kHz = 50% reduction in size
At UI-Chicago (1995-2004)
Founded and directed world’s first industry-
university-government consortium to improve
power supply reliability (1998-2004)
K. Shenai, IEEE Spectrum, vol. 37, No. 7, pp. 50-55, July 2000 (invited paper)
Power Supply Arcing
P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001
Power Supply Arcing
P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001
Partial Vacuum Test – identifies
location of failure
Power Supply Arcing
P. Singh et al, IEEE APEC Digest, pp. 225-229, 2001
Zinc Whisker Spray Test – identifies
minimum spacing between features
Safe Operating Area (SOA) Degradation
Zero voltage transition (ZVT) boost converter High turn-off dv/dt and avalanche stress on Q2
Degradation of transfer curve of Q2 with time Degradation of SOA of Q2 with time
N. Keskar, M. Trivedi and K. Shenai, IEEE IAS Digest, pp. 1098-1102, 1999
Silicon MOSFET Failures due to Dynamic Avalanching
N. Keskar, M. Trivedi and K. Shenai, IEEE IAS Digest, pp. 1639-1645, 1999
Accelerated HTRB Stress Test
“Good’ MOSFET
“Bad’ MOSFET
K. Shenai, 12th Annual Automotive Reliability Workshop,
Nashville, TN, May 2007
SEB Failure Rates of 1000V Silicon Power MOSFETs
Power MOSFET Failure: Effect of Die Size
Actual measured data down to 83% of rated voltage
No data for lower stresses due to very low failure rates
Low-level
leakage results
in significant
device de-rating.
K. Shenai, 12th Annual Automotive Reliability Workshop, Nashville, TN, May 2007
Field-Failures of Power
MOSFETs in Power Supplies
Residual material defects in silicon caused field-failures of
power MOSFETs in high-end server power supplies
K. Shenai, IEEE NAECON 2010, Dayton, OH, July 2010
Silicon IGBT Failures During
Short-Circuit and Inductive
Switching Conditions –
Simultaneous High-Voltage
and High-Current Situation
Short Circuit Failure in Silicon IGBTs
600V/50A
punch-through IGBT
Failure after 18 μs
M. Trivedi and K. Shenai, IEEE Trans. Power Electronics, vol. 14, no. 1, pp. 108-116, Jan. 1999
Silicon IGBT Failure Under Clamped Inductive Stress
M. Trivedi and K. Shenai, IEEE Trans. Power Electronics, vol. 14, no. 1, pp. 108-116, Jan. 1999
Physics of “Hot Spot” Formation Poynting vector S = E x H
δ𝑢
δ𝑡= −𝑔𝑟𝑎𝑑 𝑺 − 𝑱 . 𝑬
𝑢 = 1
2 (E.D + B.H)
Energy conservation law
u = electromagnetic
energy density
Dipole radiation pattern
Electric field strength (color)
Poynting vector (arrows)
K. Shenai et al, IEEE Proceedings, vol. 102, no. 1, Jan. 2014, pp. 35-52
Si vs. WBG Power Devices –
Future of WBG Power Devices
System-Level Benefits of WBG Power Devices
Shenai’s Figure of Merit -
2400x improvement
QF2
AEM
Shenai’s Figure of Merit -
2400x improvement
QF2
AEM
Silicon Power Switch
WBG Power Switch
50,000 cm3
18 kg
4,500 cm3
0.2 kg
Shenai et al, IEEE TED, pp. 1811-1823, 1989
At the system level, the objective should be to increase power and cooling densities
WBG Power Switch
for
• Increased energy savings
• Reduced system cost
• Robust & reliable system
Smaller Chip : Lower Cost
Si vs. WBG Material Properties
W
H
Y
W
B
G
?
Increased Energy Efficiency
Smaller Converter Profile
Replace Si with WBG
Increased switching frequency and
system integration
Higher Junction Temperature Improved package and thermal
management
Wide bandgap (WBG) semiconductors, such as SiC and GaN devices, offer
superior electrical and thermal performances compared to silicon
K. Shenai et al, "Optimum Semiconductors for High-Power Electronics," IEEE Trans. Electron
Devices, vol. 36, no. 9, pp. 1811-1823, September 1989.
Status of Commercial WBG
Power Devices • Vertical SiC JBS Power Diodes
( 300 V < VBR < 1700 V)
• Lateral Low-Voltage ( VBR < 650 V) GaN
Power Transistor
• Vertical High-Voltage ( 900 V < VBR < 1700 V)
SiC Power MOSFET
Cost of WBG device is 2-3X higher than that of Si device
Circuit design complexities – Gate driver issues
System-level benefits of WBG devices are minimal
WBG device field-reliability is unknown
SiC Wafers – Current & Future
Parameter Silicon SiC
Growth
Temperature
< 1000°C > 2000°C
Method Czochralski PVT
Defect Density < 1/cm2 Very High
Cost Low Very High
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
2000 2002 2004 2006 2008 2010 2012 2014
Dislocation Density (cm-2)
BPD
TSD
TED
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
2000 2002 2004 2006 2008 2010 2012 2014
BPD Density (cm-2)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
2000 2002 2004 2006 2008 2010 2012 2014
TSD Density (cm-2)
0
500
1000
1500
2000
2500
3000
3500
4000
4500
5000
2000 2002 2004 2006 2008 2010 2012 2014
TED Density (cm-2)
Defect Engineering of 4H-SiC Wafers
Micropipe
s
(a) (b) (c) High resolution synchrotron monochromatic X-ray topographs recorded at Argonne’s
Advanced Photon Source (APS) facility. (a) Back-reflection X-ray topograph (g = 0004)
images of close-core threading screw dislocations (TSDs) and basal plane
dislocations (BPDs) in a (0001) 4H SiC wafer; (b) Grazing incidence X-ray topograph
(g = 11-28) of 4H-SiC substrate showing TSDs (right and left handed) and TEDs; (c)
Transmission X-ray topograph showing the images of BPDs.
Defects in State-of-the-Art
Commercial 4H-SiC Wafers
Formidable Material Challenge
SiC seed
Vertical (c-axis) 4H-SiC boule
growth proceeds from top surface
of large-area seed via hundreds to
thousands of threading screw
dislocations (TSDs).
Crystal grown at T > 2200 °C --> High thermal gradient & stress --> More dislocations
Growth in the c-axis direction, enabled by screw-dislocations providing steps!
After Ohtani et al. J. Cryst. Growth 210 p. 613.
Threading screw dislocation growth spirals
(THE sources of steps for c-axis growth)
found at top of grown 4H-SiC boule.
Contention: Elimination of screw dislocations from power devices not possible while
maintaining commercially viable crystal quality and growth rate and via this approach.
c-a
xis
Role of Crystal Defects on the
Electrical Characteristics of PiN Diode
Avalanche Testing of Power Devices
First proposed by Shenai C. S. Korman et al, in Dig. Int. High-Frequency Power Conversion, pp. 128-139, 1988
Measured Avalanche Energy (EAVL) of Power Diodes
600V/8A 4H-SiC JBS diode 600V/6A silicon MPS diode
Most WBG data sheets do not list EAVL
First demonstrated by Shenai K. Shenai et al, Proc. IEEE, Feb. 2014
Why EAVL of SiC Power Diodes < EAVL of Si Power Diodes?
Measured dv/dt of SiC Power Diodes
dv/dt (V/nsec.)
Tc = 25°C
0
1.5
3
4.5
6
0 15 30 45 60
SW2
PD2
Peak D
iode C
urr
ent
(A)
Failure Instant
SW2: 600V/6A 4H-SiC JBS diode
PD2: 600V/8A silicon MPS diode
SD1: 300V/10A 4H-SiC JBS diode
SW1: 200V/12A silicon MPS diode
Most WBG data sheets do not provide dv/dt ratings
First demonstrated by Shenai’s group K. Acharya and K. Shenai, Power Electronics Technology, pp. 672-677, Oct. 2002
Why dv/dt Capability of SiC Power Diodes < dv/dt Capability of Si Power Diodes?
Safe Operating Area (SOA) of Power MOSFETs
Why SiC SOA is
smaller than
silicon?
K. Shenai et al, Proc. IEEE, Feb. 2014
IXFB30N120P C2M0080120D
4H-SiC Material Defects and
MOS Gate Oxide Reliability
K. Yamamoto et al, “Influence of threading dislocations on lifetime of gate thermal oxide, ”Mat. Sci. Forum, vols.
717-720 (2012), pp. 477-480.
Commercial SiC JBS Diodes
Measured @ 25ºC
Punch-through Design Measured @ 25ºC
Silicon SiC
Avalanche breakdown Punch-through (leaky)
Minimum on-resistance Not optimized
Too much fat left in SiC diodes
K. Shenai and A. Chattopadhyay, IEEE Trans. Electron Devices, vol. 62, no. 2, pp. 359-365, Feb. 2015.
Field-Induced Lattice Deformation in 600V 4H-SiC JBS Diode
Defect delineation study
performed using hard X-rays
at Argonne’s Advanced Photon
Source (APS).
At 900V reverse bias, TSDs in
the vicinity of the metal-
semiconductor junction were
excited and acted as charge
generation centers that led to
diode breakdown.
Collaborators:
Stony Brook University
Brookhaven National Labs
K. Shenai – unpublished work, 2014
Lessons from the Past:
Higher chip cost and thermal
limitations rendered GaAs
chip technology always a
technology of the future
Why Power Electronics
Converters Fail in the Field?
Why Power Supplies Failed in the Field?
Output capacitor leakage; reactive charge dumping from transformer
leakage inductance; power supply arcing caused by zinc whiskers
K. Shenai, IEEE Spectrum, vol. 37, No. 7, pp. 50-55, July 2000 (invited paper)
Failures in Electronic Systems
What is the
Junction Temperature Tj ?
Industry Response
Question: Please indicate which components you consider most important to be
addressed by future research to improve the reliability of power electronics converter systems?
J. Falck al, IEEE Industrial Electronics Magazine, vol. 12, no. 2, June 2018, pp. 24-35
Industry Response
Question: Please rank the following options for achieving high
reliability for power electronics systems?
J. Falck al, IEEE Industrial Electronics Magazine, vol. 12, no. 2, June 2018, pp. 24-35
Physics of Failures
What is the role of material defects
on cost, performance and
reliability of a semiconductor
power switch?
Power Semiconductor Switch
L
O
A
D
A
B C
vSUP
iL(t) , diL(t)/dt
vAB(t), dvAB(t)/dt
on
off
• Low on-state resistance (RDS(ON))
- to reduce conduction power loss
(I2 RDS(ON))
• Low capacitances
- to reduce switching power losses
(CV2f)
• Good reliability
• Low chip cost
Load is
inductive
Losses in a Power MOSFET
Power converters are designed by considering mainly PON
Source (S)
n- epi
Drain (D)
p-body RW
CDS
CGD
CGS
RD
Gate (G)
RG
Drift-Region Design
n buffer
n+ substrate
n+ source
On-State Power Dissipation
Δ𝑇 = 𝑇𝑗 − 𝑇𝑐 = 𝐼𝑂𝑁2 𝑅𝑂𝑁 𝑅𝑗𝑐
Tjmax = 150°C is “industry standard”
> 200°C is desired
Thermal Management of
WBG Power Devices
Silicon Substrate (~ 200 microns)
2DEG
GaN (< 1 micron)
AlGaN Buffer
(a few microns)
S D G Heat Source
Lateral GaN Power Transistor
SiC Substrate (~ 350 microns)
SiC (few microns)
S G
Vertical SiC Power Transistor
D
S
Moving Forward:
How to design power converters
with “built-in” field-reliability?
Industry-Driven Consortium
Major OEMs
• EV
• Grid
• Aerospace
• Computer/Telecom
OEMs
Research
Labs Academia
Inverter &
Converter
Manufacturer
Component
Suppliers
Need to Account for
Material – Device – System Interactions
Top-down
systems-driven
reliability
engineering
approach
OEMs Motor Control, Utility Grid, EVs,
Power Supplies, etc.
Converter
Suppliers
Inverters & DC-DC Converters
Switch & Module
Manufacturers
Power Semiconductor
Chips & Modules
Material
Suppliers
SiC & GaN Wafers Defect Density
(Growth Rate & Wafer Size)
Current Density
(Tjmax & VBD)
Power Density
(Cooling Density)
Cost
(Ta, MTBF)
Systems-Driven Reliability
Engineering
QUESTIONS?
Thank You
OPTIONAL SLIDES
Reliability =
measure of continuous service accomplishment (or time to failure)
Metrics
Mean Time To Failure (MTTF) measures reliability
Failures In Time (FIT) = 1/MTTF, the rate of failures
Traditionally reported as failures per 109 hours of operation
Ex. MTTF = 1,000,000 hours FIT = 109/106 = 1000
Mean Time To Repair (MTTR) measures Service Interruption
Mean Time Between Failures (MTBF) = MTTF+MTTR
What is Reliability?
The Famous “Bathtub” Curve
Accelerated HTRB Stress Test
K. Shenai, 12th Annual Automotive Reliability Workshop, Nashville, TN, May 2007
Weibull Probability Model
Acceleration Parameters
Predicted Failure Rate = Acceleration Factor (AF) x Acceleration Test Failure Rate
MOSFET “Field-Reliability” Model
SiC MOSFET Gate Oxide Failures
Role of Bulk Material Defects on SiC
MOSFET Gate Oxide Reliability