defects impact on time dependent dielectric …neil/sic_workshop/presentations...the sweet spot will...
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Defects Impact on Time dependent dielectric breakdown in SiC MOSFET
Z. Chbili , K. P. Cheung
Semiconductor Electronics Division, NIST, Gaithersburg, MD
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Motivation
S. Arthur, ARL workshop 2013
1. High density of defects in SiO2/SiC.
2. TDDB: Early and “extrinsic” failures are a serious reliability issue in SiC
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Motivation
• Collective TDDB data: • ~ 500 devices
• All stress conditions
normalized
• What are these early failures, and what are the available solutions?
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Motivation
• Traditional causes of early failures:
• Particles, Contamination, Local
thinning of the oxide … • Traditional solutions:
1. Clean-up the process
2. Screening
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Motivation
• Screening in Weibull distribution:
• Possibility of screening
• Impossible screening
` `
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Motivation
• What if the cause of early failures in SiC is different? • Do the traditional solutions still work ?
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TDDB background
well known TDDB facts: 1) For thick oxides, there is a critical
charge to breakdown QBD.
2) Increase in tunneling current will
achieve a certain QBD in a shorter TBD
3) Trap-Assisted-Tunneling (TAT) increases tunneling current.
JTAT+F.N > JF.N
TTAT+F.N< TF.N
+
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Defects and breakdown in SiC
• What defects will increase the
tunneling current and result in a shorter lifetime?
• A defect band can also result in TAT at a certain depth.
• SiC : large density of defects above the band edge
• The distribution of defects can be broad.
± kT
ED ± kT
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Defects and breakdown in SiC
• For a uniform distribution of defects
1e13 cm-2 eV-1
• We only consider ~ 5e11 cm2 defects above the band edge
• The average distance between defects is > 10nm
• The possibility of lined up defects is still low
• It is possible not to consider multiple-TAT
Tox
± kT
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Defects and breakdown in SiC
• How does one defect enhance the
tunneling current locally?
Tox
? ?
• TAT tunneling is a two-step process : the slower step determines the total probability
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Defects and breakdown in SiC
• There is a sweet spot where the
probability of tunneling through each barrier are equal
• The additional tunneling current is maximum if the defect is at the sweet spot.
• The sweet spot location is field dependent.
Tox
?
x0
x1
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Defects and breakdown in SiC
• At the sweet spot, the current
enhancement coefficient is field dependent.
Tox
?
x0
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Defects and breakdown in SiC
• For Eox = 8.2 MV/cm:
• X (η) = 1.025nm
• η = 2.5e4
• Need to consider the avg around the size of the defect wavefunction:
• η! = 8e3
Tox
?
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Defects and breakdown in SiC
• For a sample of intrinsic
SiO2/SiC capacitors
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• What is the effect of one defect at the sweet spot on the weibull distribution
• Tbd = Tbd0/ η
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• But the 1 defect effect is only local : we should consider area scaling around the area of defect.
• weakest link.
• We consider the size of the defect to be roughly 1nm2
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• What happens if we have a large number of defects at sweet spot : 1e6, 1e7, 1e8 ?
• Area scaling statistics
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• For a sample of intrinsic
SiO2/SiC capacitors
• What if the defects are not in
the sweep spot ?
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• What is the effect of one defect at 0.5nm from the sweet spot.
• Tbd = Tbd0/ η
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• Area scaling for the defect 0.5nm away from sweet spot
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• What happens if we have a large number of defects at 0.5 nm from sweet spot : 1e6, 1e7, 1e8 ?
Tox
• For Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• Effect of defects at sweet spot.
Eox = 8.2 MV/cm:
• Effect of defects 0.5nm from
sweet spot.
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Defects and breakdown in SiC
• A distribution of defects in and away from
the sweet spot will result in a continuum of failure distributions shorter than intrinsic.
Tox
Eox = 8.2 MV/cm:
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Defects and breakdown in SiC
• At smaller fields, the lieftime
continuum is more spread.
• This effect has a limit beyond
which lower failures are caused by traditional causes.
Eox = 6.2 MV/cm
Eox = 8.2 MV/cm
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Motivation
• Can trap assisted tunneling explain the broad failure distribution in our collective TDDB data?
• If this is the case, screening is impossible.
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Early failure screening
• Can trap assisted tunneling explain the broad failure distribution in our collective TDDB data?
• If this is the case, screening is impossible.
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Early failure screening
• Can trap assisted tunneling explain the broad failure distribution in our collective TDDB data?
• If this is the case, screening is impossible.
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Early failure screening
• Can trap assisted tunneling explain the broad failure distribution in our collective TDDB data?
• If this is the case, screening is impossible.
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Conclusion
• We presented enough indications that TAT in SiO2 defects could be the
cause of early failures in SiC MOS devices.
• Such early failures are not possible to screen out.
• This problem will not be possible to solve by traditional means of “cleaning” the oxidation process.
• A new method of oxide growth should be adopted.
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E. Wu, IEEE Trans. Electron Devices 2002
Time Dependent Dielectric Breakdown
• A uniformly reliable dielectric result in a Weibull distribution of failures