PDSOI and Radiation Effects:
An Overview
Josh Forgione
NASA GSFC / George Washington
University
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Abstract
• Silicon-On-Insulator (SOI) Complementary Metal-OxideSemiconductor (CMOS)
– An alternative to bulk CMOS for radiation environments
– SOI dielectric isolates device layer from substrate.
– Long-time appeal to military and aerospace IC designers.
• Discussion focus:
– Investigate behavior of Partially-Depleted SOI (PDSOI) device
with respect to three common space radiation effects:
• Total Ionized Dose (TID)
• Single-Event Upsets (SEUs)
• Single-Event Latchup (SEL).
– Test and simulation results from literature and bulk comparisons
facilitate reinforcement of PDSOI radiation characteristics.
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Introduction
• Traditional CMOS is fabricated on a bulk
substrate.
+ VS
n+
VG
+ VD
+ VS
n+
p+
VG
+ VD
p+
n well
P-type, ‘Bulk’’
Substrate
Figure 1: Bulk CMOS device structure
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Introduction
• SOI MOSFETs are isolate from the substrate and
each other via a thick insulator, or ‘buried oxide’
(BOX)
+ VS
n+
VG
p
+ VD
+ VS
p
n+
VG
n+
+ VD
p
SiO2 Insulator – ‘Buried Oxide’
p Substrate
Figure 2: SOI CMOS device structure
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Introduction
• Device-layer thickness (tsi) determines one of two main SOI types
– Partially-Depleted SOI (PDSOI): tsi > 2*xdmax
• Focus of discussion
– Fully-Depleted SOI (FDSOI): tsi < xdmax
B: FDSOI
Front Gate
Depletion
Fully
Depleted
Body
tSi
Front Gate Oxide
xdmax
Front Gate Oxide
tSi
xdmax
A: PDSOI
p Body
Back Gate Buried
Oxide (BOX)
Back Gate Depletion
Back Gate Buried
Oxide (BOX)
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Figure 3: Distances between front and back-gate depletion regions
define PDSOI (A) and FDSOI (B).
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Introduction
• Equivalent PDSOI circuit model:
VG
+ VS
n+
VG
+ VD
p
+ VS
n+
+ VD
p
n+
Buried Oxide (BOX)
Buried Oxide (BOX)
p substrate (back gate)
p substrate (back gate)
VBG
n+
VBG
Figure 4: Transistors (left), capacitors, and diodes (right) embedded in the PDSOI MOSFET
structure.
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Introduction
• PDSOI ‘floating body’ effects
– Results from inherent, un-terminated body
– Body-source junction can forward-bias
parasitic Bipolar Junction Transistor (BJT)
• Current transient
– Body-source potential affects threshold voltage
– Can be alleviated (but not cured!) using body ties
• Body ties = Body potential connected to known reference
• Difficult to implement for each transistor in a device
• A must for space-flight designs
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SOI and Single Event
Latchup (SEL)
• Incoming charged particles can activate parasitic thyristor structure in
bulk CMOS devices
– Can result in permanent device failure
• SOI’s adjacent device isolation eliminates the possibility of SEL
entirely.
Figure 5: Cross section of a p-well CMOS inverter, with parasitic elements pertinent to
latch-up[1]
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PDSOI and SEUs
• Significant SOI cross-sectional area decrease
– Deposited charge in substrate cannot return to depletion region
– Theoretical result:
• Decreased current transients
• SOI devices less susceptible to SEU
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Figure 6: Conceptual distribution of charge funnels in bulk (a) and SOI (a)
devices. The SOI device dramatically reduces the area for collected
charge[via 3].
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PDSOI and SEUs
Figure 7: Sensitivity of the
68020 microprocessor, in
CMOS/thick SOI and CMOS/epi
taxial technologies, for two
different test programs[4 via 5].
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PDSOI and SEUs
Figure 8: Peak transient current as a function of strike Linear Energy
Transfer (LET) and technology scaling for bulk and SOI technologies[6]
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SEU and SOI
• Parasitic BJT increases SEU sensitivity
– Amplifies deposited charge
– Can completely null SOI SEU advantages due to reduced
cross-sectional area[2]
– Spaceflight designs require body ties
• Decreases BJT gain (β)
Figure 9: Parasitic BJT in PDSOI
Table I: Experimental and Calculated LET Threshold in SOI
SRAMs in MeV/(mg/cm2), Bipolar Amplification Factors β*[4]
VG
+ VS
n+
+ VD
p
n+
Buried Oxide (BOX)
p substrate (back gate)
VBG
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PDSOI and TID
• Gate-oxide charge traps can disable a bulk or SOI
device
– Increased leakage currents lead to threshold voltage
shifts.
• PDSOI buried-oxide charge traps can activate the
back-gate transistor
– May increased front-gate leakage currents and cause
threshold voltage shifts.
• Process and device design techniques can reduce
the effects of trapped charge in the buried oxide[7].
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PDSOI and TID
Figure 10: Drain-to-Source (DS) I-V characteristics for (a) a back-gate transistor irradiated to 1 Mrad (SiO2) and its
effect on (b) the top-gate transistor leakage current. The transistors were irradiated in the OFF (VGS = VS = 0V; VDS
= 5V) bias condition[8].
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Conclusions
• SOI often used for low-power, high-speed
devices
– Desired features in military and aerospace
designs
• SOI SEL immunity does not infer a a cureall for radiation-tolerant designs
– Inherent parasitics lead to TID and SEU
sensitivities in PDSOI.
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Conclusions
• Radiation effects omitted from discussion:
– Proton secondary effects
– Analog and Digital Single-Event Transients
(ASETs, DSETs, respectively)
– Dose rate
– Etc., etc.
• All effects and appropriate mitigation
methods must be considered during design
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References
[1]
R.S. Muller, T.I. Kamins, Device Electronics for Integrated Circuits. New York:
John Wiley & Sons, Inc, 2003.
[2]
A. G. Holmes-Siedle, Len Adams, Handbook of radiation Effects. Oxford
University Press, 2002.
[3]
J.P. Coligne, Silicon on Insulator Technology: Materials to VLSI. Boston:
Kluwer, 2003.
[4]
P. Lestrat, et. al, “SOI 68T020 Heavy Ions Evaluation,” IEEE Transactions on
Nuclear Science, vol. 41, p. 2240, 1994.
[5]
O. Musseau, “Single-Event Effects in SOI Technologies and Devices,” IEEE
Transactions on Nuclear Science, vol. 43, p. 603. April 1996.
[6]
P. Dodd, et. al, “Production and Propagation of Single-Event Transients in HighSpeed Digital Logic ICs,” IEEE Transactions on Nuclear Science, vol. 51, p. 3278, December
2004.
[7]
K. Bernstein, SOI Circuit Design Concepts. Hingham, MA: Kluwer, 2000.
[8]
J.R. Schwank, “Radiation Effects in SOI Technologies,” IEEE Transactions on
Nuclear Science, vol. 50, p. 522. June 2003.
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PDSOI and Radiation Effects:
An Overview
Josh Forgione
NASA GSFC / George Washington
University
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