forgionemapld 2005/p1041 pdsoi and radiation effects: an overview josh forgione nasa gsfc / george...

Forgione MAPLD 2005/P1041

PDSOI and Radiation Effects: An Overview

Josh Forgione

NASA GSFC / George Washington University


Abstract• Silicon-On-Insulator (SOI) Complementary Metal-Oxide-

Semiconductor (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.


Introduction

P-type, ‘Bulk’’ Substrate

+ VDVG

n+

+ VS

n+

n well

+ VDVG

p+

+ VS

p+

• Traditional CMOS is fabricated on a bulk substrate.

Figure 1: Bulk CMOS device structure


Introduction• SOI MOSFETs are isolate from the substrate and

each other via a thick insulator, or ‘buried oxide’ (BOX)

p Substrate

+ VDVG+ VS + VDVG+ VS

n+ n+ n+ p p p

SiO2 Insulator – ‘Buried Oxide’

Figure 2: SOI CMOS device structure


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

t S

i

A: PDSOI

p Body

x dm

ax Front Gate Depletion

B: FDSOI

Front Gate Oxide

Back Gate Buried Oxide (BOX)

Back Gate Depletion

Fully Depleted

Body

Front Gate Oxide

Back Gate Buried Oxide (BOX)

t S

i

x dm

ax

Figure 3: Distances between front and back-gate depletion regions define PDSOI (A) and FDSOI (B).


Introduction• Equivalent PDSOI circuit model:

n+ p n+

p substrate (back gate)

+ VD

VG

+ VS

VBG

Buried Oxide (BOX)

n+ n+


+ VD

VG

+ VS

VBG

Buried Oxide (BOX)

p

Figure 4: Transistors (left), capacitors, and diodes (right) embedded in the PDSOI MOSFET structure.


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


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]


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

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].


PDSOI and SEUsFigure 7: Sensitivity of the 68020 microprocessor, in CMOS/thick SOI and CMOS/epi taxial technologies, for two different test programs[4 via 5].


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]


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)

Table I: Experimental and Calculated LET Threshold in SOI SRAMs in MeV/(mg/cm2), Bipolar Amplification Factors β*[4]

Figure 9: Parasitic BJT in PDSOI

n+ p n+


+ VD

VG

+ VS

VBG

Buried Oxide (BOX)


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].


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].


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 cure-all for radiation-tolerant designs – Inherent parasitics lead to TID and SEU

sensitivities in PDSOI.


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


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 High-

Speed 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.


PDSOI and Radiation Effects: An Overview

Josh Forgione

NASA GSFC / George Washington University

forgionemapld 2005/p1041 pdsoi and radiation effects: an overview josh forgione nasa gsfc / george...

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