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SOI CMOS

EECS 277A

Aishwarya Sankara

17723777

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Today

What is SOI?

Characteristics of SOI

Fabrication methods Basic categorization

Electrical anomalies

Advantages and Disadvantages

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What is SOI?

-SOI Silicon-on-Insulator

-Si layer on top of an

insulator layer tobuild active devicesand circuits.

-The insulator layeris usually made ofSiO2

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Characteristics

Include:

- High speed- Low power

- High device density

- Easier device isolation structure

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SOI Fabrication

Processes

-SOS Silicon-on-Sapphire

-SIMOX Separation by

Implantation of Oxygen

-ZMR Zone melting and

recrystallization

-BESOI Bond and Etch-

back SOI

-Smart-cut SOI Technology

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Categorization

-Categorization based on thethickness of the silicon film.

-The first is a partially-

depleted device and the latter

is a fully-depleted device.

-Each has its own advantages

and disadvantages.

-PD device threshold voltage

is insensitive to film thickness.

-FD device has reduced short

channel and narrow channel

effects.

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Electrical anomalies

Floating-body effect:-Usually seen inPartially-Depleteddevices.

- As shown in figure, theMOS structure isaccompanied by aparasitic bipolar devicein parallel.

-The base of this deviceis floating.

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Electrical anomalies

Kink Effect:

-Sudden discontinuity in draincurrent.

-Seen when the device is biasedin the saturation region.

-The bipolar device is turnedon.

Solution:

-Provide a body contact for the

device.

- Use FD devices.

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Electrical anomalies

Self-heating effect:

- Thermal insulation is provided by the oxide surface.

- Heat dissipation is not efficient.

- This happens only when there is logic switching in the device.

In fully-depleted devices, the threshold voltage is sensitive to

the thickness of the silicon film. Manufacturing process is comparatively difficult.

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Advantages of SOI

Suitable for high-energy radiation

environments.

Parasitic capacitances of SOI devices are much

smaller.

No latch-up.

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Advantages

-Easier deviceisolation

-High devicedensity

-Easier scale-down of

threshold voltage.

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Uses in digital and analog circuits

A combination of FD and PD devices are used

in digital circuitry.

Superior capabilities of SOI CMOS technology

usage in memory cell implementation.

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Uses in digital and analog circuits

SOI technology is useful for implementing

high-speed op-amps given its low Vt.

Higher transconductance (especially of FD)

implies higher gain.

Lower power consumption compared to bulk

devices at low current level.

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Disadvantages

Major bottleneck is high manufacturing costs

of the wafer.

Floating-body effects impede extensive usage

of SOI.

Device integration dopant reaction with the

oxide surface.

Electrical differences between and SOI nad

bulk devices.

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Conclusion

Due to its characteristics, SOI is fast becoming

a standard in IC fabrication.

Several companies have taken up SOI

manufacturing.

High-volume production of SOI is yet to

become common.

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References J. Kuo, Low- Voltage SOI CMOS VLSI Devices and Circuits. New York, John Wiley, Sept 2001.

J.Kuo, CMOS VLSI Engineering(SOI). Kluwer Academic Publishers, 1998.

Vivian Ma, SOI VS CMOS. University of Toronto.

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www.chips.ibm.com

THANK YOU!

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