SOI CMOS EECS 277A

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SOI CMOSEECS 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 to build
  active devices and circuits.
• The insulator layer is usually made of SiO2

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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 the thickness 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 in Partially-Depleted devices.
• As shown in figure, the MOS structure is
  accompanied by a parasitic bipolar device in
  parallel.
• The base of this device is floating.

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Electrical anomalies

• Kink Effect
• Sudden discontinuity in drain current.
• Seen when the device is biased in the saturation
  region.
• The bipolar device is turned on.
• 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 device isolation
• -High device density
• -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.
• www.google.com
• www.chips.ibm.com
• THANK YOU!