Today…
• What is SOI?• Characteristics of SOI• Fabrication methods• Basic categorization• Electrical anomalies• Advantages and Disadvantages
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
Characteristics
Include:- High speed- Low power - High device density- Easier device isolation structure
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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’.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
Advantages of SOI
• Suitable for high-energy radiation environments.
• Parasitic capacitances of SOI devices are much smaller.
• No latch-up.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
Advantages
-Easier device isolation
-High device density
-Easier scale-down of threshold voltage.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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.
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE
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!
04/19/23 UNIVERSITY OF CALIFORNIA, IRVINE