SOI stands
for silicon on insulator. This technology offers the possibility
of building electronic devices in a thin layer of silicon that
is electrically isolated from the thick semiconductor substrate
through the use of a buried insulating layer. In the standard
silicon technology the semiconductor substrate is associated with
undesirable effects such as high leakage currents, parasitic bipolar
components, and, more importantly, interference between individual
active devices or circuits built in the same integrated chip.
In addition, the use of SOI technology in CMOS (complementary
metal oxide semiconductor) circuits may also shrink the dimensions
of MOSFET (metal oxide semiconductor field effect transistor)
devices and further push the frequency limit of silicon technology.
The first confirmation that SOI technology is becoming the state-of
the art technology in low-power IC's came in 1999, when IBM launched
the first fully functional SOI mainstream microprocessor. The
IBM specifications predict a 25-35% improvement over similar bulk
CMOS technology, which is equivalent to about two years of progress
in bulk CMOS design and fabrication. Besides the fast speed, other
benefits of the new SOI chip are reduced power consumption (up
to 3 times) and a small soft error rate.
While SOI
has already become a mature technology for low power CMOS circuits
and justifiably represents the main manufacturing technology for
the next generation of ULSI (Ultra Large Scale Integration) low
power circuits, it is still an emerging technology for high voltage
power integrated circuits. An increased effort is now directed
towards developing advanced SOI power device structures for integrated
circuits. Compared to bulk junction isolated (JI) devices, SOI
devices and circuits offer improved isolation, reduced leakage
currents and faster switching speed. Nevertheless, power devices
made in SOI technology may suffer from reduced breakdown levels,
self-heating effects and latch-up. Several solutions to address
these drawbacks partially or totally have been proposed in this
paper.
This paper
gives an introduction to SOI technology and devices, reviews major
steps in developing SOI power devices for integrated circuits,
discusses advantages and drawbacks of SOI devices compared to
bulk silicon devices and finally reveals the most recent developments
in high voltage SOI structures.
SOI TECHNOLOGY
Creating
defect free films of single crystal silicon on an insulating layer
is a technological challenge. Several techniques have been developed
to do this, but of these only a few have been commercially successful:
silicon-on-sapphire (SOS), separation by implantation of oxygen
(SIMOX), bonded and etched back SOI (BESOI) and uni-bond wafers
produced by the smart cut process. Each of these in turn are discussed
briefly:
Until the
1980's silicon-on-sapphire was the Most mature of all SOI materials.
It is produced by growing thin films of single crystal silicon
on a sapphire wafer by chemical vapor deposition from silane gas
at 1000°C. such films have the advantage of lying on a substrate
which is extremely good conductor of heat; also they are highly
immune to radiation. However, cost of silicon-on-sapphire material,
the relatively poor quality of silicon films, and the emergence
of new SOI technologies such as SIMOX and wafer bonding which
are based purely on silicon have prevented it from becoming a
mainstream SOI technology; its use remains a niche market.
