October 2000 Newsletter - Naval Postgraduate School

RESEARCH CENTER
RAD-HARD SEMICONDUCTOR CHIPS, continued from page 14
components have been tested for strategic radiation levels.
Additionally several of the newer programs, National Missile
Defense (NMD) and TBM and many of the surveillance
programs require radiation hardened-components. Further
underground tests have been eliminated and many of the
radiation testing facilities for weapons effects have been taken
out of service.
Technical Issues
Each successive generation of transistors shrinks in size. Ten
years ago each individual transistor “pushed” about 5-10
million electrons from circuit node to circuit node. Today, the
average transistor switches about 100,000 electrons. In the
next 5 years or fewer, it will be 1,000 electrons. Cosmic
particles from space or byproducts of radioactive isotopes in
everyday materials interact with transistors and ionize millions
of electrons. These “single particle” events are known as “soft
errors” which randomly turn transistors on or off causing
computer programs to fail. The density of cosmic particles at
the earth’s surface is much less than in space (approximately
10 cosmic particles pass through your body a day), however
many more low energy particles exist than high energy ones at
sea level, thus as each new generation is released the ICs
become more susceptible to natural radiation. Before 1979,
the relatively “large” transistors showed no susceptibility for
single particle radiation anomalies, but each following
generation has shown more problems. Unknown to most, at
50,000 feet in the upper atmosphere a neutron cloud is
produced from cosmic particle interaction. In the last two
decades integrated circuits in avionics, and in computer
systems at high elevations have shown increased susceptibility
to natural radiation. Much closer to everyone’s desktop, in the
last two years IBM and INTEL have had to switch completely
to alpha particle-free solder due to prevent upsets from
decaying isotopes in the lead solder. Studying the present
roadmap for semiconductors, almost all ICs will become
susceptible in the next decade to “natural” radiation. Imagine
the difficulty for space and strategic system engineers to
design for harsher radiation environments.
In oxides, ionized electrons can get “trapped” in the insulator
and over years (in space applications) or nanoseconds
(weapons applications), charge builds up interfering with the
transistor operation. This “total dose” of radiation to initiate
failure is known as Total Dose and is in units of Rads(10 -7 J/
gm). Rad-hard ICs are designed with special oxides that can
withstand Megarads. Commercial ICs at best can survive to
20K Rads. One recent worry from DSWA (Defense Special
Weapon Agency) is that a rogue nation could launch one
small nuclear weapon into the upper atmosphere, which
could produce sufficient ionizing radiation to pump up the
Van Allen radiation belts from underneath and engulf many
of the Low Earth Orbit (LEO) satellites in radiation. Almost
all commercial satellites at LEO utilize unhardened commercial
parts. Such an event would cause hundreds of satellites to
fail due to Total Dose effects within 30-60 days and this
expanded radiation belt would last on the order of a year.
Previous U.S. and Soviet upper atmosphere nuclear tests in
the 50s and 60s reeked havoc on early satellite systems.
Today’s semiconductor chips in some respects are more
vulnerable, with many more satellites at stake.
The same particles that can ionize electrons, with more
energy these particles can displace atoms in a semiconductor
crystal or in insulating oxides. Each displacement of an atom
introduces a charged point defect, essentially an atom out of
place in the crystal. In the near future, the capability of a
single particles displacing one Oxygen or one Si atom in a sixto-eight
atom thick Silicon Dioxide layer would permanently
damage the transistor.
--continued on page 16
An example of a two-dimensional simulation of a cosmic
particle ionizing charge in a MOSFET transistor. The
simulation code allows researchers to track how ionized
charge moves through a semiconductor structure.
NPS Research page 15
October 2000