20-07-2012, 03:22 PM
Radiation-Induced Soft Errors in Advanced Semiconductor Technologies
2005Induced Soft Soft Error.pdf (Size: 381.47 KB / Downloads: 166)
Abstract
The once-ephemeral radiation-induced soft error has
become a key threat to advanced commercial electronic components
and systems. Left unchallenged, soft errors have the potential
for inducing the highest failure rate of all other reliability
mechanisms combined. This article briefly reviews the types of
failure modes for soft errors, the three dominant radiation mechanisms
responsible for creating soft errors in terrestrial applications,
and how these soft errors are generated by the collection of
radiation-induced charge. The soft error sensitivity as a function
of technology scaling for various memory and logic components is
then presented with a consideration of which applications aremost
likely to require soft error mitigation.
INTRODUCTION
AS the dimensions and operating voltages of computer
electronics are reduced to satisfy the consumer’s insatiable
demand for higher density, functionality, and lower
power, their sensitivity to radiation increases dramatically.
There are a plethora of radiation effects in semiconductor
devices that vary in magnitude from data disruptions to permanent
damage ranging from parametric shifts to complete
device failure [1], [2]. Of primary concern for commercial
terrestrial applications are the “soft” single-event effects (SEEs)
as opposed to the “hard” SEEs and dose/dose-rate related
radiation effects that are predominant in space and military
environments. As the name implies, SEEs are device failures
induced by a single radiation event. The author uses the term
soft error throughout the text to encompass the key SEE that
affects commercial semiconductor technologies. But it is useful
to be aware of the different failure modes.
THE TERRESTRIAL RADIATION ENVIRONMENT
What Radiation Does in Silicon
The terrestrial environment is dominated by three different
mechanisms (described in the next section) that generate (either
directly or as secondary reaction products) energetic ions that
are responsible for inducing soft errors. The magnitude of the
disturbance an ion causes depends on the linear energy transfer
(LET) of that ion (typically reported in units of megaelectron
volt square centimeter per milligram). In a silicon substrate, one
electron hole pair is produced for every 3.6 eV of energy lost
by the ion. With a simple conversion, the LET can be plotted in
more convenient units of charge loss per distance as illustrated
in Fig. 1. The LET is dependent on the mass and energy of
the particle and the material in which it is traveling. Typically,
more massive and energetic particles in denser materials have
the highest LET.
High-Energy Cosmic Rays
The second significant source of SER is related to cosmic
ray events. Primary cosmic rays are thought to be of galactic
origin. They react with the Earth’s atmosphere via the strong
interaction and produce complex cascades of secondary particles.
These in turn continue on deeper into the atmosphere,
creating tertiary particle cascades, and so on. At terrestrial
altitudes (as opposed to flight or satellite altitudes), less than
1% of the primary flux reaches the sea level where the flux
is isotropic and composed of muons, protons, neutrons, and
pions [13]. Neutrons are one of the higher flux components,
and since neutron reactions have higher LETs, they are the most
likely cosmic radiation to cause upsets in devices at terrestrial
altitudes (assuming 10B and alpha emitting impurities have
been minimized). The “accepted” cosmic differential neutron
flux at sea level is shown in Fig. 4. This curve defines how
many neutrons over the given energy range are incident on a
device at sea level. Recent work has been published improving
the accuracy of this data [14], [15]. The neutron flux is strongly
dependent on altitude with the intensity of the cosmic ray
neutron flux increasing with increasing altitude.
CONCLUSION
Ionization collected from terrestrial radiation events can
cause data errors leading to failures in electronic devices.
At terrestrial altitudes, three mechanisms are responsible for
soft errors: the reaction of high-energy cosmic neutrons with
silicon and other device materials, the reaction of low-energy
cosmic neutrons with high concentrations of 10B in the device,
and alpha particles emitted from trace radioactive impurities
in the device materials. The soft error sensitivity of various
memory and logic devices used to create advanced commercial
electronic systems as a function of technology scaling has been
presented. The author has shown that while the SER of DRAM
in a system is relatively unchanged by scaling, SRAM and
peripheral logic system SER are increasing rapidly with each
new technology node. The cost and efficacy of various methods
to mitigate soft errors have also been reviewed along with the
conclusion that the most effective way to improve memory
system soft error reliability is to employ EDAC techniques,
while sequential logic robustness can best be improved by
design hardening and spatial and time redundancy.