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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 18 NO. 2
16
SOFT-ERROR SUSCEPTIBILITY OF FinFET SRAMs
Anthony S. Oates and Yi-Pin Fang, TSMC Ltd., Hsinchu, Taiwan
aoates@tsmc.comEDFAAO (2016) 2:16-27
1537-0755/$19.00 ©ASM International
®
INTRODUCTION
For technology scaling beyond 20 nm, FinFET transis-
tors will replace the conventional planar geometry. The
driving force for the introduction of FinFET architecture
is the superior immunity to short-channel effects and the
reduction of the effects of process variation on device
performance exhibited by the FinFET.
[1-4]
Figure 1 shows
a comparison of architectures of the planar FET and the
FinFET. One of the major concerns with the introduc-
tion of a new transistor architecture is ensuring that the
transistor maintains the high level of reliability required
for silicon circuits. Fortunately, in terms of reliability, the
transition to FinFETs has been smooth; FinFETs do not
introduce new reliability issues beyond those with which
we are familiar.
[4]
One of the most important reliability concerns for
silicon circuits is soft errors in SRAM circuits, which
involves electrical upsets generated by the interaction of
energetic atomic and subatomic particles with the silicon
substrate material. SRAMs are particularly sensitive to
radiation-induced soft errors due to the relatively low
amount of charge at the storage nodes. Errors are gener-
ated by the impact of alpha particles emitted from trace
amounts of uranium in solder and packagingmaterials of
the circuit, andbyneutrons that originate in the cosmic ray
shower in the Earth’s atmosphere. Alpha particles gener-
ate charge in the silicon substrate as they lose energy.
[5]
Neutrons, because they are electrically neutral, induce
errors when they collide with the silicon atoms of the
substrate, leading to the generation of charged secondary
ions. These ions generate charge as theymove through the
silicon.
[6]
There is a secondmechanismwhereby neutrons
induce errors: Cosmic ray neutrons have a wide energy
range (unlike alpha particles emitted frompackagemate-
rials), and low-energy (slow) neutrons may be captured
by naturally occurring B
10
atoms close to the surface of
the silicon substrate.
[7]
The decay of the B
10
resulting from
the neutron capture produces an energetic alpha particle
together with a lithium ion, both of which can gener-
ate charge in the silicon. For the most advanced silicon
process technologies, B
10
is commonly incorporated in
the contact regions of transistors due to the use of B
2
H
6
as
a carrier gas during tungsten contact deposition.
[8,9]
Soft
errorsmay also be generated by cosmic raymuons, which
are actually more abundant in the cosmic background
radiation than neutrons. However, the authors’ recent
work
[10]
indicates that errors due to muons are negligible
compared to alpha particles and neutrons, and somuon-
induced errors will not be discussed here.
Fig. 1
3-D schematic of a bulk FinFET compared with a planar FET