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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 18 NO. 2
Fig. 5
(a) Comparison for drain current transients of planar and FinFET SRAMs when an ion of 30 MeV-cm
2
/mg strikes horizontally
through the drain. (b)
Q
coll
of planar and FinFET SRAMs from the horizontal track crossing the drain with different LETs
available for charge collection is smaller than that for
planar devices, so a smaller amount of deposited charges
canbe expected todiffuse to the FinFETdrain. Simulations
of the transistor response to energetic ion strikes were
performed using the Synopsys Sentaurus 3-D tool. Details
of the simulation procedure are described in Ref 14.
Two ion-strike conditions are considered in these simu-
lations. The first involves ion strikes to the drainwhere the
ion track either crosses the drain along its length in the
center of the drain body or occurs at normal incidence
fromthe top and vertically crosses the drain. In the second
condition, the ion strikes the bulk substrate at a distance
approximately 0.5 μm under the drain, and the ion track
does not cross the drain. The collected charge,
Q
coll
, of the
drain node of the
n
-FET is then calculated by integrating
the transient current over this time interval. An upset of
the data stored in the SRAM cell occurs when the
Q
coll
is
collected over a threshold value,
Q
crit
, which results in a
flip of the stored data. Figure 4 shows the drain voltage as
a function of time for both bulk FinFET and planar SRAMs
when the ion track crosses the drain along its length at
a time of 400 ps in the simulation. In this instance, an
incident ion linear energy transfer (LET) = 30MeV-cm
2
/mg
is chosen. In both cases, the drain voltage falls after the
ion strikes, while the voltage of the gate increases.
Q
crit
is
obtained by integrating the drain transient current that
results from a collision of an energetic particle with the
silicon substrate over the time at which the falling drain
voltage crosses the increasing gate voltage. The simulated
Q
crit
of the FinFET SRAM is 1.53 fC, and that of the planar
SRAM is 1.46 fC. The similarity of the
Q
crit
values of FinFET
and planar SRAMs is reasonable because the effective
W
gate
,
L
gate
, and
T
ox
, as well as the operation voltage, are
identical.
Figure 5(a) shows the comparison of the drain current
transients arising froma horizontal ion strike through the
n
-FET drain with an LET of 30 MeV for the bulk FinFET and
planar SRAMs. The time-dependent drain current pulses of
(a)
(b)