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Volume 40, Number 1, 1996
Terrestrial cosmic rays and soft errors |
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Table of contents:  HTML |
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Copyright info |
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Critical charge calculations for a bipolar SRAM array |
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by L. B. Freeman |
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The critical charge,
Qcrit,
of a memory array storage cell is defined as
the largest charge that can be injected without changing the cell's logic
state. The
Qcrit
of a Schottky-coupled complementary bipolar SRAM array
is evaluated in detail. An operational definition of critical charge is
made, and the critical charge for the cell is determined by circuit
simulation. The dependence of critical charge on upset-pulse wave shape,
statistical variations of power supply voltage, temperature gradients,
manufacturing process tolerances, and design-related influences due to
word- and drain-line resistance was also calculated. A 2×
range in
Qcrit
is obtained for the SRAM memory array cell from simulations of normal
(±3 )
variations in manufacturing process tolerances, the shape of
the upset current pulse, and local cell temperatures. Using SEMM, a 60×
variation of the soft-error rate (SER) is obtained for this range in
Qcrit.
The calculated SER is compared to experimental values for the
chip obtained by accelerated testing. It is concluded that a range in
values of the critical charge of a cell due to normal manufacturing and
operating tolerances must be considered when calculating soft-error rates
for a chip.
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