Superconducting Technology Assessment - nitrd

ISSUES AFFECTING RSFQ CIRCUITS
Margins and Bit-Error-Rates
Any successful digital circuit technology requires large margins and low bit-error-rates (BER). Margins are the
acceptable variation for which the circuit performs as designed. BER is the error-incidence per device per operation.
Achieving high margins at high speed in RSFQ circuit technology depends critically on the symbiotic marriage
of circuit design and chip fabrication. Margins and, consequently, BER in superconductors are affected by many
factors, including all parameter spreads and targeting, noise, gate design, power (current) distribution cross-talk,
signal distribution cross-talk, ground plane return currents, moats and flux trapping, and timing jitter.
Bit Error Rate and Integration Scale
It is necessary to balance performance and manufacturing issues. IC values fall in the range 0.1-0.2 mA, to ensure
adequate noise immunity while keeping the total current and power dissipation as small as possible. A high J C value
improves speed, but requires smaller junction size, so parameter spreads may be more difficult to control.
Characteristic parameter spreads must be many times smaller than the operating margins of the circuits in order
to achieve VLSI densities. The requirements for petaflops computing are especially restrictive in that a very low BER
is required. If one assumes one error per year is acceptable, then a 64 bit processor with 100 gates/bit, capable of
petaflops throughput, would require a BER ~ 10 -26 . This BER can be used to quantify the requirements for foundry
IC process control.
BER
0.5
1e-20
1e-26
-------------------------------------------------------------------------------------------------------------------------------------
1e-52
0.7 0.9 1 1.1 1.3
Effective Margin
Figure 1. Calculated BER of a 2-JJ comparator as a function of operating margin, for IC =0.2 mA (lower curve), and IC =0.1 mA (upper curve).
Horizontal line is for 10-26 BER, which is corresponds to one error/year for petaflops capacity
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