Superconducting Technology Assessment - nitrd

Various moat protocols have been proposed and tried: long narrow channels, an array of circular holes, and random
sizes and shapes. Studies at NEC and UC Berkeley concluded that long, narrow moats surrounding each gate are
best, particularly if the area enclosed by the moat is smaller than the area of one flux quantum in the ambient field.
However, moats cannot fully surround a gate because that would require wideband data lines to cross a region
without a ground plane. Therefore, moats have breaks.
For a high probability of trapping flux in the moat, the moat inductance should be large to minimize the energy
require to trap the flux. Thus, E SFQ = Φ 02
/2L moat should be small. Narrow moats increase packing density, but have
low inductance per unit length; long moats increase inductance. Therefore, long, narrow moats appear to be the
best option. When flux is squeezed into the moat, it creates a corresponding shielding current in the surrounding
superconducting film. Just as ground-plane currents are not confined either under a microstripline or to the edge
of the ground plane, the shielding currents surrounding a moat are not confined to a London penetration depth
from the edge of the moat. They are spatially distributed on the ground plane as required to meet the boundary
condition that the normal component of magnetic field is zero. Assuming the length factor for shielding ground
currents is approximately the width of the moat, the moat should be as narrow as litho permits and at least one
width’s distance removed from the nearest circuit inductor.
A second source of trapped flux is the equivalent of ESD in semiconductors. Transient signals that are not well
filtered in any lead can produce trapped flux in the superconducting films and shift operating margins. The solution
here is to employ isolation techniques in addition to filtering, including optical isolators in input/output lines and
transmitting bias current to the shielded cryogenic environment by narrow band filtered RF.
Chips are usually flipped and bonded to a substrate with a superconducting film directly below the chip. This brings
the active circuits at most a few microns from another superconducting ground plane. Although there is no data
that supports this conjecture, one should be concerned and it needs to be addressed. EM simulations should help,
but it will have to be resolved experimentally.
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