Superconducting Technology Assessment - nitrd

3D Packaging an Alternative
An alternative to planar packaging on MCMs and boards is 3-D packaging. Conventional electronic circuits are
designed and fabricated using a planar, monolithic approach in mind with only one major active device layer. More
compact packaging technologies can bring active devices closer to each other allowing short Time-of-Flight (TOF),
a critical parameter for systems with higher clock speeds. In systems with superconducting components, 3-D packaging
enables higher active component density, smaller vacuum enclosures, and shorter distances between different
sections of the system. As an example, 3-D packaging will allow packing terabytes to petabytes of secondary
memory in a few cubic feet (as opposed to several hundred cubic feet) and much closer to the processor.
Power and I/O Cables Needed
SCE circuits for supercomputing applications are based on DC-powered RSFQ circuits. Due to the low voltage (mV
level), the total current to be supplied is in the range of few Amperes for small-scale systems and can be easily
increased to kilo-Amperes for large-scale systems. Serial distribution of DC current to small blocks of logic has been
demonstrated, and this will need to be accomplished on a larger scale in order to produce a system with thousands of
chips. However, the panel can expect that the overhead of current-supply reduction techniques on-chip will drive
the demand for current supply into the cryostat as high as can be reasonably supported by cabling. Additionally,
high serial data rate in and out of the cryostat is expected for a petaflops-scale system. This necessitates RF cabling
that can support high line count to service thousands of processors while maintaining the high signal integrity and
low losses required for low bit error rate.
Refrigeration
The technology for the refrigeration plant needed to cool large systems is understood. Small space and commercial
cryocoolers are available, but engineering changes are needed to enlarge them for larger-scale systems. One key
issue is the availability of manufacturers. Development funding may be needed for U.S. companies to insure that
reliable American coolers will be available in the future. Development toward a 10 W or larger 4 K cooler would
be desirable to enable a supercomputer with a modular cryogenic unit.
System Testing Required
A petaflops-scale superconducting supercomputer is a very complex system and offers major challenges from a system
integrity viewpoint. A hierarchical and modular testing approach is needed. The use of hybrid technologies—
including superconducting components, optical components and conventional electronic components and system
interfaces with different physical, electrical and mechanical properties—further complicates the system testing. This
area requires substantial development and funding to insure a fully functional petaflops-scale system.
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