Superconducting Technology Assessment - nitrd

Supplying DC current to all of the SCE chips in parallel would result in a total current of many kiloAmperes. Several
methods may be used to reduce this total current and heat load. One technique is to supply DC current to the SCE
chips in series, rather than in parallel (current recycling) taking advantage of very low resistances of lines in SCE
circuits. However, it would require true differential signal propagation across floating ground planes. High
temperature superconductor (HTS) cables may be used to bring in DC current from 77 K to the 4 K environment.
Another solution is to use switching power supplies: high voltages/low currents can be brought near SCE circuits
and conversion to low voltages/high currents, all at DC, can occur at the point of use. However, this method
employs high power field-effect transistor switches, which themselves can dissipate significant power.
Of these methods, current recycling shows the most near-term promise. Laboratory level experiments were
conducted for small scale systems. The demonstration of a large-scale SCE system with kiloAmperes of current was
not performed yet. For smaller IO counts, coaxial cables can be used for both high and medium-speed lines. Coax
cables have different sections. The middle section is a short length of stainless steel coaxial cable that has a
high thermal resistance and a bottom section of non-magnetic Cu coaxial cable for penetration into the
magnetically-shielded chip housing. For higher-frequency input signal up to 60 GHz and clock lines, Gilbert-
Corning GPPO connectors are used. Flexible BeCu microstrip ribbon cables were also considered for medium speed
output lines. At 1 GHz, electrical attenuation and heat conduction of these ribbon cables were measured to be
nearly identical to the tri-section coax cable. Modified connector assemblies were tested at room temperature and
at liquid nitrogen temperature and found to be within specification for both reflection and attenuation.
For systems with hundreds or thousands of I/O lines, coaxial cables are not practical. To meet this challenge flexible
ribbon cables have been developed 8 . These cables consist of two or three layers of copper metallization separated
by dielectric films, typically polyimide. With three copper layers, the outer two layers serve as ground planes and
the inner layer forms the signal lines, creating a stripline configuration. Stripline cables provide excellent shielding
for the signal lines. The dielectric layers consisted of 2 mil thick polyimide films. The ground planes were fabricated
from 0.6 micron-thick sputtered-copper films, while the signal lines were patterned from a 4-micron-thick
plated-copper film. The signal line width was 3 mils for 50Ω impedance with a pitch of 70 mils and each cable has
more than hundred leads. In addition to large I/O count and multi-GHz connections, flexible ribbon cables offer the
advantage of low thermal conduction to the cold stage. Cables can be manufactured with copper films the
thickness of the electrical skin depth for a particular frequency of interest. Successful cable-attach procedures with
high reliability and low cost were also demonstrated. Signal speeds up to 3 GHz were experimentally demonstrated.
The construction of the cable should allow operation up to 20GHz after minor modifications.
Interposer Board
LNA Board
Interposer Strip
Dewar Wall
Breakout
Board
Heat stationed
at cold pedestal
Heat stationed
at 1st stage
Substrate with traces connecting
input and output lines
Interposer Board
Interposer Strip
Cable
Metal pads
Indium bumps
Figure 12. A high-speed flexible ribbon cable designed for modular attachment (Ref: NGST).
8
“Cryogenic Packaging for Multi-GHz Electronics” T. Tighe et al, IEEE Tran. Applied Superconductivity, Vol 9 (2), pp3173-3176, 1999.
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