Flexible Coaxial Ribbon Cable for High-Density Superconducting Microwave Device Arrays
Jennifer Pearl Smith, Benjamin A. Mazin, Alex B. Walter, Miguel Daal, J. I. Bailey, Clinton Bockstiegel, Nicholas Zobrist, Noah Swimmer, Sarah Steiger, Neelay Fruitwala
11 Flexible Coaxial Ribbon Cable for High-DensitySuperconducting Microwave Device Arrays
Jennifer Pearl Smith , Benjamin A. Mazin , Alex B. Walter , Miguel Daal , J. I. Bailey, III , ClintonBockstiegel, Nicholas Zobrist , Noah Swimmer , Sarah Steiger , Neelay Fruitwala
Abstract —Superconducting electronics often require high-density microwave interconnects capable of transporting signalsbetween temperature stages with minimal loss, cross talk, andheat conduction. We report the design and fabrication of super-conducting 53 wt% Nb-47 wt% Ti (Nb47Ti) FLexible coAXialribbon cables (FLAX). The ten traces each consist of a I I I Index Terms —Superconducting cables, Superconducting mi-crowave devices, Superconducting filaments and wires, Supercon-ducting resonators, Arrays, Microwave technology, Time domainreflectometry, Microwave power transmission.
I. I
NTRODUCTION S UPERCONDUCTING devices are revolutionizing a widerange of research and technological fields including quan-tum computing [1]–[4], nanowire single-photon detectors [5],X-ray microcalorimeters [6], submillimeter bolometers [7],and Microwave Kinetic Inductance Detectors (MKIDs) [8]–[11]. These applications require increasingly large supercon-ducting arrays, which present the common technical challengeof transporting microwave signals from the cold device stageto room temperature without losing or corrupting the signal orconducting excess heat to the cold stage. Low thermal conduc-tivity is especially important for detector arrays in the field orin space using adiabatic demagnetization refrigerators (ADRs)which have less cooling power than dilution refrigerators butoffer smaller form factors and simpler operation.Commercially available superconducting coaxial cables areoften used below 4 K; however, they are either semi-rigid andcumbersome to use in small cryogenic volumes, have largecross-sections yielding excessive heat loads, or both. Anotheroption is flexible superconducting circuits fabricated usinglithography techniques. These laminated cable technologiesboast low thermal conductivity and high-density interconnectsbut lack the length, durability, and signal isolation needed formany applications [12]–[16].
Fig. 1. Photographs showing a.) Close-up of cable end where NbTi centerconductors connect to center trace of GCPW transition board via stainlesssteel capillary tubing. b.) Fully assembled cable end with protruding microspot-welded, shared Nb47Ti ground shield. c.) Fully assembled cable spanningthe 3.4 K stage and the 90 mK cold ADR stage with a thermal sink at 800mK halfway down the length of the cable in the MKID Expolanet Camera(MEC) experiment [15], [17].
An optimal solution should be made from superconductingmaterial with a transition temperature well above 4 K tomaximize transmission with an encompassing ground shieldto minimize cross talk and pickup. It must have a small cross-section and be made from a low thermal conductivity material.Lastly, it should be flexible, durable, and ideally cheap andeasy to manufacture. Such a structure is difficult to realizebecause few materials have the desired properties and oftenare difficult to work with and interface with connectors.In this paper we present a superconducting FLexible coAX-ial ribbon cable (FLAX) which uniquely satisfies the afore-mentioned criteria. We developed this solution to carry broad-band signals for 10 000+ pixel multiplexed Microwave KineticInductance Detector (MKID) arrays for exoplanet detectionoperating at 90 mK [17], [18]. We expect this technology to beespecially relevant for superconducting technologies requiringhigh detector isolation and low thermal load. a r X i v : . [ phy s i c s . i n s - d e t ] J u l RT Duroid6010LMGold-PlatedCopper
G3PO ConnectorsVias
NbTiPFA Micro Spot Welds
Stainless SteelCapillary Tubing
Fig. 2. Exploded view of cable-end assembly diagram with key dimensionsshown in mm. Drawing is not to scale. From top/back to bottom/front: G3POhalf-shell connectors are soldered to the transition board. Two ground tabswith via borders and an intervening signal trace create a 50 Ω groundedcoplanar waveguide. The FLAX cable center conductors are crimped intostainless steel capillary tubing and soldered to the center traces. The FLAXground shield is spot welded to the ground tabs. The cable cross-section showsthe PFA (blue) insulated NbTi (grey) wire set in semicylindrical crimps madein the shared Nb47Ti foil ground shield. The two sides of the shield aremechanically and electrically bonded with micro spot welds less than λ { » II. FLAX D
ESIGN AND M ANUFACTURE
The FLAX cables are fabricated using I I . The shared outer coaxialconductor is formed with 0.025 mm [0.001”] Nb47Ti foilpurchased and rolled by ATI and HPM . The wires are heldin ten, I „ Ω characteristic impedanceand 3.56 mm standard trace pitch density used by G3POconnectors available from Corning Gilbert (compatible withSMP-S) (see Fig. 1, 2). The two sides of the ground shieldare mechanically and electrically bonded by micro spot weldswhich run the length of the cable between each trace. Thewelds are approximately every 2 mm which is less than λ { “ I Ω grounded coplanar waveg-uide (GCPW) geometry for increased signal isolation. Betweeneach trace, the Nb47Ti outer conductor foil is micro spotwelded to the ground tabs of the transition board while surfacemount coaxial G3PO push-on connectors are soldered to theother end of the GCPW (Fig. 1a). The cable end assembly is Supercon Inc., 830 Boston Turnpike, Shrewsbury, MA. ATI Specialty Alloys & Components, 1600 Old Salem Rd., Albany, OR. Hamilton Precision Metals, 1780 Rohrerstown Rd., Lancaster, PA. Corning Optical Communications, 4200 Corning Place, Charlotte, NC. -3dB-3dB
Capillary Tube FLAX Cable G3POConnector -3dB-3dBSMA-G3POAdapter Coax SMA-G3PO Adapter CoaxCoaxCalibrationThroughDUT
Transition Board
Fig. 3. Schematic diagram depicting FLAX attachment to the G3PO push-onconnectors via a capillary tube soldered to a coplanar waveguide transitionboard and the device under test (DUT) circuit at 4 K. clamped in a 3 ˆ ERFORMANCE C HARACTERIZATION
Transmission loss ( S ), cross talk ( S ), and time domainreflectometry measurements were performed in a dilutionrefrigerator under vacuum at 4 K with a Keysight N9917Anetwork analyzer. The device under test circuit consisted of theassembled FLAX cable with a 3 dB cryo-attenuator obtainedfrom XMA and a 25 cm nonmagnetic SMA-to-G3PO adaptercoaxial cable obtained from Koaxis on either end (see Fig. 3).A Crystek braided, semi-rigid coax through line was used asa calibration reference. Repeated handling through the testingprocess revealed the cables have a minimum inside bend radiusclose to 2 mm and are robust to cryogenic cycling. A. Transmission
Ripples in the FLAX transmission suggest standing wavemodes are present on the traces which is indicative of animpedance mismatch between the FLAX cable and the 50 Ω circuit (see Fig. 4). The transmission ripples are not uniformlyharmonic which suggests the impedance is changing withlength along each trace. This could be explained by flawsin micro spot welding placements along the cable whichdetermine the distance between the inner and outer coaxialconductors and therefore the characteristic impedance. Thecharacteristic impedance of the traces were probed usinga time domain reflectometry measurement adjusted for loss(see [19] for details on loss correction) which confirmed theimpedance varies from 55–65 ˘ Ω along the traces (seeFig. 5). This mismatch at various points in the cable launchesreflected waves which contribute to the observed ripple.We hypothesize an additional factor contributing to theimpedance mismatch originates in the intermediate regions of XMA Corporation-Omni Spectra, 7 Perimeter Road, Manchester, NH.P/N: 2082-6040-03-CRYO Koaxis RF Cable Assemblies, 2081 Lucon Road, Schwenksville, PA.P/N: AO10-CC047C-YO18 Obtained through Digikey. P/N: CCSMA18-MM-141-12 -8-6-4-202 P o w e r [ d B ] Frequency [GHz] -80-70-60-50-40 P o w e r [ d B ] S S Fig. 4. Top: S (transmission) measurement of sample FLAX traces from various cables at 4 K. Bottom: S (nearest neighbor forward cross talk)measurement of sample FLAX traces from the same cable at 4 K. The average cross talk level is given by the dashed red line. the cable ends where the center conductor exits the foil sheathand transitions onto the GCPW transition board (see Fig. 1, a.).After exiting the ground shield, the exposed wire can act as aninductor. Previous work done by our group shows inductanceon the input and output of a transmission line causes rippleswhich increase in magnitude at higher frequencies [15]. Thisis because the impedance of a perfect inductor grows linearlywith frequency, i.e., Z L “ jωL . With each successive cableiteration, manufacturing techniques improved, the length ofexposed wire was shortened, and the frequency-dependentripple amplitude diminished. The use of a capillary tube topin the hair-like center conductor close to the transition boarddramatically reduced the cable end inductance.Using the peak of the ripple, we report the loss of the 30 cmcable at 8 GHz to be roughly 1 dB which is slightly higherthan the 0.5 dB/m loss reported by commercially availablesuperconducting coaxial cables [20], [21]. This differencecannot be explained by a difference in cable materials orgeometry [22]. Likely, the source of our additional loss is theimpedance mismatch caused by manufacturing imperfectionswhich produce reflections in the cable and off the ends asdescribed above. B. Cross Talk
We found the average nearest-neighbor forward cross talkto be -60 dB (see Fig. 4). This is roughly 30 dB lowerthan what we previously achieved using flexible laminatedNbTi-on-Kapton microstrip cables [15]. Since the cable’sinstallation in the MKID Exoplanet Camera (MEC) at Subaru
Fig. 5. A typical time domain reflectometry (TDR) measurement of thecryogenic signal path showing the characteristic impedance at lengths alongthe signal path. Commercially available 50 Ω standard coaxial cables borderthe FLAX cable highlighted by the double arrow. Note the TDR measurementis accurate to ˘ Ω . Observatory, this enhanced isolation has increased our pixelyield „
20% [17]. We suspect this large improvement isbecause the exposed microstrip geometry allows trace-to-tracecoupling whereas the coaxial nature of the FLAX shieldsthe center conductors thereby preventing signal corruption. In
TABLE IS
UMMARY OF THERMAL , MECHANICAL , AND MICROWAVE PROPERTIES OF SUPERCONDUCTING COAXIAL RIBBON CABLE , LAMINATED MICROSTRIPCABLE , AND BEST COMMERCIALLY AVAILABLE SUPERCONDUCTING COAXIAL CABLES
Thermal Load Mechanical MicrowaveCable per trace [nW] All Dimensions [mm] Values at 8GHz100mK to Trace OD Min. Inside Conductor Dielectric Cross Talk Attenuation I ) Bend Radius Material Material [dB] [dB]FLAX
16 800 3 .
556 0 .
376 2
Nb47Ti PFA ´
60 1
CryoCoax
26 1400 ą . . NbTi PTFE N/A ă . KEYCOM
34 1800 ą .
860 8
NbTi PTFE N/A ă . Nikaflex
16 460 3 .
556 0 . . Nb47Ti Nikaflex ´
25 1 Computed using dimensions available from [15], [20], [21] and assuming a cable length of 30 cm. Estimated with ripple peak. For the microstrip geometry this is the total cable thickness. Kapton polyimide film manufactured by Dupont, see [15] for details.Fig. 6. We computed a cable thermal conductivity G p T q in units of µ WcmK ´ by summing the thermal conductivity of each constituent mate-rial weighted by the cross-section [15], [23]. The cable previously developedby our lab (Nikaflex, gold) is compared with the subject of this paper (FLAX,salmon), and two commercial options by KEYCOM (P/N: NbTiNbTi034,burgundy) and Cryocoax (P/N 5139-P1NN-611-100P, pink). Solid lines arecomputed using literature values for Nb47Ti [24], [25], PTFE [23], Nikaflex(Kapton polyimid film) [15], [26], and Pyralux [24]. PTFE values were usedto estimate the PFA dielectric in the FLAX cable [22]. Dashed lines indicateextrapolation. early iterations of the cable, we found infrequent or failedmicro spot welds in the ground shield lead to much higherlevels of cross talk. This leads us to conclude incorporatingmicro spot welds less than λ { apart between the tracesreduces electromagnetic coupling. C. Thermal Conductivity
Following previous convention, a cable thermal conductiv-ity, G p T q , was computed by summing literature values of con-stituent materials weighted by their cross-sections (see Fig. 6)[15], [23]. We compare our superconducting coaxial ribboncable to two commercially available superconducting coaxial cables as well as our lab’s previously developed laminatedNbTi-on-Kapton microstrip cables [15]. We estimate the ther-mal conductivity of the PFA dielectric present in the flexiblecoaxial ribbon cables using PTFE; the same dielectric used inthe two commercial solutions [22]. The smallest commerciallyavailable superconducting coaxial cables from KEYCOM and CryoCoax were chosen for comparison. The electricaland thermal properties of the cables are summarized in table I.The heat load from one temperature stage to another canbe computed by integrating values in Fig. 6 from T to T ( T ă T ) and dividing by the cable length. The ten-traceFLAX cables are currently installed in the MEC experimentwhere they span 33 cm from the 3.4 K stage to the 90 mKcold ADR stage with a thermal sink at 800 mK about halfwaydown the length of the cable [18]. We estimate they generate athermal load of „
200 nW on the 90 mK cold ADR stage. Thisis about equivalent to the thermal load created by the Nikaflexcables and approximately half the computed heat load of eithercommercial option. IV. C
ONCLUSION
We have manufactured a superconducting flexible coax-ial cable capable of delivering microwave signals betweentemperature stages with minimal loss, cross talk, and heatconduction. Strong signal isolation is especially importantfor our application of moving 4-8GHz servicing 10 000+multiplexed sensors across temperature stages. The FLAXcable represents a 30 dB improvement in cross talk as com-pared to our group’s previously developed NbTi-on-Kaptonmicrostrip cables. This enhanced isolation facilitated a „ KEYCOM Corp. 3-40-2 Minamiotsuka,Toshima-ku Tokyo.P/N: NbTiNbTi034 CryoCoax - Intelliconnect, 448 Old Lantana Road, Crossville, TN.P/N: 5139-P1NN-611-100P push-on, small form factor connectors and reduced trace pitchallow for increased detector density in a cryogenic system.We found an attenuation of dB at 8 GHz with „ CKNOWLEDGMENT
J. P. Smith is supported by a NASA Space TechnologyResearch Fellowship under grant number 80NSSC19K1126.R
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