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JBenford CAAllenJAChervenakMMFreund ASKutyrevSHMoseleyRAShaferJGStaguhn NASA GoddardSpaceFlightCenterCode685GreenbeltMD20771 ENGrossmanGCHiltonKDIrwin JMMartinisSWNamCDReintsema NIST BoulderMS810 BoulderCO8005 Abstr

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MULTIPLEXEDREADOUTOF SUPERCONDUCTINGBOLOMETERS D.J.Benford ,C.A.Allen,J.A.Chervenak,M.M.Freund, A.S.Kutyrev,S.H.Moseley,R.A.Shafer,J.G.Staguhn NASA GoddardSpaceFlightCenter,Code685,Greenbelt,MD20771 E.N.Grossman,G.C.Hilton,K.D.Irwin, J.M.Martinis,S.W.Nam,C.D.Reintsema NIST Boulder,MS81#.0%, Boulder,CO80%05 Abstract Studies of emission in the far-infrared and submillimeter from astrophysical sources require large arrays of detectors containing hundredsto thousands of elements. A multiplexed readoutisnecessaryforpracticalimplementationofsucharrays, and can be developed using

S/UIDs, such that, e.g., a %2 %2 array of bolometers can be read out using 100 wires rather thanthe 2000neededwithabruteforceexpansionofexisting arrays. These bolometer arrays are made by micromachining techniques,usingsuperconductingtransitionedgesensorsasthe thermistors. We describe the development of this multiplexed superconductingbolometerarrayarchitectureasasteptoward bringing about the 3rst astronomically useful arrays of this design. ThistechnologywillbeusedintheSAFI45instrument on SOFIA, and is a candidate for a wide variety of other spectroscopicandphotometricinstruments. keywords:

Bolometers,S/UIDs,multiplexing,transitionedge sensors,farinfrared,submillimeter ThisworkwasperformedwhiletheauthorheldaNationalResearchCouncil-Goddard SpaceFlightCenterResearchAssociateship.
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Introduction Advances in bolometer fabrication have made possible the construction of submillimeter-wavelength bolometer arrays with up to 100 detectors 6e.g., CSO S7A4C [1] ,8CMT SCUBA [2] , I4AM %0m MAMBO [3] 9. Currently, the sensitivity of these instruments is background-limited, so deep- and wide-3eld surveys are limited by the number of detectors and the amount of observing time

available. In order to achieve a leap to 10,000detectors6oforderthelargestsizeusableoncurrentandforeseen telescopes9,ascalabledetectorarchitecturemustbedemonstrated. Inthis paper,wepresentademonstrationofanarchitecturewhichcanbescaledto kilopixelarraysusingsuperconductingsensorsandamultiplexedampli3er technique [4] to reduce the wiring overhead. In our implementation, we choose to use the close-packed geometry, which yields an improvement in mapping speed per focal plane area [5] . 7owever, the superconducting detectorscanbeusedregardlessofarraygeometry,andareequallyfeasible for arbitrary

array implementations. Detector arrays of this type are currentlybeingdeveloped foruseintheSAFI45instrumentforSOFIA [6] andforupgradedground-basedspectrometersFIB45 [7] andSPIFI [8] SuperconductingBolometers The transition between the superconductingand the normal state can be used as an extremely sensitive thermometer 6a Transition 5dge Sensor, or T5S9. A thin 3lm, held at its transition temperature, requires only a tiny additional heat input to warm it above its transition, increasing the resistancebyalargefraction. Infact,thesuperconductingtransitioncanbe

verysharp,yieldingadimensionlesssensitivity log R/d log 1000. 4ecently, we have fabricated thin 3lm superconducting bilayers of molybdenum and gold [9] and molybdenum and copper [10] .Onesuch transition is shown in Figure 1= it features a bilayer with #00 Aof molybdenum and 750 A of gold, yielding a normal resistance of %%0mΩ. Nearitstransitiontemperatureof##0mK,thesensitivityreaches 1100. Because the transition region is narrow 6 1mK9 compared to the temperature above the heat sink 6150mK above a 7e refrigerator at %00mK9, the T5S is nearly isothermal across the transition. In use as a

detector, the power applied to raise the T5S into its transition region is nearly constant. This has the eAect that the response becomes linear to better than 1B, substantially better than the typical linearity achievedwithsemiconductingbolometerthermistors. Typically,additional devices can be fabricated with transition temperatures reproducible to Int. 8.I4MMWaves–2
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0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.400 0.410 0.420 0.430 0.440 0.450 0.460 0.470 0.480 0.490 0.500 Resistance ( Temperature (K) = 0.3277 = 0.4521 K Sensitivity = 1100 Fig.

1.—SuperconductingtransitionofaMoAubilayer. 2B, repeatable normal state resistances, and stray resistances of less than %mΩ. The bilayer process allows the transition temperature to be tuned by varying the relative thicknesses of the normal metal 6gold or copper9 and superconducting metal 6molybdenum9 layers. In this manner,detectorsoptimizedforperformanceinavarietyofdiAerentoptical loads and operating temperatures 6e.g. broadband imaging, narrowband spectroscopy9canbeproduced. In order to bias the T5S, a voltage source is provided by passing a current through a parallel arrangement of a

small shunt resistor and the T5S 6Figure 29. Because the sensitivity of a T5S is large 6 1100 for the 3lm in Figure 1 vs. 5 for a semiconducting thermistor9, a voltage-biased T5S is stabilized by strong electrothermal feedback [11] In this mechanism, an increase in temperature yields a sharp increase in resistance, which reduces the current Eowing through the T5S, lowering the bias power and decreasing the temperature. This enables the devices torespondveryquickly6timeconstantsof 1ms9comparedtoitsphysical timeconstant. MultiplexedSQ,IDAmplifier

Alow-impedancedetectorsuchasasuperconductingT5Siswell-matched by a superconducting quantum interference device 6S/UID9 ampli3er. Fromafundamentalstandpoint,aS/UIDampli3erfunctionsasamagnetic Eux to voltage converter, with extremely low output voltage noise. A Int. 8.I4MMWaves–%
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voltage-biasedT5SinserieswithaFpickupGinductorplacednearaS/UID will induce a changing magnetic Eux through the S/UID when the T5S resistancechanges. AsinWeltyHMartinis [12] ,weusea3rststageS/UID todriveaseriesarrayofS/UIDs. ThisFseriesarrayGS/UIDcanproduce

afull-scaleoutputvoltageswingoforder#mI,readilyampli3edbyroom- temperatureelectronics. A S/UID can be switched rapidly between its superconducting state and an operational state by biasing the S/UID with roughly 100 Aof current. If we stack S/UIDs in series with J1 electrical FaddressG leads as shown in Figure 2, driving current between an adKacent pair of leadswillresultinonlyoneS/UIDbeingoperationalatatime. Withthe otherS/UIDsinthesuperconductingstate,theoutputvoltageacrossthe entire array is exactly the voltage across the one active S/UID. In this manner,onlyoneampli3erisnecessaryfor

detectors,althoughatadata rate times faster. Adding in connections for a common T5S bias and feedbacksignal,atotalof J7wiresareneeded. IN TES 1 IN TES 2 IN TES N ... ... Address 1 Address 2 Address N-1 SQUID Output Voltage Address 0 Address N shun shun shun Fig. 2.—Simpli3edschematicofaS/UIDmultiplexer. TheS/UID responsefunction is a voltage output which is a periodic functionoftheinputmagneticEux. Inordertosimplifyoperation,wehave builtadigital feedbackloop tolinearize theoutput. Foreach multiplexed device, the instantaneous value of the Eux is stored in memory= the next

timethisdeviceisreadout,theoppositeoftheEuxisappliedtotheS/UID loopthroughafeedbackcoil6notshowninFigure29. Inthismanner,the Int. 8.I4MMWaves–#
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total Eux through the S/UID loop is nulled, and the feedback signal is proportionaltotheinputsignal. We have built a 1 8 S/UID multiplexer which we tested using the circuit described in Chervenak at al. [4] . One sine wave and one triangle wave inputeach were fed into a cold electronics setup so as to mimic the modulation ofa signal frominfraredlight. Themultiplexed ampli3erwas switched between these inputs, ampli3ed, digitized, and

demultiplexed to recovertheoriginalinputwaveforms6Figure%9. Theperformancefeatures lowdistortionwith 1Btypicalcrosstalk,highlightingtheexcellent3delity oftheampli3er. ItshouldbepointedoutthattheT5Sisbiasedatalltimes, andislow-pass3lteredusinganinductorwithtimeconstant L/R 20 to a response time slower than the multiplex switching time. 5Aectively, theinductorandT5Sintegratethesignal,sothatthemultiplexersamples anintegratedsignal=nolossofsignal-to-noiseisintroducedeventhoughthe signalfromeach T5Sisreadoutforashortertime. Thisistrueprovided

thatthenoisecontributionoftheS/UIDandroomtemperatureelectronics is substantially less 6by a factor of more than 9 than that ofthe T5S. Furthermore,inordertoremainstable,thedevicesmustbesampledfaster than L/R L6%J2 29 TES 100k7z [13] -3 -2 -1 0 50 100 150 200 250 300 350 400 450 500 Signal (arbitrary units) Time (ms) Fig. %.—TimeseriesofdatafromeightdemultiplexedS/UIDmultiplexer channels. OneS/UIDchannelhasasinusoidalinputEux6diamonds9,one hasatriangle-wave inputEux6squares9,andtheresthavenoappliedEux 6no points9. Only the two channels with applied signal show substantial response,exceptfora

2Bcrosstalktothesubsequentchannels. Int. 8.I4MMWaves–5
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.pticalPerformance Optical performance was measured in a test setup designed to calibrate low-background detectors for SPI45 [14] . This setup used a helium-cooled blackbody consisting of a textured, black, carbon-loaded epoxy 65potek M209wallinagold-coatedcavity. Selectableaperturesallowthethroug hput to the blackbody to be chosen. The blackbody can be heated to cover temperaturesbetween2Kand#0K.Ametal-meshbandpass3lterat%50 wavelength 6850G7z9 with a fractional bandpass of 1N10 reduced the total transmitted power to be

within the range of our bolometers, which were designed to saturate 6i.e., be driven normal9 at 5pW. The result of theblackbodycalibrationisshowninFigure#,wherethemeasuredpower has been corrected for a narrowband bolometer absorptivity of M0B. The measuredresponsefollowsthetheoreticalpowerverywelluptoasaturation powerof 2pW. 0.5 1.5 2.5 0 5 10 15 20 25 Incident Power (pW) Blackbody Temperature (K) Saturation Power Fig. #.— Photometry of a single T5S bolometer exposed to blackbody radiationat%50 m,correctedforM0BabsorptioneOciency. In addition to calibration with the blackbody, the test setup

permits an external source to be used. In order to reduce the optical load to an acceptable level, a 1B transmissive neutral density 3lter is placed in the beam. Thetimeconstantwasmeasuredbyusingarapidlychoppingblade with a hotNwarm load. An upper limit of 2ms was found, limited by the speed of the chopper. A Fourier transform spectrometer was used to measurethefrequencyresponse,whichwaslimitedbythebandpass3lter. Int. 8.I4MMWaves–6
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No bandwidth degradation due to ineOciencies in the absorbing coating were seen. Also, abeammapwas made,and excellent reKection of out-of-

beampowerwasfound. During the chopping measurements, a set of 8 bolometers were read outwithaframerateof10k7z, providingademonstration ofmultiplexed readout of an infrared signal. The detection of the chopped signal in multiplexed bolometers is shown in Figure 5. A %00KN77K source was modulated at 27z in the beam from all detectors simultaneously. The opticalsignalforeachdetectorwasfedbackusingthedigitalfeedbackloop tomaintaineachS/UIDataconstanttotalEux. Afterdemultiplexing,it isapparentthatthesignallevelsaremaintained withhigh3delity. 0.0 1.0 2.0 3.0 4.0 Signal (arbitrary units) Time (s) 0,0

0,1 1,0 1,1 2,0 2,1 3,0 3,1 Fig. 5.— Demultiplexed signal from a chopped hotNcold load seen by 8 detectorssimultaneously. Therelativeamplitudesareuncertainata %0B level. Conclusions We have demonstrated superconducting transitions in molybdenum Ngold and molybdenumNcopper bilayers, which look promising for use as T5S 3lms on sensitive bolometers. 5xcellent linearity and fast response are seen, and optical eOciency of M0B has been achieved. A multiplexed S/UID ampli3er has been fabricated and is shown to provide low-noise, Int. 8.I4MMWaves–7
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high-3delity readoutof several

T5Sdetectors with asingle signal output. Thisarchitecture canbeextendedtotwo-dimensional arrayswithmodest increase in the total number of wires. We believe that a large-format 6thousandsofdetectors9bolometerarraycanbemadewiththistechnology, havingapplicationinfuturefar-infraredinstruments. REFERENCES P1Q Wang,N.W.etal.1MM6, AppliedOptics,%5,p.66%M. P2Q 7olland,W.S.etal.1MM6,Int8I4MMWaves, 17,p.66M. P%Q Kreysa,5.etal.1MM8,Proc.SPI5R%%57,FAdvancedTechnology MMW,4adio,andTerahertzTelescopesG, T.G.Phillips,ed.,p.%1M. P#Q Chervenak,8.A.,Irwin,K.D.,Grossman,5.N.,Martinis,8.M.,

4eintsema,C.D.H7uber,M.5.1MMM,AppliedPhysicsSetters,7#, pp.#0#%-#0#5. P5Q Bock,8.8.,Glenn,8.,Grannan,S.M.,Irwin,K.D.,Sange,A.5., SeDuc,7.G.HTurner,A.D.1MM8,Proc.SPI5R%%57,T.G.Phillips, ed.,p.2M7. P6Q Shafer,4.A.etal.2000,inSPI5Proceedings,FAstronomical TelescopesandInstrumentation2000G, Munich,Germany,inpress. P7Q MaAei,B.etal.1MM#,InfraredPhys.Technol.,%5,2,p.%21. P8Q Swain,M.4.,Bradford,C.M.,Stacey,G.8.,Bolatto, A.D.,8ackson, 8.M.,Savage,M.S.HDavidson,8.A.1MM8, Proc.SPI5R%%5#, A.M. Fowler,ed.,p.#80. PMQ Benford,D.8.etal.2000, Proc.5leventh Intl.SymposiumonSpace TerahertzTechnology, inpress.

P10QIrwin,K.D.,7ilton,G.C.,Martinis,8.M.,Deiker,S.,Bergren,N., Nam,S.W.,4udman,D.A.,HWollman,D.A,2000,Nucl.Instr.and Meth.A###,pp.18#-187. P11QIrwin,K.D.1MM5,Appl.Phys.Sett.66,p.1MM8. P12QWelty, 4.P.HMartinis,8.M.1MM%,I555TransactionsonApplied Superconductivity,%,p. 2605. P1%QIrwin,K.D.,7ilton,G.C.,Wollman,D.A.,HMartinis,8.M.,1MM8, 8ourn.Appl.Phys.8%,p.%M78. P1#Q7argrave,P.C.etal.1MMM, Proc.SowTemperatureDetectorsR8,in press. Int. 8.I4MMWaves–8