A Fabrication Route for Arrays of Ultra-low-Noise MoAu Transition Edge Sensors on Thin Silicon Nitride for Space Applications

0.57 MB·

by  D. M. Glowacka

JournalofLowTemperaturePhysicsmanuscriptNo.
(willbeinsertedbytheeditor)
D. M.Glowacka1 M.Crane1 D. J.
Goldie1 S.Within·gton1 ·
·
A Fabrication Route for Arrays of
Ultra-Low-Noise MoAu Transition Edge
Sensors on thin Silicon Nitride for Space
4
1
Applications.
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2

n 18.07.2011
a
J

0 Keywords TransitionEdgeSensor,bolometer,far-infraredimagingarray,space
1 telescope

Abstract We describe a process route to fabricate arrays of Ultra-Low-Noise
]
t MoAu Transition Edge Sensors (TESs). The low thermal conductance required
e
forspaceapplicationsisachievedusing200nm-thickSiliconNitride(SiN )pat-
d x
- terned to form long-thin legs with widths of 2.1m m. Using bilayers formed on
s SiN islands from films with 40 nm-thick Mo and Au thicknesses in the range
n x
30to280nmdepositedbydc-sputteringinultra-highvacuumwecanobtaintun-
i
. able transition temperatures in the range 700 to 70 mK. The sensors use large-
s
c area absorbers fabricated from high resistivity, thin-film b -phase Ta to provide
i impedance-matchingtoincidentradiation.Theabsorbers arepatternedtoreduce
s
y theheatcapacityassociatedwiththenitridesupportstructureandincludeAuther-
h malisingfeaturestoassisttheheatflowintotheTES.Arraysof400detectorsatthe
p pixelspacingrequiredforthelong-wavelengthbandofthefar-infraredinstrument
[
SAFARIarenowbeingfabricated.Deviceyieldsapproaching99%areachieved.

1 PACSnumbers:85.25Oj,95.55Fw
v
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8
1 Introduction
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. The far-infrared (FIR) band from 300m m to 20m m is of crucial importance for
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astrophysics,particularlywith regardto studying the formation and evolution of
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galaxies,starsandplanetarysystems.Itencompassesthewavelengthrangeofthe
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1 plannedspacemissionsSPICA1,FIRI2 SAFIR, SPECS, andSPRIT; anditcov-
: ersthewavelengthrangeoftheballooninterferometersBETTII3andFITE.4The
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new generation of space missions covering this range will require large-format
i
X
1:DetectorandOpticalPhysicsGroup,CavendishLaboratory,
r
a UniversityofCambridge,J.J.ThomsonAvenue,Cambridge,CB30HE,UK
Tel.:[phone]
E-mail:[email protected]
2
arrays of extremely low noise detectors with Noise Equivalent Powers (NEPs)
of order 10 19W/√Hz or better. This requirement has driven the development
−
of sensitivesuperconducting detectors,such as TransitionEdge Sensors (TESs).
WhileitistechnicallyfeasibletomanufacturesingleTESshavingthissensitivity,
it is challenging to create an ultra-low-noise TES technology that can be engi-
neered into complete imaging arrays, with the required optical packing and uni-
formity of performance. The NEPs required for these sensitive detectors can be
(a)
20
18
(cid:17)
z 16
H
√ 14
/
W 12 Gb=0.73 pW/K
19 10
0
1 8
×
(cid:16) 6 Gb=0.17 pW/K
P
4
E
N 2
0
100 101 102
Frequency(Hz)
(b)
Fig.1 (Coloronline)(a)OpticalimageofanearlierMoCuTES.(b)DarkNEPforanearlier
MoCuTESreportedinRef.[7].
achievedbythermallyisolatingtheTESusingfourlong( 500m m)andnarrow
( 4m m)support legsfabricatedusingthinSiN .Wehav≥epreviouslymadeand
x
≤
characterizedsmallarraysofMoCuTESswithT 100mKbothwithandwith-
c
∼
outIRabsorbersaspartofadevelopmentefforttomovefrommicrostrip-coupled
polarization-sensitiveTESswithNEP 2 10 17W/√Hz tailoredforground-
−
∼ ×
based astronomy5,6 to the ultra-low-noise performance required for space appli-
cations.7,8 Figure 1(a) shows an imageof one of theseMoCu devices fabricated
3
withoutanadditionalabsorberstructure.ThecurvatureinthecentralSiN island
x
isoforder1m moutofthefigureandarisesfromresidualstressintheSiO layer
2
needed to passivate the Cu. The SiO also contributes heat capacity and hence
2
possibly additional noise features. MoAu TESs do not require passivation and
our initial devices reported elsewhere in these proceedings9 have demonstrated
excellentlong-term stability.Figure 1(b) shows the measured dark NEP for two
of theseTESs. The lowfrequency NEP 4 10 19W/√Hz for a conductance
−
∼ ×
to the bath G =0.17pW/K is within a factor of order √2 of the phonon limit
b
calculatedwithnoisemodifiergf =0.5.10
Inthispaperwedescribethefabricationrouteforultra-low-noiseMoAuTES
detectorsoperatingcloseto100mKwithabriefreportonthestatusofthelatest
measurements.
2 Experimentaldetails
(a) (b) (c)
Fig.2 (Coloronline)Opticalimagesof:(a)Asingle100 100m m2Mo/AuTESwithlongitu-
dinalandpartiallateralAubarsacrossthebilayerandTaab×sorber.The200nmthickSiN island
x
undertheTEShasanarea110 110m m2.TheTaabsorberis320 320m m2.Thesupporting
legsare2m mwideand540m m×long.(b)Astripedabsorberwithsev×en25m m-wideTabarsand
(c)ameshedTaabsorberpatternedintoa25m m-widegridtoinvestigatetheeffectontheheat
capacity.
The array described here has 388 single devices each consisting of a MoAu
bilayerTESwith Au banks and fingers partiallyacross the TES,11 a Ta FIR ab-
sorber, Au thermalizing structures and Nb connections all formed on a SiN is-
x
land.TheislandisthermallyisolatedfromtheSiwaferbyfourSiN legsofwidth
x
2.1m m andlength540m m,whichdeterminethethermalconductanceandhence
theresponsivity,saturationpowerandreadoutnoisespectrumofthedevice.Pho-
tographsofthedevicetypesareshowninFig.2.
Thesensorsaremanufacturedon50mmdiameter,225m mthick 100 ,double-
h i
sidepolishedSiwafers.Thesewafershave,onbothsides,a50nmfilmofthermal
SiO anda200nm filmspellofslightlyoff-stochiometricSiN ,formedbylow-
2 x
pressure chemicalvapour deposition.Ultimately,the SiN becomes a suspended
x
membrane,producedbyback-sideetchingofthesupportingsiliconsubstrateun-
dertheTESandabsorber,togivetherequiredthermalisolation.
4
Fig.3 (Coloronline)Schematicdiagramshowingtheprocessflowforasinglearray.Thecross
sectionisalongthelinemarkedA-AinFig.2(a).
The first two steps of device fabrication are Reactive Ion Etching (RIE) of
theSiN /SiO usingCHF gastooutlinethearrayanddefinethemembraneand
x 2 3
legs (done in two separate steps, one on the front of the wafer and one on the
back):Fig.3(a)and(b).ThentheTaabsorberlayerisdepositedbyDCmagnetron
sputtering in a system with a base pressure of 10 10Torr as in Fig. 3(c). The
−
next step is deposition of superconducting bilayer by DC magnetron sputtering
alsounderultra-highvacuumconditions.Thebilayerdescribedhereconsistsofa
40 nm Mo layer followed by a 195 nm Au layer, deposited in quick succession
toavoidinterfacecontaminationandtherebyachievinga reproducibleproximity
effect.The bilayeris patternedby wet etching: first the Au in a 50% solution of
commerciallyprepared gold etchand water,and then the Mo in a commercially
prepared aluminum etch: Fig. 3(d). The next processing step is the deposition
of a 200 nm-thick Au thermalizing layer on the back of the wafer: Fig. 3(e). A
second 200 nm Au deposition forms thermalizing bars on the Ta absorber, and
Au banks and lateral bars on the bilayer to improve thermalization of the TES:
Fig.3(f).AAuCu deposition,not shown,forms resistiveheatersontheSiframe
and Johnson noise thermometers on selected SiN islands in place of the TES.
x
TheTESelectricalconnectionsandcontactpads areformedfrom 250 nmofNb
(notshownontheprocessflow).Ateachstage,theAuandNblayersareremoved
fromunwantedareasbylift-off.Thewaferisthenreadyforbondingfacedownto
5
acarrierwafer:Fig.3(g).Thelaststepisthefabricationofthemembrane,which
requiresremovalofthesupporting Si from thewindowusing DeepReactiveIon
Etching, thus leaving the TES and absorber membrane suspended on the nitride
legs: Fig. 3(h). Finally the wafer is demounted, cleaned in an O plasma and is
2
readyfortesting.
ThepixelspacingfortheprototypearraywasmatchedtotheSAFARIL-band
requirementof1.6mm.ThearrayincludesfullTaabsorbersasinFig.2(a),striped
(Fig.2(b))andmeshedabsorbers(Fig.2(c)).OnselectedcalibrationpixelstheTa
was omitted completely and on others both the absorber and its support nitride
was removed. Johnson noise thermometers were also included to assist with es-
timates of stray-light. The target T was in the range 110 120mK. The array
c
−
was cooled in a closed-cycle,sorption-pumped dilution refrigerator mounted on
a pulse-tube cooler giving a base temperature of 68 mK.12 The array was en-
closedinaAu-platedCuboxtheinsideofwhichwascoatedwithlight-absorbing
SiCgranulesandcarbonblackmixedinStycast2850tominimizescatteredlight.
A photograph of the experimental enclosure is shown in Fig. 4(b). The sample
spacewassurroundedbymultiplelayersofNbfoilandMetglastoprovidemag-
netic shielding. We used NIST SQUIDs with analogue electronics readout that
keepsthemultiplexerinafixedstatebutnone-the-lesspermitsreadoutofmultiple
channels.FortheseteststheSQUIDsandbiasresistorshaveaseparatelight-tight
enclosurewithlight-tightelectricalfeedthroughtothearrayenclosure.
3 ResultsandDiscussion
(a) (b)
Fig.4 (Coloronline)(a)Completed arraychip.(b)Completearraymountedinalight-tight
enclosureforlowtemperaturetesting.
The first 388-element arrays have now been completed and low temperature
measurementsareinprogress.Thermalcycletestsalreadyindicatethatthemount-
ingschemeshowninFig.4(b)isrobustandnodeviceshavebeendamagedonre-
peatedthermalcyclesdespitetheremovalofalargefractionofthesupportSi(of
order36%).Overalldeviceyieldwas99%.AllTESsareclearlyvisibleinFig.4(a)
includinginthelowerleftportionaverysmalltestpixelwheretheabsorbingTa
6
andsupportSiN wasremovedandthestripedabsorbersinthelowerrightsector.
x
OnasubsetofTESsnormalstateresistancesweremeasuredtobeR =31 1mW
n
andT =113 5mK.Conductancetotheheatbathwas0.19 0.2pW/K±giving
c
± ±
a predicted phonon-limited NEP of 2.4 10 19W/√Hz calculated with noise
−
modifiergf =0.5. ×
Further measurements are in progress to assess uniformity of the character-
istics across the array, to measure in detail the dark NEP and response times as
a function of absorber geometry, and to use the frame heaters to assess wafer
heatsinkingandpixel-to-pixelthermalcross-talk.Wearealsodevelopingafully
micro-machined wafer backing-plate incorporating metallized back shorts and
wiringfanoutforintegrationintoafullfocal-plane.Resultswillbereportedsep-
arately.Theresultsalreadyobtainedsuggestanimportantstep-forwardinthede-
velopment of the next generationof space-basedTESs and from the perspective
offabricationasignificantstep.
4 Acknowledgments
ThisworkwassupportedinpartbyESATechnologyResearchProgrammeCon-
tract No. 22359/09/NL/CP. We are also very grateful to colleagues working on
that contract within the Astronomy Instrumentation Group, Cardiff University,
theSpaceResearchOrganizationNetherlands,theNationalUniversityofIreland,
MaynoothandtheSpaceScienceandTechnologyDepartmentofRutherfordAp-
pletonLaboratoryfornumerousstimulatingdiscussions.
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