Recent results of the ATLAS Upgrade Planar Pixel Sensors R&D Project

2.5 MB·English

by  Giovanni Marchiori

Recent results of the ATLAS Upgrade Planar Pixel
Sensors R&D Project
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Giovanni Marchiori∗
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fortheATLASUpgradePlanarPixelSensorsCollaboration
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RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
1. Introduction
The Large Hadron Collider (LHC) at CERN is a proton-proton collider designed to reach
√
an unprecedented high center-of-mass energy ( s=14 TeV) and luminosity (1034 cm−2s−1). It
operates smoothly since spring 2010 at a center-of-mass energy of 7 TeV and a luminosity which
has constantly increased up to around half the design value, and is expected to attain the nominal
performanceintheyearsfollowingtheshutdownplannedfor2013and2014. ATLAS(AToroidal
LHC ApparatuS) is a general purpose detector built around one LHC interaction point in order to
reconstruct the products of the highly energetic pp collisions [1]. Its primary goals are to probe
the Standard Model predictions at high energy, to discover the Higgs boson and to search for
beyond-Standard Model evidences, like supersymmetric particles or other exotic processes. The
Inner Detector (ID) is the ATLAS subdetector closest to the interaction point. It is made of three
nestedsubsystemsimmersedina2Taxialmagneticfield: asiliconpixeldetector[2]atsmallradial
distancer fromthebeamaxis(50.5<r<150mm),doublelayersofsingle-sidedsiliconmicrostrip
detectors (299<r<560 mm), and a straw tracker with transition radiation detection capabilities
(563<r<1066 mm). The ID allows an accurate reconstruction of tracks from the primary pp
collisionregionandfromsecondaryverticesduetophotonconversionsorheavyflavordecays.
To extend the physics reach of the LHC [3], a two-step upgrade of the accelerator is planned
which will increase the peak luminosity by a factor 5 to 10 towards the end of this decade. The
first step (phase I, around 2017) will require relatively small upgrades (linac, collimation) and
will bring the LHC luminosity to 2−3×1034 cm−2s−1. The second step (phase II, around 2021)
will require a major effort (new final focusing design) and will raise the luminosity further up to
5×1034 cm−2s−1 (includingluminosityleveling).
The ATLAS pixel detector is composed of 250 µm thick n-in-n DOFZ-Si pixel sensors with
cellsizesof50×400µm2,readoutbytheDC-coupled(bump-bonded)FEI3chips[4]. Ithasbeen
designedtowithstandafluenceof1015 n /cm2,whichwillbeexceededintheinnerlayerbefore
eq
the end of phase I. For this reason, a fourth pixel layer (IBL [5]) at r =3.7 cm will be inserted
during the 2013-2014 shutdown and is designed to withstand a total fluence of 2×1015 n /cm2
eq
(5×1015 n /cm2 including safety factors). The whole pixel detector will be replaced before
eq
the start of phase II, in order to cope with the increased occupancy (between 100 and 200 pile-
up collisions, an order of magnitude higher than at the nominal LHC luminosity) and radiation
damage,withtheinnermostlayerbeingexposedtoatotalfluenceof2×1016 n /cm2 (Fig.1).
eq
2. TheATLASUpgradePPSCollaboration
TheLHCupgradeposesseveralchallengestotheATLASpixeldetector:
• radiationdamage: themuchhigherfluencesimplyincreasedleakagecurrents, ahigherbias
voltage needed to (at least partially) deplete the devices, and larger charge trapping (lower
chargecollectionefficiency). S/Nwilldecrease.
• datarate: thehigherinteractionraterequiressmallerpixelstokeepoccupancyundercontrol.
Pixels of size 50×250 µm2 are being produced for the IBL, even smaller pitches may be
used for the inner layers for the phase II upgrade. For the same reason, the replacement of
2
RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
Figure1:Left:pile-upeventsataluminosityof1×1035cm−2s−1. Right:fluencesinn /cm2expectedfor
eq
aphaseIIintegratedluminosityof3000fb−1. Asafetyfactorof2isapplied.
theinnermostshortstriplayer(s)bypixellayer(s)mightbedesirable,however,asignificant
costreductionofpixelsensorsisnecessarytocompetewithstripsensorcosts.
• tightgeometricalconstraints: withtheadditionofafourthpixellayerclosertothebeampipe,
thelimitedspacebetweenthefirstlayerswillnotallowthe"shingling"ofthedetectormod-
ules along the beam direction. In addition, flat staves ease cooling and production. To
minimize the geometric efficiency losses the edges of the modules have to be active or at
leastslimmer(<450µm)thanthecurrentones. Theinactiveedgeofthepresentmodulesis
1.1mm,hosting16guardrings(totalwidth≈600µm)tomanagethepotentialdropbetween
the cutting edge of the sample and the pixel electrodes, and an additional safety margin of
500µmtoinsulatetheguardringareafromdamagesofthelateralsurfaceduetothedicing.
Toinvestigatethesuitabilityofpixelsensorsusingtheprovenplanartechnology,alreadyexploited
successfully in ATLAS and other high-energy-physics (HEP)experiments at colliders, for the up-
graded tracker, the ATLAS Planar Pixel Sensor R&D Project was established [6]. It is formed by
morethan80scientistsfrom17European,USandJapaneseinstitutes. Mainareasofresearchare
the performance of planar pixel sensors at phase I and II fluences, the exploration of possibilities
for cost reduction to enable the instrumentation of large areas, the achievement of slim or active
edges to provide low geometric inefficiencies without the need for shingling of modules and the
investigation of the operation of highly irradiated sensors at low thresholds to increase the effi-
ciency. Severaltestprototypeshavebeenproducedatvariousfoundries(CiS,Micron,Hamamatsu,
MPI-HLL)incollaborationwithRD50[7]andcharacterizedbyprobestation(IV/CVcurves),with
radioactive sources, and in testbeams with pions at CERN and electrons at DESY (hit efficiency,
charge collection), before and after irradiations. Both neutron irradiation (with the Triga reactor
inLjubljana)andprotonirradiation(usinga23MeV/cbeaminKarlsruheandthe24GeV/cbeam
of the CERN PS) have been performed. The optimization of some of the sensor parameters, for
instance the geometrical guard ring (GR) configuration (number of GRs, widths, distances), has
beensupportedbytheuseofTCADelectricalsimulationtools[8,9]. Inthefollowingsectionswe
summarizethemostrecentresultsoftheseongoingR&Dstudies.
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RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
3. Radiationhardnessofn-typeand p-typepixelsensors
Onlyelectron-collectingsensorsdesigns(n-in-pandn-in-n)havebeenconsideredinthePPS
R&D, for two radiation damage related reasons: the hole trapping time is unacceptably short at
largefluences,andp-in-n(aftertypeinversion)andp-in-psensorscannotbeoperatedefficientlyat
partialdepletionbecauseoftheundepletedvolumeneartheelectrodes.
The radiation hardness of n-in-n prototypes has been tested after neutron irradiations up to
fluences of 2×1016 n /cm2. Operating temperatures of −30 C have been chosen to reduce the
eq
leakage current and therefore shot noise. Testbeam data on devices connected to readout chips
tunedforthresholdsof1500eshowthatabouthalf(10ke)ofthechargedepositedbyaMIPin250
µmofSicanbecollectedwithabiasvoltageof1kV,whichisthemaximumallowedfortheIBLat
theendoftheLHCphaseI,after5×1015 n /cm2 (Fig.2,left). Thistranslatesintoasignal-over-
eq
tresholdratioS/T >6andasignal-to-noiseratioS/N>45. Similarstudiesshowthatsomecharge
(around5.5ke)canbecollectedevenat2×1016 n /cm2 byincreasingthebiasvoltageto1.5kV
eq
(Fig.2,right). Preliminarymeasurementsofthehitefficiencyin250µmthick,50×250µm2n-in-
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Bias Voltage [V] Bias Voltage [V]
Figure2:Chargecollectionvsbiasvoltageforn-in-npixelandstripsensorsatfluencesof5×1015n /cm2
eq
(left)and2×1016n /cm2(right). Stripsensordataarefrom[10].
eq
nirradiatedsensorsperformedintheJune2011testbeamattheCERNSPS[11](Fig.3)showthat
alreadyatV =600V anefficiencygreaterthan97%canbeachievedafterneutronirradiationto
bias
4×1015 n /cm2.
eq
In recent years the interest for p-bulk devices as possible sensors for HEP experiments has
rapidly increased as high-resistivity p-type wafers have become available and since they do not
undergotypeinversionafterheavyirradiation. Moreover,sinceguardringsarelocatedonthesame
side as the pixel electrodes, the layout of these sensors requires a single sided production process
withlessstepsthanforn-in-nsensors,andpossiblecostreduction. Apotentialissueisthatsincethe
GRsareonthesamesideasthepixels,whichareconnectedtothefrontendelectronicchipwhich
is at ground, the full bias voltage is right at the edge of the sensor and only a few µm away from
thechip. Thehighbiasvoltagesneededtodepletethesensorsmaythusgeneratesparksthatwould
destroy the chip. With an additional passivation layer (≈ 3 µm) of a highly insulating material
like benzocyclobutene or parylene on top of the sensors this effect has not been observed for bias
voltagesupto1kV.Recently285µmthickn-in-psensorswereproducedatCiSandbump-bonded
to FEI3 chips at IZM-Berlin. Prior to irradiation they exhibited breakdown voltage much higher
than the full depletion voltage (60 V), leakage currents around 500 nA/cm2 (unaffected by bump-
bonding), narrow threshold tuning with a mean of 3200 e and dispersion 22 e and 100% charge
4
LUB2: PPS 250μm, n-3.8E15, Φ=15 o#
Cell Efficiency Maps:
More loss on bias side.
Bias dots see at 400V.
RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
HV=-1000V
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Philippe Grenier L o InBgL SSeindseo r[ µRmev]iew – 4/5 July 2011 23#
Figure3: Hitefficiencyasafunctionofthehitpositionin250µmthick,n-in-nsensorswith50×250µm2
pixel cells, irradiated with neutrons at an equivalent fluence of 4×1015 n /cm2. The dashed rectangle
eq
(125<x<375 µm,25<y<75 µm)correspondstoonefullpixelcell,whileonlyhalfpixelsareshown
for neighbouring pixels. The low-efficiency vertical region near x=125 µm corresponds to the bias line
betweentwopixelcolumns.
collectionefficiencyatfulldepletion. S/N(≈19)andhitefficiency(>99%,measuredintestbeams)
were similar to that of n-in-n pixels (and of n-in-p sensors produced by HPK and Micron). These
devices have been subsequently tested after irradiation with either protons or neutrons. After a
fluence of about 5×1015 n /cm2, the breakdown voltage is greater than 750 V and the leakage
eq
current lower than 200 µA at −10 C (Fig. 4, left). A significant fraction (>30%) of the charge
generated by a MIP is collected at a high bias voltage (>600 V) and yields a detectable signal
abovethreshold(Fig.4,right). Thechargecollectionefficiencyissimilartothatofn-in-ndevices
exposedtosimilarhadronfluences.
kage current [μA]211111086420000000 624...243085eee111444 nnneeeqqq///cccmmm222 ected charge [ke]111186420 123355ne.eeee-ein111111-555555n n nnnn n5eeeeeeeqqqqqq/////1ccccc/c5mmmmmm n222222 e((((q----/5656c0000m CCCC2))))
Lea 80 Coll 6
60
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0 100 200 300 400 500 600 700 800 0 200 400 600 800 1000 1200
Bias Voltage [V] Bias Voltage [V]
Figure4: IVcurves(left)andcollectedchargevsV (right)atvariousfluencesforn-in-ppixelsensors
bias
irradiated with neutrons. The horizontal red line corresponds to the threshold set in the front-end chip
discriminator.
4. Sensorthinning
Recent strip-sensor data show that thinner sensors yield more charge than thicker ones after
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RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
fluences of 5×1015 n /cm2 for bias voltages of 1 kV or higher, because of the higher electric
eq
fieldandpossiblyalsoduetochargemultiplicationinsidethebulk[10]. ThinnersensorsforHEP
experiments have the additional advantage of the lower material budget, which improves momen-
tumresolutionandreducesenergylossinfrontofdownstreamdetectors. RecentlyCiShasstarted
to produce for the PPS R&D n-in-n sensors with thicknesses between 150 and 250 µm, in steps
of 25 µm. Part of the 250 µm sensors have been delivered, with good wafer yield (90%) and
sensor-on-waferyieldcloseto100%. AtV =1kVtheycollectmorechargethan300µmthick
bias
strips (Fig. 5). Similar studies will be performed on the thinner sensors as soon as they become
available. Alson-in-psensorswiththicknessesof150and200micronsarebeingproducedbythe
samevendorandwillbetestedinthenearfuture.
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Bias Voltage [V]
Figure5:CollectedchargevsV forn-in-nsensorsofdifferentthicknessesexposedtoaneutronfluenceof
bias
5×1015n /cm2.Thehorizontalredlinecorrespondstothethresholdsetinthefront-endchipdiscriminator.
eq
5. Reducingtheinactiveareawithslim-edgeoractive-edgedevices
Toavoidthatthedepletionregionspreadingfromtheactiveareareachesthedamagedlateral
surface, an undepleted region is usually present at the sensor’s edge. This inactive area reduces
chargecollectionneartheedge. Toincreasetheactiveareaweareexploringtwotechniques.
Edges can be made "slim" by reducing both the width of the guard ring area, optimizing the
numberandspacingoftheGRs,andthesafetymargin,exploitingdicingtechniquesthatcreateless
surfacedamage. Basedonsimulationsandonseveraldicingtrialsshowingagoodyield,areduced
number of 13 guard rings with a safety margin of 100 µm was proposed for the IBL sensors. A
further gain for n-in-n pixels can be obtained by extending the pixel implants, which are located
on the side opposite to the GRs, to partially overlap the guard rings (Fig. 6, left). In the overlap
regionthebulkis(atleastpartially)depletedandsufficientchargeiscollected,withareductionof
the inactive area to less than 1.5%. This has been investigated with n-in-n slim-edge sensors in a
recent testbeam at CERN. Preliminary results show a high efficiency (close to 100%) up to about
250 µmfromthecutlinefornon-irradiateddevicesaswellasfordevicesirradiatedwithneutrons
up to 4×1015 n /cm2 and biased at 1 kV (Fig. 6, right) [12]. Given the good performance and
eq
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RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
yields achieved, the slim-edge design has been chosen as the IBL candidate technology for the
planarpixelsandiscurrentlybeingproduced.
Figure 6: Slim-edge sensors. Left: slim-edge design. The pixels (horizontal blue rectangles) close to the
edgearelonger(500µm)andpartiallyoverlaptheguardringregion(verticalnarrowgreyrectangles)onthe
oppositeside. Right: hitefficiencymapofslim-edgesensorsirradiatedwithneutronsat4×1015 n /cm2
eq
and biased at 1000 V. Data was taken with an incidence angle of 15◦. The sensor extends in the region
125<x<625µm. Biasdotsareclosetox=125µm. Theguardringregionislocatedatx>380µm.
A second technique for the reduction of the inactive area is the active-edge concept. Within
the PPS R&D two technologies are being investigated. The former, proposed in [13] and avail-
ableatseveralfoundriescollaboratingwiththePPSgroup(CNM,FBK,VTT),usesDeepReactive
IonEtching(DRIE)toobtaindeeptrenchesaroundthedetectors, thuseliminatingtheneedofthe
cutting procedure. These trenches are heavily doped so as to behave like ohmic contacts (Fig. 7,
left). Thefabricationprocessrequiresasupportwaferandpresentstechnologicalchallengesinthe
excavationofdeep(200-230µm)trencheswithaspect-ratioaround1:20andinfillingthetrenches
withpolysiliconwithgoodplanarsurface,asneededforproperdepositionofthephotoresistforthe
laterproductionstages. PreliminarymeasurementsontestdiodesproducedatFBKwiththistech-
niqueshowreasonablylowleakagecurrentsandbreakdownvoltageshigherthanthefulldepletion
value. [14]. A production of planar pixel sensors with this technique is currently being planned.
The second edgeless technology is based on laser scribing and cleaving, which result in signifi-
cantly less surface damage [15] thus eliminating the need for a safety margin, and on passivation
of the lateral surface with either silicon oxide or nitride (for n-type bulk sensors) or atomic layer
deposition (ALD) of aluminum oxide (for p-type bulk). This creates an interface charge (positive
forsiliconoxideandnegativeforaluminumoxide)onthelateralsurfacethatisable,ifitsdensity
issufficientlyhigh(around1012cm−2),toguaranteethatthepotentialdropattheedgeisnegligible
even without guard rings. Using the U.S. Naval Research Laboratory facilities for laser scribing
andALD,diodeswithoutguardringsandadistanceofonly14µmbetweentheactiveareaandthe
cutting edge were produced (Fig. 7, right). These diodes show no evidence of breakdown up to 1
kVandachargecollectionefficiencycloseto100%evenneartheedge. Inthefutureseveraltests
areplanned,inparticulartoassesstheradiationhardnessofthesetechnologies.
6. Otheractivities
The phase II ATLAS Inner Detector may exploit an additional pixel layer at larger radius in
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laserdicingofonelateralsurface. Allguardringshavebeenremoved.
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Device IV (HPK)
order to improve tracking performances; some of the strip layers may also be replaced by pixels
“6th  Trento  Workshop”,March 2-4, 2011 Laser-Scribing & Al2O3Sidewall Passivation of P-Type Sensors 27
to reduce the occupancy. Ttrhenech'planaFrieltde'pclathe'nology could be exploited since the expected fluences a1t3$
SSLLIIDD iinntteerrccoonnnneeccttiioonn toohoffs eFFradEEii!--w"#IIi$33l%l&'(n )o!pp*t+$eiixxxceeeelldssa f((ewIIII1))015 neq/cm2. Becausetheinstrumentedareawillbemuchlarger
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a sensors. However,mostofthecostoftheATLASpixeldetectorwasduetothebumpbondingofthe
pgr (cid:131) After electroplating, FE-I3 chips 21f/r7o'n&t--e.n0(d+e&l+e(c0"8t#(r$o%-&n''ic(c&*hi.p&2s0-#1.t(o('*&.t11h0#e-' s)e3n8s(o&r,s.-F.o(r+thisreason, analternativetechniquebasedonSolid- ,-$
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xl are singularized and reconfigured Liquid InterDiffusion (SLID [16]) has started to being investigated recently, in collaboration with
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x on a 6” handle wafer. This is Fraunhofer Institute (IZM Munich). A (pixelated) thin (5 µm) layer of copper (Cu) is deposited
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removed after the interconnection throughmaskelectroplatingonboththesensorsandthechip. Inaddition,a3µmlayeroftin(Sn)is
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AT acontrolledenvi4ro5nm6%e"n&t7w/h9e(re8'atemperaturearound300Candapressurearound5barisreached.

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r t chips interconnected with SLID to a a melting temperature greater than 600 C and therefore does not liquify during the process. This
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V f 75 (cid:80)m thick sensor wafer, has been technique should be cheaper than standard bump-bonding as it requires less processing steps. It
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T finished last week. should also be exploitable with small pitches and stacking. First results with thin (75 µm) n-in-

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n p pixels connected to FEI3 chips showed promising results, with threshold dispersion, noise and
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n (cid:131) Due to the misalignment (˜20 (cid:80)m) chargecollectionefficiencycomparabletothoseofbump-bondedsensors. Sporadicproblemswith
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Figure 8: Left: SLID bonding tests performed using a dummy sensor and a FEI3 chip. Right: charge
collectedfromaMIPsourcewitha75µmthickn-in-psensorconnectedtoaFEI3chipusingSLID.
8
RecentresultsoftheATLASUpgradePlanarPixelSensorsR&DProject GiovanniMarchiori
7. Conclusion
Extensive R&D by the PPS group has demonstrated that the proven planar technology of
the ATLAS pixel detector is well suited even at the fluence (5×1015 n /cm2) predicted for the
eq
innermost layer in the phase I upgrade. The planar pixel sensors have been shown to operate and
collect some charge even at phase II fluences (2×1016 n /cm2). Encouraging results have been
eq
obtainedalsointermsofreductionofcostsandoftheinactiveregionsattheedge. Furtheractivities
areplannedinthefutureinordertoevaluatetheperformancesofthinnerdetectorsandtoestablish
theradiationhardnessupto2×1016 n /cm2 ofthevarioustechnologiesunderconsideration.
eq
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