Table Of Content9
0 Signal height insiliconpixel detectors irradiatedwithpions and
0
2 protons
n
a T.Rohea,1,J.Acostab,A.Beanc S.Dambacha,d W.Erdmanna,U.Langeneggerd,
J
C.Martinc,B.Meiera,V.Radiccic,J.Sibillec,P.Tru¨ba,d
2
2 aPaul ScherrerInstitut, Villigen,Switzerland
bUniversityof Puerto Rico, Mayagu¨ez
] cUniversityof Kansas, Lawrence KS,USA
t
e dETH Zu¨rich, Switzerland
d
-
s
n
i Abstract
.
s
c Pixeldetectorsareusedintheinnermost partofmultipurposeexperimentsattheLargeHadronCollider (LHC)
i
s andarethereforeexposedtothehighestfluencesofionisingradiation,whichinthispartofthedetectorsconsists
y mainly of charged pions. The radiation hardness of the detectors has thoroughly been tested up to the fluences
h expected at the LHC. In case of an LHC upgrade the fluence will be much higher and it is not yet clear up to
p which radii the present pixel technology can be used. In order to establish such a limit, pixel sensors of the size
[
of one CMS pixel readout chip (PSI46V2.1) have been bump bonded and irradiated with positive pions up to
1 6×1014neq/cm2 atPSIandwithprotonsupto5×1015neq/cm2.Thesensorsweretakenfromproductionwafers
v of the CMS barrel pixel detector. They use n-type DOFZ material with a resistance of about 3.7kW cm and an
2 n-side read out. As the performance of silicon sensors is limited by trapping, the response to a Sr-90 source was
2 investigated. The highly energetic beta-particles represent a good approximation to minimum ionising particles.
4 Thebias dependenceof thesignal fora widerange of fluenceswill bepresented.
3 Keywords: LHC,superLHC,CMS, tracking, pixel,silicon, radiation hardness
.
1
0
9
1. Introduction rellayersandtwoenddisksateachside.Thebar-
0
: rels are 53cm long and placed at radii of 4.4cm,
v
i The tracker of the CMS experiment consists of 7.3cm, and 10.2cm. They cover an area of about
X only silicon detectors [1]. The region with a dis- 0.8m2 with roughly 800 modules. The end disks
r tance to the beam pipe between 22 and 115cm is are located at a mean distance from the interac-
a
equippedwith10layersofsinglesidedsiliconstrip tionpoint of34.5cmand46.5cm.The areaofthe
detectors covering an area of almost 200m2 with 96turbinebladeshapedmodulesinthediskssums
about107 readoutchannels.The smallerradiiare up to about 0.28m2. The pixel detector contains
equippedwithapixeldetectorwhichwasinserted about6×107readoutchannelsprovidingthreepre-
intoCMSinAugust2008.Itconsistsofthreebar- cisionspace points up to a pseudo rapidity of 2.1.
Theseunambiguousspacepointsallowaneffective
patternrecognitioninthe denstrackenvironment
1 Correspondingauthor; e-mail:[email protected]
Preprintsubmitted toElsevier Science 22 January2009
close to the LHC interaction point. The precision the main production run for the CMS pixel bar-
of the measurement is used to identify displaced relwhichwereprocessedonn-dopedDOFZsilicon
verticesforthe taggingofb-jets andτ-leptons. according to the recommendation of the ROSE-
The two main challenges for the design of the collaboration [5]. The resistance of material prior
pixeldetectorarethehightrackrateandthehigh to irradiation was 3.7kW cm. The approximately
level of radiation. The former concerns the archi- 285m mthick sensorshadthe sizeof asingle read-
tecture of the readout electronics while the high out chip and contain 52×80 pixels with a size of
radiationlevelmainlyaffectsthechargecollection 150×100m m2each.Incontrasttopreviousstudies
propertiesofthe sensor,whichdegradessteadily. [6]thestandardbumpbondandflipchipprocedure
A possible luminosity upgrade of LHC is cur- describedin[7]wasappliedtothesamples.Asthis
rentlybeingdiscussed.Withaminorhardwareup- includesprocessingstepsatelevatedtemperature,
grade a luminosity above 1034cm−2s−1 might be this was done before irradiation which simplified
reached. Later major investments will aim for a the whole procedure considerably and resulted in
luminosity of 1035cm−2s−1 [2]. The inner regions a very good bump yield. In return it means that
of the tracker will have to face an unprecedented the readout chips were also irradiated. Although
trackrateandradiationlevel.Thedetectorsplaced the operation of irradiated readout circuits poses
ataradiusof4cmhavetowithstandthepresently a major challenge and source of measurement er-
unreachedparticle fluence ofΦ≈1016n /cm2 or rors, it gives a realistic picture of the situation in
eq
must be replaced frequently. However, the opera- CMSaftera few yearsofrunning.
tion limit of the present type hybrid pixel system Thesandwichesofsensorandreadoutchipwere
using “standard” n-in-n pixel sensors is not yet irradiatedatthePSI-PiE1-beamlinewithpositive
seriously tested. The aim of the study presented pions of momentum 280MeV/c to fluences up to
is to test the charge collection of the CMS bar- 6 × 1014n /cm2 and with 26GeV/c protons at
eq
relpixelsystematfluencesexceedingthespecified CERN-PSupto5×1015n /cm2.
eq
6×1014n /cm2 [3]. Allirradiatedsampleswerekeptinacommercial
eq
freezer at −18◦C after irradiation. However the
pionirradiatedones were accidentally warmedup
to room temperature for a period of a few weeks
2. Sensorsamples
(due to anundetected powerfailure).
The sensorsforthe CMSpixel barrelfollowthe
socalled“n-in-n”approach.Thecollectionofelec-
trons is of advantage in a highly radiative envi- 3. MeasurementProcedure
ronmentastheyhaveahighermobilitythanholes
and therefore suffer less from trapping. Further- The aim of the study was to determine the
more,thehighestelectricfieldafterirradiationin- amount of a signal caused by minimum ionising
duced space charge sign inversion is located close particle(m.i.p.)asafunctionofsensorbiasandir-
to the collecting n-electrodes. The need of a dou- radiationfluence.Forthistheresponseofthesam-
ble sided processing leading to a significant price ples to a Sr-90 source was investigated. The end-
increasecomparedtotrulysinglesidedp-in-nsen- pointenergyofthebetaparticlesisabout2.3MeV
sorsisusedasachancetoimplementaguardring whichapproximatesam.i.p.well.Howeverthereis
scheme keeping all sensor edges on groundpoten- alsoalargenumberof“lowenergy”particleswhich
tial.Thisfeaturesimplifiesthedesignofthedetec- are stopped in the sensor and cause much larger
tor modules considerably. For n-side isolation the signals. Those have to be filtered during the data
socalledmoderatedp-spraytechnique[4]hasbeen analysis.
chosenandapunchthroughbiasinggridhasbeen The samples were mounted on a water cooled
implemented. Peltierelementandkeptat−10◦C.Thesourcewas
The sensor samples were taken from wafers of placedinsidethe boxabout10mmabovethe sen-
2
sor.Asthecompactsetupdidnotallowtheimple- offurther investigations.
mentation of a scintillator trigger a so called ran- After these steps data is taken using the Sr-90
dom trigger was used. In this method the FPGA source. The sensor bias was varied over a wide
generating all control signals for the readout chip range. The maximum voltage applied was 250V
stretchesanarbitrarycycleoftheclocksenttothe for the unirradiated samples, 600V for the sam-
readout chip by a large factor, and, after the la- plesirradiatedupto1×1015n /cm2,and1100V
eq
tency,sendsatriggertoreadoutthedatafromthis for the samples which received a fluence of 2.8×
stretchedclockcycle.Thestretchingfactorwasad- 1015n /cm2. The change of the sensor bias has
eq
justed in a way that about 80% of the triggers noeffectonthecalibrationperformedbefore.The
showedhitpixels. temperature can be kept stable during the bias
◦
A measurementsequence consists of the follow- scanwithin0.2 C.Theeffectofsuchsmalltemper-
ing steps: aturevariationshas beentestedto be negligible.
– Cool down the sample while flushing the box Thedatawasanalysedoffline.Firstallanalogue
withdry nitrogen. pulseheightinformationwereconvertedintoanab-
– The “pretest” adjusts basic parameters of the solutechargevalue,usingthe parametrisationde-
readoutchip. scribedabove.Afterthisapixelmaskisgenerated
– The “full test” checks the functionality of each which excludes faulty pixels. A pixel was masked
pixel. if it shows much less (“dead”) or more (“noisy”)
– Fine tune the thresholdin eachpixel to a value hitsthanitsneighbours,andifthepulseheightcal-
of4000electronsasuniformaspossible(“trim” ibration failed. In addition a manually generated
the chip). list of pixels can be excluded. In a second step all
– The pulse height calibration relates for each clustersofhitpixelsarereconstructed.Ifacluster
pixelthepulseheighttotheDACvaluesusedto touchesamaskedpixelorthesensoredge,itisex-
inject test pulses. The analogue response is fit- cluded from further analysis. Clusters of different
tedtoanhyperbolicarc-tangentfunction[8]and size(onepixel,twopixels,etc.)areprocessedsep-
the four fit parameters are calculated for each arately.Tomeasurethepulseheight,thechargeof
pixel.Withprocedureanabsolutecalibrationof a cluster is summed and histogrammed. To those
eachpixelis possible. histograms a Landau function convoluted with a
Thisprocedurewasidenticaltowhatisusedtotest Gaussianis fitted. The quoted chargevalue is the
andcalibratethemodulesinstalledintheCMSex- mostprobablevalue (MPV)ofthe Landau.
periment.Itwasperfectlyadequateforallsamples Due to the low thresholdof only 2000electrons
upto afluence of1×1015n /cm2. thehighlyirradiatedsamples(2.8×1015n /cm2)
eq eq
Forthesamplesirradiatedto2.8×1015n /cm2 showedahighernumberofnoisypixels,especially
eq
thefeedbackresistorofthepreamplifierandshaper atthesensoredgewherethepixelsarelarger.How-
had to be adjusted manually to compensate for ever, also some “good ” pixels showed a certain
the radiation induced change of the transistor’s number of noise hits which lead to a second peak
transconductance. The DAC which controls this in the pulse height spectra. It was well separated
settingisnotimplementedinthetestingsoftware. from the signal for voltagesabove 200V. The ori-
Thenthestandardcalibrationprocedurewasused ginofthe 2 peakscouldeasilybe distinguished:
with the exception that the pixel threshold was – Thesignalpeakmoveswithhigherbiastohigher
loweredto about2000electrons(insteadof4000). valueswhilethenoisepeakstaysatthesamepo-
Anadditionalfeatureofthereadoutchip,theleak- sitionbutbecomesmoreprominent(morenoise
age current compensation, which might be useful hits athigher bias).
forsuchhighlyirradiatedsamples,wasnotused. – The spatial distribution of the signalshows the
The readout chips of the samples irradiated to intensity profile of the source, while the noise
5×1015n /cm2 showedsomefunctionality,how- hits arerandomlydistributed.
eq
evera calibrationandquantitative analysisofthe – The signal peak has a typical Landau shape,
data was not yet possible and will be the subject while the noisepeak ismoreGaussian.
3
hit pixels. Figure 1 shows the distribution of the
40000
cluster size for four irradiation fluences. Naively
Not irradiated, 150V
35000 onewouldexpectaspectrumdominatedbyone-hit
6.1E14, 600V
30000 1.1E15, 600V clusterswithasmallfractionofclustersofsizetwo
2.8E15, 800V
tofourcausedbyparticlespassingjustin-between
25000
two pixels or close to a pixel corner. However, as
20000
visible in Fig. 1, there is a tail of events with ex-
15000
tremely large clusters, which does not dependent
10000 on irradiation or bias voltage. This supports the
5000 hypothesis of secondary particles. Therefore it is
not surprising that the signal is a function of the
0
0 1 2 3 4 5 6 7 8 9 10
Cluster Size clustersizes.Figure2showsthepulseheightdistri-
butionofanunirradiatedsensorfordifferentclus-
Fig. 1. Distribution of cluster size for four irradiationflu-
ter sizes. In particular clusters with more than 4
ences.
hitpixelstendtohaveverylargesignalsandtheir
distribution can no longer be described by a Lan-
dau function. More surprising is the fact that al-
All Clusters ready in small clusters with less than four pixels
1 Pixel
103 2 Pixels themostprobablevalueofthepulseheightdistri-
3 Pixels butionclearlydependsontheclustersize.Inorder
4 Pixels
toreduceacontaminationofthedatafromlowen-
> 4 Pixels
ergy particles, the pulse height is only extracted
fromclustersofsize one.
Figure 3 shows the bias dependence of the sig-
102 nalforallmeasuredsamples.Fortheunirradiated
samplesthesuddenriseofthesignalatthefullde-
0 200 400 600 800 1000 1200 140S0ign1a6l0 H0ei1g8h0t 0[a.u20.]00 pletionvoltageofVdepl ≈55Visnicelyvisible.The
signal then saturates very fast. The samples irra-
Fig.2. Pulseheight distribution ofan unirradiatedsensor diated to fluences in the 1014n /cm2-range also
eq
inarbitraryunits (1 unitis about 65 electrons).
showanicesaturationofthesignalaboveroughly
300V. The onset of the signal in the “low” volt-
ThequotedsignalisagaintheMPVofaconvoluted
agerangeclearlydisplaystheincreaseofthespace
Landau-Gaussfit.
chargeduetoradiation.Thereisastrongvariation
of the saturated signal for samples with the same
irradiationfluencewhichcannotbeexplainedwith
4. Results differences in the sensor thickness.The reasonfor
thisisprobablytheimperfectionofthepulseheight
Because the radiation of the Sr-90 source con- calibration, which relies on the assumption that
tains a large graction of low energy betas which theinjectionmechanismfortestpulsesisequalfor
causemuchhighersignalthanaminimumionising allreadoutchips,whichisnotthecase.Variations
particleandasthe setupwasnotequippedwitha oftheinjectioncapacitorarelargerthan15%,and
scintillator which triggered only if a particle pen- also the resistor network in the DAC shows vari-
etrated the sample, the contamination of the low ations,which are,however,much smaller.For the
energy particles had to be reduced using the of- samplesirradiatedtofluencesabove1015n /cm2,
eq
flineanalysis.Aparticlestoppedinthesensorusu- no saturation of the signal with increasing bias
ally causes part of the ionised electrons to travel is visible. It is remarkable that even after a flu-
intheplaneofthesensorionisingfurtherelectrons ence of 2.8×1015n /cm2 a charge of more than
eq
in the flight path. This results in large clusters of 10000 electrons can be achieved if it is possible
4
]
-
e
k 25
[
l
a Not irradiated
n
g 4.3E14, Pions
i
S
20 6.2E14, Pions
6.1E14, Protons
1.1E15, Protons
2.8E15, Protons
15
10
5
0
0 200 400 600 800 1000
Voltage [V]
Fig.3. Signal fromsinglepixel clusters as afunction of the sensor bias.Each linerepresents one sample.
Inordertodisplaythedevelopmentofthesignal
-]nal [ke 25 NPiootn isrr,a 6d0ia0tVed, 250V h60ei0gVhtwaassaexfturnaccttieodnfoorf etahcehflsuaemnpcele, (t2h5e0cVhafrogrethaet
Sig 20 Protons, 600V unirradiated ones) and plotted in Fig. 4. In addi-
Protons, 800V
Protons, 1000V tionthevaluesfor800Vand1000Vareplottedfor
15 the highest fluence. Apart from the large fluctua-
tions,whichareduetothecalibrationoftheread-
10
out electronics, the reduction of the charge with
fluence is nicely visible. Further it becomes obvi-
5
ousthat itpays to goto veryhighbias voltagesif
the fluencesexceeds 1015n /cm2.
0 eq
0 5 10 15 20 25 30
F [1014 n /cm2]
eq
Fig.4.Mostprobablesignalasafunctionoftheirradiation
fluence. Eachpointrepresentsonesample(apartfromthe
5. Conclusion
highestfluence whereeach ofthetwosamplesisshownat
three bias voltages).
In order to estimate the survivability of the
to apply a bias voltage above 800V. This nicely presentCMSbarrelpixeldetectorinaharshradi-
complements the results for n-in-p strip detectors ation environment, single chip detectors (sensors
showninthis conference[9,10]. bumpbondedtoareadoutchip)havebeenirradi-
5
atedtofluencesupto5×1015neq/cm2 andtested [1] TheCMSCollaboration,TheCMSexperimentatthe
with a Sr-90 source. The samples that received CERN LHC, Journal of Instrumentation 3 S08004
fluences up to about 1015n /cm2 could be used (2008) 26–89, http://www.iop.org/EJ/journal/-
eq
page=extra.lhc/jinst.
without any modification of the chip calibration
procedure and obtained a signal charge of above [2] EP-THfaculty meeting, CERN,17.01.2001.
10000 electrons at a bias voltage of 600V. From [3] The CMS Collaboration, CMS Tracker, Technical
this point of view their performance is perfectly Design Report LHCC 98-6, CERN, Geneva,
Switzerland (1998).
adequatefortheCMSexperiment,evenatfluences
twice as high as the 6×1014n /cm2 specified in [4] J. Kemmer, et al., Streifendetektor,
eq
Patentoffenlegungsschrift DE 19620081 A1.
theTechnicaldesignreport[3].Thesamplesirradi-
atedto2.8×1015n /cm2 couldbeoperatedwith [5] G. Lindstro¨m, et al., Radiation hardsilicondetectors
eq
– developments by the RD48 (ROSE) collaboration,
slightly adjusted chip settings and also showed a
Nucl. Instrum. Methods A466 (2001) 308–326.
signalofabout10000electrons,howeveratabias
voltageof1000V.This indicates the suitabilityof [6] Y. Allkofer, et al., Design and performance of the
siliconsensorsforthecmsbarrelpixeldetector, Nucl.
such devices for a use at an upgraded LHC. The
Instrum. Methods A 584(2008) 25 –41.
sampleswhichreceived5×1015n /cm2couldnot
eq
[7] C. Bro¨nnimann, et al., Development of an indium
yet be operated. Their examination is subject of
bump bond process for siliconpixel detectors at PSI,
further studies.
Nucl. Instrum. Methods A565 (2006) 303–308.
[8] S.Dambach, CMSpixelmodulereadoutoptimization
andstudyoftheB0 lifetimeinthesemileptonicdecay
mode, Ph.D. thesis, ETH Zu¨rich, Switzerland, ETH
Diss.No. 18203 (2009).
[9] A. Affolder, P. Allport, G. Casse, Studies of charge
Acknowledgement
collectionefficienciesforp-typeplanarsilicondetectors
ThepionirradiationatPSIwouldnothavebeen afterthermalneutronand24MeVprotondosesupto
possible without the beam line support by Dieter 2×1016neq/cm2,these proceedings.
RenkerandKonradDeiters,PSI,thelogisticspro- [10]I. Mandi`c, V. Cindro, G. Kramberger, M. Miku˘z,
videdbyMauriceGlaser,CERN,andthegreatef- Observation of fullcharge collection efficiency inp+n
fortofChristopherBetancourtandMarkGerling, stripdetectorsirradiatedupto3×1016neq/cm2,these
proceedings.
UC Santa Cruz (both were supported by a finan-
cialcontributionofRD50 andPSI).
The proton irradiation was carried out at the
CERNirradiationfacility.The authorswouldlike
to thank Maurice Glaser and the CERN team for
the outstanding service.
The work of J.Acosta, A.Bean, C.Martin,
V.RadicciandJ.SibilleissupportedbythePIRE
grantOISE-0730173ofthe US-NSF.
The work of S.Dambach, U.Langenegger, and
P.Tru¨bissupportedbytheSwissNationalScience
Foundation(SNF).
ThesensorswereproducedbyCiSGmbHinEr-
furt,Germany.
References
6
This figure "clusterSize.jpg" is available in "jpg"(cid:10) format from:
http://arXiv.org/ps/0901.3422v1
This figure "spectrum.jpg" is available in "jpg"(cid:10) format from:
http://arXiv.org/ps/0901.3422v1
