For the barrel part of the CMS pixel tracker about 800 silicon pixel detector
modules are required. The modules are bump bonded, assembled and tested at the
Paul Scherrer Institute. This article describes the experience acquired during
the assembly of the first ~200 modules.Comment: 5 pages, 7 figures, Vertex200

B. Meier

Ch. Hörmann

H.Chr. Kästli

König

R. Horisberger

R. Weber

S. König

S. Streuli

T. Rohe

W. Erdmann

English

arXiv

Crossref

Building CMS pixel barrel detector modules

For the barrel part of the CMS pixel tracker about 800 silicon pixel detector modules are required. The modules are bump bonded, assembled and tested at the Paul Scherrer Institute. This article describes the experience acquired during the assembly of the first ~200 modules

König, S

Hormann, Ch.

Horisberger, R.

Meier, B.

Rohe, T.

Streuli, S.

Weber, R.

Kastli, H.Chr.

Erdmann, W.

CERN Document Server

arXiv:physics/0702181v1   21 Feb 2007 Building CMS Pixel Barrel Detectur ModulesS. Ko¨nig a,1 Ch. Ho¨rmann a,b R. Horisberger a B. Meier c T. Rohe a S. Streuli c R. Weber a,cH.Chr. Ka¨stli a W. Erdmann aaPaul Scherrer Institut, 5232 Villigen PSI, SwitzerlandbPhysik-Institut der Universita¨t Zu¨rich, 8057 Zu¨rich, SwitzerlandcInstitut fu¨r Teilchenphysik, ETH Zu¨rich, 8093 Zu¨rich, SwitzerlandAbstractFor the barrel part of the CMS pixel tracker about 800 silicon pixel detector modules are required. The modules arebump bonded, assembled and tested at the Paul Scherrer Institute. This article describes the experience acquiredduring the assembly of the first ∼200 modules.Key words: CMS, pixel detector, module assembly1. The CMS Pixel Barrel ModulesThe barrel part of the CMS pixel detectorconsists of about 800 detector modules that aremounted in three layers on a structure of thin-walled aluminum pipes and carbon fibre cross-pieces [1]. While the majority of the modules (672)are full modules as seen in fig.1 on the right, theedges of the six half-shells are equipped with 16half-modules each (96 in total, see fig.1 on theleft). A half-module has 8 readout chips and ishalf the size of a full module.As seen on fig.1 a module is composed of thefollowing components:– 250 µm thick silicon nitride basestrips– 8 to 16 readout chips (ROCs) with 52x80 pixelsof size 150x100 µm2– Sensor made from 285 µm thick DOFZ-silicon.1 Corresponding author; e-mail: stefan.koenig@psi.chfull−module = 16 ROCs ^half−module = 8 ROCs ^ROCsSensorHDIPowercableTBMSMD−ComponentsBasestripsSignalcableFig. 1. Exploded view of a barrel pixel detector full module(right) and picture of an assembled half module (left)– High Density Interconnect (HDI), a flexible lowmass 3 layer PCB (flex print)– Token Bit Manager chip (TBM) controling thePreprint submitted to Elsevier Science 22 February 2007readout of the ROCs– Signal & Power cableA completed full-module has the dimensions66.6x26.0 mm2, weight 2.2 g plus up to 1.3 g forcables, and consumes 2 W of power. More detailscan be found in [2].2. Assembly ProcedureThe assembly procedure of a CMS barrel moduledivides in four parts (see fig 2):(i) assembly of the HDI(ii) processing of the ROC- and sensor- wafers(iii) bump bonding of the raw module(iv) final assembly of the module2.1. HDI AssemblyThe flex print is delivered pretested andequipped with all passive electrical components bythe vendor. Afterwards the power and signal-cableand the TBM-chip are attached and connectedwith aluminum wirebonds at PSI. The assembledHDI is then tested for the functionality of theTBM and any shortcuts or broken traces in thepower line or token passage. Up to 16 HDIs canbe assembled per working day.In the early stage of the production the bond-ability of the HDIs after severalmonth of storage atroom conditions was not satisfactory. It was thenimproved by a mechanical cleaning step with a softrubber. Unfortunedly this method was likely tocreate shortcuts between adjacend bondpads andcaused a considerable amount of extra work in test-ing and debugging of assemledHDIs and fabricatedmodules (23 % of HDIs not useable, 22 % of mod-ules needed repairs). The storage of the flexes in adry nitrogen atmosphere introduced directly afterthe first problems were discovered eliminated theneed for cleaning the bondpads and led to an ex-celent yield of the assembled HDIs. The fraction offailing HDIs was reduced to less than <2%.The different parts are addressed in the followingsubsections.HDI + SMD’sGlue SignalcableSolder PowercableGlue TBMTest HDI & TBMTBM/SignalcableWirebondCleaningROC’sReflowCleaningSensorReflowTest Raw ModuleReworkGlue BasestripsModule Dr.Glue TBM CoverChip UnderfillGlue HDIWirebond ROC’sFinal Module TestBump Bonding(Tests each ROC)GelpackSignalcablePowercableTBMBasestripsFinal Module AssemblyWafer ProcessingBumpbonding of Raw ModuleHDI−AssemblyFig. 2. Flowchart showing the assembly procedure2.2. Processing of ROCs and WafersThe sensor and ROC wafers have to undergocertain processing and manipulation steps beforethey can be used for the module production:First of all an acceptance test of sensors andROCs is conducted on the wafer. After 72 ROC-wafers tested the overall yield is about 70%. Theacceptance-tests on sensor wafers reflect just theresults of the vendor. Then follows a compositionof photolithographic steps, UBM 2 sputtering andindium evaporation and a final liftoff step to pre-pare the wafers for the subsequent bump bondingprocess [6] The ROC wafers are then thinned downto 170 µm and ROC and sensor-wafers are dicedand picked based on the results of the acceptancetests. Finally the raw devices are tested again. Inthe case of the ROCs this is done as part of thebump bonding process.Only chips with less than 1% statistically dis-tributed dead or noisy pixels and without pixel2 Under Bump Metal - for ROC wafers only, sensor wafershave UBM by the vendor2masking defects are used in the following assemblysteps. A sensor is discarded when either the darkcurrent is above 2 µA at 150 V and room temper-ature or the ratio of the dark currents measured at100 V and 150 V is greater than two.The sensors are then cleaned and undergo the re-flow process where the deposited indium is meltedup to prepare the indium bumps needed for thelater bump bonding. After another optical inspec-tion the sensors are stored unter a dry nitrogenatmosphere untill they are used.Initial problems during the liftoff-process lead-ing to very work intensive procedures could besolved by modifying the fotolithographic masks.Now the openings in the fotoresist are distributedover the whole surface of the wafer. This reducedboth the duration and complexity of the liftoff pro-cedures and reduced the use of the ultrasonic baththat caused partial removing of the deposited in-dium bumps on several sensors to an absolute min-imum. Another problem with corrosion like effectson the aluminum bondpads of the ROC chips wassolved by consequently renewing the liftoff solvendafter each liftoff step, especially before the use ofthe ultrasonic bath.2.3. Raw Module AssemblyThe sandwich of sensor and the 8 to 16 ROCchips needed to build a module is called ’raw mod-ule’. Building it involves the following steps:First the set of ROCs is cleaned and arrangedwith the help of a special jig that ensures theirdefined positions on the gelpacks which are thenplaced on the bump bonding machine. Sensor andROCs are then bump bonded using the automaticbump bonding machine developed at PSI [6] (seefig. 3). The individual ROC chips are tested dur-ing the bump-bonding process to ensure that onlyelectrically working chips are bump-bonded to thesensor. To form a mechanical and electrical con-nection between the indium bumps of sensor andROCs themodule is reflowed in the reflow oven. Af-terwards the completed rawmodule is tested again.The so called ’raw test’ gives a very fast feedbackon the bump bonding quality and was upgradedseveral times to enhance its abilities. It finally mea-measure sensor positiontest a chipmeasure chip positionbumpbond a chipFig. 3. Automatic bump bonding machine at PSIheatable vacuum tableheatable vacuum headFig. 4. Reworking jig at PSIsures the ROC power consumption, does a thresh-old scan and measures the level of the analog out-put. In addition to that it does an IV curve mea-surement on the sensor and makes a bump yieldtest. Based on the result of the test the modules arereleased for further assembly or undergo a rework-ing step to remove defective chips or improve thebump yield if possible and usefull. Raw modulesdrawing a high current (>15 µA @ 200V) are notrepaired and are not used further on. The ability torework single chips of a bare module with a specialreworking jig (see fig. 4) has constantly improved3Fig. 5. Bump yield plot of a completely reworked module(16 chips reworked) only very few bump defects (coloreddots) could be foundduring the module production. The rework qualityis either excellent (less than 12 bump defects perchip) or very poor (∼1000 defects per ROC). Theoverall chip replacement yield is about 85% now(60 of the 71 replaced chips have been good) andeven full modules (16 chips) were reworked withalmost perfect results (see fig. 5).2.4. Complete Module AssemblyWhen the raw module has passed the last test,the construction of the module is finalised on thePSI module assembly line (see fig. 6) in the follow-ing steps:First the base strips are glued to the ROC sideof the raw module. After the glue has cured anda small amount of glue has been applied betweenthe edge of the sensor and the ROCs to reinforcethe connection near the wirebonding area the pre-assembled HDI is glued on the sensor side of theraw module. After the electrical connection be-tween the flex and the ROCs is formed with wirebonds the module assembly is complete. Each glue-ing step is done on a separate jig that ensures theexact placement of the parts and keeps them inplace with vacuum until the glue has cured.Instead of using a glue disperser to apply the de-sired small quantities of glue, the adhesive is ap-plied with a stamp that is lowered into a glue bathand matches the form of the glueing area. Thismethod is a very easy but well defined way to ap-ply the glue exactly where it is needed.In order to apply the very small amount of gluein between the ROCs and the sensor, the stampis replaced with KaptonR© [5] flaps that bring theglue in the opening between the sensor and theROCs.The assembly procedure has proven itself to bevery reliable and fault proof during the productionso far and has not been changed even after morethan 200 modules build. The number of modulesthat after assembly are worse than before is onlyabout 8, which is less than 4% of the modules pro-duced.At the end the completed module undergoes thefinal testing. This includes thermocycling betweenroom temperature and -20 ◦C. The fully automatictest setup (see fig. 7) can handle up to 4 modulesin parallel and up to 2 testloads per working day.The modules are then graded according to theirtest results. The major criterias are the following:Grade A modules are excellent with less than1 permill defectice or unbonded pixels per chip andsensor dark currents of less than 2 µA at 17 ◦C and150 V.Grade B modules are useable modules with lessthan 4 percent defectice or unbonded pixels perchip and sensor dark currents up to than 10 µA at17 ◦C and 150 V.All other modules are graded as C if they can-not be repaired by the ’module doctor’, which isthe case for sensor defects (high dark current) orchip defects. If only a bond has loosened or a shorton the HDI has developed or even the TBM ceasesto function the module will be repaired and testedagain. The ’module doctor’ was introduced tounderstand and possibly repair the HDI-inducedmodule defects and has proven to be very usefullin the first phase of the production, where manymodules needed to be repaired. Due to the im-proved quality of the assembled HDIs the ’moduledoctor’-station was used only very frequent duringthe production of the last 100 modules.The module assembly procedure is capable ofa constant rate of 4 full and 2 half modules perworking day. In parallel to that 16 HDIs can beassembled together with the processing of ROCsand sensors for the bump bonding. The manpowerrequired to maintain this rate is about 5 FTEs 3including the final testing of the modules.More Details about the module assembly can befound in [7].3 Full Time Equivalents4glue applierglue HDIchip underfillsurvey microscopeglue/solder cablesglue baseplateFig. 6. Assembly line at PSItest/thermocycle 4 modules in parallelFig. 7. Setup for final module test including thermocycling3. Production ResultUp to end of November 2007, 286 Modules werebump bonded, but 30 modules were in differentstages of production and testing at this time. Outof the 256 completed moduls 210 (82%) modulesare graded A(177) or B(33) (59 after repair or re-work). Half of the 46 modules with grade C failedin the state of a raw module. About half of themodules were lost due to problems and errors inthe startup of the production. The others are re-jected due to sensor problems.References[1] The CMS Collaboration. CMS Tracker. TechnicalDesign Report LHCC 98-6, CERN, Geneva,Switzerland, 1998.[2] CMS Barrel Pixel detector Overview. H.C. Kaestlithese proceedings.[3] H.C. Kaestli. Design and performance of the CMS pixeldetector readout chip. NIM A 565(2006) 188-194[4] T.Rohe et al.Fluence Dependence of Charge Collectionof irradiated Pixel Sensors. arXiv:physics/0411214 v34 Jan 2005[5] KaptonR© is a registrated trademark of DuPont for itspolyimide film[6] Ch. Bro¨nnimann et al. Development of an IndiumBump Bond Process for Silicon Pixel Detectors at PSI.These proceedings.[7] S. Ko¨nig. Assembly of the CMS pixel barrel modules.NIM A 565(2006) 62-665

Building CMS Pixel Barrel Detectur Modules

Building CMS Pixel Barrel Detectur Modules

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