3
Fine pitch and low material readout bus in the Silicon Pixel Vertex Tracker for the PHENIX Vertex Tracker upgrade Kohei Fujiwara ,1 RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan For the PHENIX Collaboration article info Available online 7 March 2010 Keywords: Pixel Flexible printed circuit board Readout bus abstract The construction of the Silicon Pixel Detector is starting in spring 2009 as project of the RHIC-PHENIX Silicon Vertex Tracker (VTX) upgrade at the Brookhaven National Laboratory. For the construction, we have developed a fine pitch and low material readout bus as the backbone parts of the VTX. In this article, we report the development and production of the readout bus. & 2010 Elsevier B.V. All rights reserved. 1. Introduction The PHENIX experiment was started in 2000 at the Relativistic Heavy Ion Collider (RHIC) of the Brookhaven National Laboratory in the United States to investigate the spin structure of nucleons and the characteristics of a hot and dense matter called Quark Gluon Plasma. The heavy quarks (b-quark and c-quark) in pp and heavy ion collisions are expected to be generated in initial parton–parton collisions and either penetrate the dense nuclear medium [1], or are absorbed, depending on the characteristics of the heavy quark. In PHENIX, the heavy quark production has been studied by measuring single electrons from decays of heavy quarks. In order to measure heavy quark production precisely, it is important to identify the heavy quarks directly. Hence, the PHENIX detector will be upgraded with a Silicon Vertex Tracker (VTX). The VTX will enable to tag charm and bottom quark production by identifying displaced vertexes with high precision tracking. In the spin program, the VTX will work as large coverage tracking equipment to identify jet. The PHENIX VTX detector has four-layers of silicon detectors. The inner two layers consist of pixel detectors and the outer two layers consist of stripixel detectors [1]. We have developed a fine pitch and low material readout bus (Pixel Bus) for the pixel detector system. In this article, we report the development and construction of the Pixel Bus in detail. 2. Silicon Pixel Detector The VTX is composed of two sub-detectors. The pixel system forms the inner two layers and the stripixel system forms the outer two layers. The pixel system consists of 30 pixel ladders of pixel detectors placed cylindrically around the beam pipe in two layers. The first layer at 2.5 cm consists of 10 ladders, and the second layer at 5.0 cm consists of 20 ladders. The length of the ladder is approximately 22 cm along the beam pipe. Each ladder is electrically divided into two independent half-ladders [1]. One half-ladder has two sensor hybrids, which is bump-bonded device with a pixel sensor and four readout chips. The sensor hybrids has four active areas, each of which has 32 columns (z) 256 rows ðfÞ with a pixel size of 425 mmðzÞ 50 mmðfÞ. The readout chips of ALICE1LHCb chip was developed by CERN EP-MIC group [2]. The chip processes a signal from a pixel sensor, then it outputs binary data. One readout chip has data of 32-bit 256 depth. The converted binary data are transferred to a SPIRO readout board [1] via a fine-pitch and low material Pixel Bus. The signal on the SPIRO board is converted to a serial optical signal and transmitted to the PHENIX DAQ system. 3. Fine pitch and low material readout bus We have successfully developed a new fine pitch, high signal density and low material readout bus. It is fabricated using Flexible Printed Circuit board technology with a chemical milling process by TOUKAI DENSHI KOUGYOU CO., Ltd. in Japan. 3.1. Requirements In principle, the ladder has five constraints in the detector and the production side. The following are the requirements of the Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods in Physics Research A 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.03.044 Tel.: + 81 3 3909 2151; fax: + 81 3 3909 2590. E-mail address: [email protected] 1 Present address: Tokyo Metropolitan Industrial Technology Research Institute, 3-13-10, Nishi-gaoka, Kita-ku, Tokyo 115-8586, Japan. Nuclear Instruments and Methods in Physics Research A 623 (2010) 483–485

Fine pitch and low material readout bus in the Silicon Pixel Vertex Tracker for the PHENIX Vertex Tracker upgrade

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Nuclear Instruments and Methods in Physics Research A 623 (2010) 483–485

Contents lists available at ScienceDirect

Nuclear Instruments and Methods inPhysics Research A

0168-90

doi:10.1

� Tel.

E-m1 Pr

Institut

journal homepage: www.elsevier.com/locate/nima

Fine pitch and low material readout bus in the Silicon Pixel Vertex Tracker forthe PHENIX Vertex Tracker upgrade

Kohei Fujiwara �,1

RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan

For the PHENIX Collaboration

a r t i c l e i n f o

Available online 7 March 2010

Keywords:

Pixel

Flexible printed circuit board

Readout bus

02/$ - see front matter & 2010 Elsevier B.V. A

016/j.nima.2010.03.044

: +81 3 3909 2151; fax: +81 3 3909 2590.

ail address: [email protected]

esent address: Tokyo Metropolitan Indu

e, 3-13-10, Nishi-gaoka, Kita-ku, Tokyo 115-8

a b s t r a c t

The construction of the Silicon Pixel Detector is starting in spring 2009 as project of the RHIC-PHENIX

Silicon Vertex Tracker (VTX) upgrade at the Brookhaven National Laboratory. For the construction, we

have developed a fine pitch and low material readout bus as the backbone parts of the VTX. In this

article, we report the development and production of the readout bus.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

The PHENIX experiment was started in 2000 at the RelativisticHeavy Ion Collider (RHIC) of the Brookhaven National Laboratoryin the United States to investigate the spin structure of nucleonsand the characteristics of a hot and dense matter called QuarkGluon Plasma.

The heavy quarks (b-quark and c-quark) in p–p and heavy ioncollisions are expected to be generated in initial parton–partoncollisions and either penetrate the dense nuclear medium [1], orare absorbed, depending on the characteristics of the heavy quark.In PHENIX, the heavy quark production has been studied bymeasuring single electrons from decays of heavy quarks. In orderto measure heavy quark production precisely, it is important toidentify the heavy quarks directly. Hence, the PHENIX detectorwill be upgraded with a Silicon Vertex Tracker (VTX). The VTX willenable to tag charm and bottom quark production by identifyingdisplaced vertexes with high precision tracking. In the spinprogram, the VTX will work as large coverage tracking equipmentto identify jet.

The PHENIX VTX detector has four-layers of silicon detectors.The inner two layers consist of pixel detectors and the outer twolayers consist of stripixel detectors [1]. We have developed a finepitch and low material readout bus (Pixel Bus) for the pixeldetector system. In this article, we report the development andconstruction of the Pixel Bus in detail.

ll rights reserved.

strial Technology Research

586, Japan.

2. Silicon Pixel Detector

The VTX is composed of two sub-detectors. The pixel systemforms the inner two layers and the stripixel system forms theouter two layers. The pixel system consists of 30 pixel ladders ofpixel detectors placed cylindrically around the beam pipe in twolayers. The first layer at 2.5 cm consists of 10 ladders, and thesecond layer at 5.0 cm consists of 20 ladders. The length of theladder is approximately 22 cm along the beam pipe. Each ladder iselectrically divided into two independent half-ladders [1].

One half-ladder has two sensor hybrids, which is bump-bondeddevice with a pixel sensor and four readout chips. The sensorhybrids has four active areas, each of which has 32 columns(z)�256 rows ðfÞ with a pixel size of 425mmðzÞ � 50mmðfÞ. Thereadout chips of ALICE1LHCb chip was developed by CERN EP-MICgroup [2]. The chip processes a signal from a pixel sensor, then itoutputs binary data. One readout chip has data of 32-bit�256depth. The converted binary data are transferred to a SPIRO readoutboard [1] via a fine-pitch and low material Pixel Bus. The signal onthe SPIRO board is converted to a serial optical signal andtransmitted to the PHENIX DAQ system.

3. Fine pitch and low material readout bus

We have successfully developed a new fine pitch, high signaldensity and low material readout bus. It is fabricated usingFlexible Printed Circuit board technology with a chemical millingprocess by TOUKAI DENSHI KOUGYOU CO., Ltd. in Japan.

3.1. Requirements

In principle, the ladder has five constraints in the detector andthe production side. The following are the requirements of the

K. Fujiwara / Nuclear Instruments and Methods in Physics Research A 623 (2010) 483–485484

Pixel Bus which has to be satisfy; � 50ms of detector readouttime, to avoid mechanical conflict with the neighboring detectorsector, to minimize the multiple scattering and photon conver-sions, and to easily produce and handle the Pixel Bus in theproduction side. In terms of the detector readout time, we havechosen to expand the number of parallel 32-bit data width fromthe original design of ALICE [5] to 128-bits. At first, we modifiedthe digital PILOT chip, which has a 2�32-bit data bus width. Thedigital PILOT [5] chip controls readout chips and multiplexes adata from the chips [1]. By using two digital PILOT chips for twosensor module, which is called as half-ladder [1], the Pixel Buswill have a 128-bit data bus width. Four readout chips (4 �32-bitdata width) can be read in parallel on the half-ladder. However,this expansion requires a circuit with a four times denser layoutfrom the original ALICE design on the bus. Moreover, to avoidmechanical conflict with the neighborhood, a bus width of13.9 mm is required. In order to minimize the multiple scatteringand the photon conversion in the detector, the Pixel Bus has to bethin as much as possible with an available technology. Fourth, tobe satisfied with the production and handling of the bus, the busshould be made up of a technology of flexible printed circuitboard (FPC), which is made from a conductor of copper andaluminum and a polymide insulator.

The total length from the farthest sensor hybrid to the SPIROreadout board, is � 60 cm. Since it is difficult to make up a finepitch and longer FPC (425 cm). Thus, the bus part is split into twopieces, the fine pitch part that contains the sensor hybrids, and anextender which we call the Bus Extender. The length of the PixelBus is 250 mm long and the length of the Bus Extender is� 350 mm. The Bus Extender is outside the acceptance ofthe detector and is fabricated using a wider pitch and a moreconventional Copper-Polymide FPC technology. Finally, for thedetector construction, two types of the Pixel Bus are necessary,i.e. the right side and left side bus. The difference of these buses isthe position of signal and power connectors. The circuit on thebuses has exactly the same functionality. The right Pixel Bus hasthe connectors on the right, the left Pixel Bus has them on the left.

The development of such a fine-pitch FPC is challengingdevelopment but key components for the project.

3.2. Pixel bus

The Pixel Bus includes a total of 188 lines, 128 data and 60control, and these lines must fit within the 13.9 mm width of thesensor hybrid. The bus is composed of a component layer, twolayer signal layer, a layer for the completion of vias, power andground layer. Fig. 1 shows the half-ladder with the Pixel Bus andthe structure.

In order to realize the 188 bus lines routed on two signallayers, we choose a design with a trace width of 30mm and aninter-trace spacing 30mm resulting in a 60mm trace pitch. For

Connectors for Extender connection

Fig. 1. Half-ladder with pixel b

maintaining a good trace shape during the etching process, withthe radiation length, we use 3mm thick copper for the signaltraces and 50mm thick aluminum for the power and groundplanes. The structural material for the bus is Polymide [3,4].

3.3. Signal integrity

The Pixel Bus is developed based on the signal integrity andquality assurance. In terms of the signal integrity, we used theHSPICE simulator [6] with a simulation model to predict atransmitted data signal via the Pixel Bus and Bus Extender. Thesimulation model for the simulation which includes the physicalparameters of the Pixel Bus and Bus Extender, the GTL [7]transmitter and receiver [3,4].

For the physical parameter, they are the matrix parameters R

(O=m) for resistance, L (H/m) for inductance, G (S/m) forconductance and C (F/m) for capacitance of lines in the bus. Theyare calculated using the 2-D electromagnetic field solver inthe HSPICE simulator. It supports the W-element which is theMulti-conductor Lossy Frequency Dependent Transmission Line.This can analyze the behavior of a transmission line includingthe skin effect in the copper film and a loss tangent in a polymidefilm. In the comparison of the simulation and measurement,they are similar behavior. In consequence, both the Pixel Bus andBus Extender can transmit data signals to a SPIRO board correctly[3,4].

3.4. Quality assurance

Next, for the quality assurance, we developed the Bus Checkerby a Linux PC which is used to check the internal connection forline by line [4]. The quality assurance procedure for the busfabrication process has been established to check the bus qualityand insure high yield. The essence is that injecting a TTL test pulseonto an input side of the Pixel Bus. The result of the receiving sideon the line is compared with the connection, and then the fail orpass is judged on the Linux PC. Additionally, to checking the signalon the expected line, all other lines are inspected for the absenceof the signal. In this way all combinations of opens and shorts canbe automatically tested. Once the inspection is completed, the testlead portion is cut away from the bus [3,4].

3.5. Improvement

At the production phase, we have attempted to improve thePixel Bus production yield. We succeeded to increase the yieldfrom very low level (o10%) to � 80% by two methods. The firstmethod is a modification of a via process and shape. The via isinterconnection among different layers, which is a through holewith a copper plating. The another is to reduce the ‘‘Micro Short’’

Power inlet Tabs

Read out chip

Silicon Pixel SensorGround (Aluminum)

Power (Aluminum)

2nd Signal layer1st Signal layer

Componant Layer

150 μm

200 μm

15.7 mm

Wire bonding

Pixel Bus(Copper-Aluminum-Polyimide)

Carbon Structure

SensorHybrid

Bump bonding

Via layer

Surface Mount Device parts

Earth Postfor H/V

us and the cross-section.

0

Freq

uenc

y

0

10

20

30

40

50

Mean = 0.174Ω/mm Mean = 0.191Ω/mm

σ = 0.017 Ω/mmσ = 0.011 Ω/mm

Freq

uenc

y

0

10

20

30

40

50

Ω/mm0.05 0.1 0.15 0.2 0.25 0.3 0

Ω/mm0.05 0.1 0.15 0.2 0.25 0.3

Fig. 2. Resistance distribution of the right and left pixel bus. (a) is the right side bus, (b) is the left side bus.

K. Fujiwara / Nuclear Instruments and Methods in Physics Research A 623 (2010) 483–485 485

between a line and a ground plane by new layout. It is a bridgemade by a residue of copper between a pattern and pattern.

In the via process, a laser drilling is done by � 10 times with afine-adjusted laser output and spot. The laser is moving androtating along the circumference. Because in a via hole drilled by alaser at the once, a polymide film is greatly melted by thegenerated heat and the reflection at the higher layer output.Consequently the via hole diameter is irregularity in a polymideand copper region. The irregular diameter region causes a defectin an Au plating process for the vias. The circular via hole ischanged to oval shape. By these modification, the plating fluidis penetrated into the via hole easily. As a result, the via quality isimproved than the first vias.

In the layout, the distance between a line and a ground planeon a signal layer increased more 100mm from the first design. Bythe layout, a residue of copper around the region is removedcompletely in the etching process, then the Micro Short on the busdisappeared.

All line resistance on the right and left Pixel Bus after theimprovements has been measured for evaluating its performanceusing Agilent 4263B LCR meter and a micro-positioner with aprobe needle. Fig. 2 shows the measured resistance divided by theline length for the both side bus. The mean value is 0:174O=mm,the sigma is 0:017O=mm for the right side bus, the mean value is0:191O=mm, the sigma is 0:011O=mm for the left side bus. Thecalculated value is 0:1320:17O=mm. In consequence thefabrication quality is excellent by the improvements.

3.6. Performance

We had a test beam using the 120 GeV proton beam in summer2008 at the Fermi National Accelerator Laboratory to check thefunctionality of Pixel Bus. In the test, we confirmed theperformance with the PHENIX DAQ. Total 19542 events werecollected and 7308 tracks were reconstructed [8].

4. Conclusion

We have successfully developed a very fine-pitch as 30mm ofline and 30mm of space in minimum and low radiation length as0.22% of X/X0 Copper-Aluminum-Polymide bus for the pixel layersof the VTX upgrade in the PHENIX experiment. Development ofthe bus has been completed and the production is starting by theyield improvement. A pre-production half-ladder, as shown inFig. 1, is produced. Finally, the ladder production is starting inspring 2009 by the Pixel Bus.

Acknowledgments

We would like to acknowledge Mr. K. Matsuo and Ms. S.Takami of TOUKAI DENSHI KOUGYOU Co., Ltd. for their greateffort to develop and produce this fine pitch Pixel Bus. This workcould not be done without their excellent technique of the finepitch FPC.

References

[1] Proposal for a Silicon Vertex Tracker (VTX) for the PHENIX Experiment, 2006,available at /http://pvd.chm.bnl.gov/twiki/pub/VTX/Proposal/VTXProposal.pdfS.

[2] W. Snoeys, et al., Nucl. Instr. and Meth. A 466 (2001) 366.[3] K. Fujiwara, Development of a silicon pixel tracker for an experiment of high

energy heavy ion and polarized proton collisions, Ph.D. Dissertation, GraduateSchool of Science and Technology., Niigata University, Niigata, Japan, 2007,available at /http://ribf.riken.jp/�fujiwara/doc/fujiwara_doctoral_dis.pdfS.

[4] K. Fujiwara, et al., IEEE Trans. Nucl. Sci. NS-56 (1) (2009) 250.[5] A. Kluge, ALICE silicon pixel on detector pilot system OPS2003—The missing

manual, CERN Internal Note/ALICE-INT-2004-030 version 1.0, 2005, availableat /http://akluge.home.cern.ch/akluge/work/alice/spd/spd_documents/ALICE-INT-2004-030.pdfS.

[6] Synopsys. Inc: HSPICE Manual, 2006.[7] B. Gunning, L. Yuan, T. Nguyes, T. Wong, A CMOS low voltage-swing

transmission-line transceiver, ISSCC Dig. Of Tech. Papers, February 1992, 58.[8] A. Taketani, Silicon vertex tracker for RHIC PHENIX experiment, Nucl. Instr.

and Meth. A, Proceedings of TIPP09 2009.