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ATLAS PIXEL FLEXHYBRID DESIGN

DOCUMENT

ATLAS Project Document. No.Project - System -- Type - Sequential No.

Institute Document No. Created1-Dec-00

Page1of 13

ATL-IP-EP-0008 Modified7-Dec-00

Rev. No.

ATLAS PIXEL FLEX HYBRID DESIGN DOCUMENT

The design requirements of the flex hybrid for the ATLAS pixel modules are described in this document.

Prepared by:R. Boyd

Checked by:

Approved by:

Distribution List

ATLAS PIXEL FLEX HYBRID DESIGN DOCUMENT of 13

ATL-IP-EP-0008 Created: 1-Dec-00 Modified: 7-Dec-00 Rev. No.: 2

History of Changes

Rev. No. Date Pages Description of changes

2 12-8-00 4, 7, 8 Corrected number of bumps, corrected Figure 1, correctedfull Pixel radiation dose, clarified statements on power requirements,corrected typo’s.



Table of Contents

1. INTRODUCTION 4

2. PIXEL DETECTOR OVERVIEW 4

3. PIXEL DETECTOR MODULES 4

4. FLEX HYBRID FUNCTION 6

5. ELECTRICAL REQUIREMENTS 7

6. PHYSICAL REQUIREMENTS 8

7. IRRADIATION TESTS 9

8. PROTOTYPE FABRICATION 9

9. ASSEMBLY 11

10. PRODUCTION ORGANIZATION 11



1. IntroductionThe ATLAS Pixel Detector Flex Hybrid provides interconnection of the various electronic components of the pixeldetector module. It also provides interconnection of the module with the cable plant leading to the off detector readoutelectronics and power supply. This document details the design requirements of the flex hybrid and theirimplementation in the layout of the flex hybrid. Since the scale of the design is large, detailed diagrams of schematicand layout will be presented during the FDR. A file is available at the same location on EDMS as this document whichcontains some basic drawings and dimensional information (drawings.pdf)

2. Pixel Detector OverviewA description of the ATLAS Pixel Detector is given in the Technical Design Report (TDR) available fromhttp://atlasinfo.cern.ch/Atlas/GROUPS/INNER_DETECTOR/PIXELS/tdr.htmlor the ATLAS Secretariat.The TDR covers the basic layout and physics goals of the pixel detector. Although some of the details of the Pixeldetector layout have changed, e.g., the adoption of a fully insertable system, the fundamental design is still the samewith respect to this FDR.

3. Pixel Detector ModulesModules are the basic building blocks of the ATLAS pixel detector system. A module consists of a silicon sensor tilewith a sensitive area of 16.4 mm x 60.4 mm, sixteen read-out Front End (FE) chips, a Module Controller Chip(MCC), performing clock and control functions, and the local signal interconnection and power distribution bussesand passive components, including a temperature sensor, resistors and capacitors. The FE's are connected to the sensorpixel elements through more than 46080 bump bonds per module.

Modules are completely independent from each other for data transmission and communicate with the ROD's(ReadOut Driver) via dual serial optical links, one for transmission of event data, another for reception of timing andcontrol signals. The optical links and their driver and receiver IC’s are located off module. Figure 1 is a block diagramof the power and signal connections of a module.

The module is built using two types of chips:• Front-End Chip (FE):Includes an array of 400 x 50 µm pixels arranged in 18columns with 160 rows each, End-of-Column (EoC)logic, which provides zero suppression and performs theL1 trigger coincidence matching, a serial transmitter anda minimum of control logic to initialize the mask andtest flip-flops in the pixel cells. Several DAC’s foranalog parameter setting are included in the chip as well.• Module Controller Chip (MCC):Performs the two main functions of event building anderror handling, and of timing and control for the FEchips in the module. This chip can also work in“transparent mode”, which allows direct access to the FEchips to support more flexible operation duringdebugging. Other diagnostic functions are embedded inthe chip both for on-chip and system tests.

The flex hybrid provides bussing of signals between theMCC and FE’s, in addition to routing power for the FE’s, MCC and sensor. A drawing of a module with the flexhybrid is shown in Figure 2.



Figure 2 . Version 2.x flex hybrid module top view and cross section of an edge including FE wire bondpads.

Disk module cut line

Test connector

Disk modulealignment holes

Disk pigtailsolder pads

Disk and barrel module cut line

Solder pads for module I/O connections to support card

Module cross section at edge

Barrel module cutline

Barrel pigtail wirebond pads



4. Flex Hybrid FunctionThe Flex Hybrid provides the routing between the 16 FE chips and the MCC on a module. All the signals that areactive during data taking use LVDS (Low Voltage Differential Signaling) standards to reduce EMI (ElectroMagneticInterference) and balance current flows. Control and other low speed signals are CMOS logic level. Theinterconnections between the MCC and the 16 FE chips in a module follow a star topology for data transfer out of theFE's, which uses unidirectional serial links. The timing and control signals utilize a bus topology. Wire bond pads onthe edge of the hybrid are used to connect power and signal lines to corresponding wire bond pads located on the FEchips, which extend beyond the edge of the sensor. On the flex hybrid, power and ground lines, as well as differentialsignals, are placed parallel to each other on the same metal layer. Figure 3 is a block diagram of all the signals andpower routed on the flex hybrid.

Other factors considered in the design of the flex hybrid include:• Signal integrity must be maintained across the entire flex hybrid, which in addition to following appropriate

layout practices, requires several resistors for the LVDS signals.• Each power supply must be capacitively decoupled to reduce noise and allow fast signal response.• The sensor bias supply (Vdet) must be routed to accommodate up to 700 V. This means that a minimum distance

must be maintained to adjacent traces and allowance must be made for hi-pot encapsulation. In addition, thebottom side traces must be electrically isolated from the sensor up to the specified voltage.

• Both the prototype and production designs require that a certain level of testability been designed into the flexhybrid.

• The mechanical envelope must be respected (see the Flex Hybrid Interface Document).• Radiation cross section must be minimized.• The flex hybrid and components must be radiation tolerant to 50 Mrads.• The flex hybrid must be reliable for up to 10 years operation.• The wire bond pads must be large enough to allow at least one re-bonding attempt (see the Flex Hybrid Interface

Document).• Connections for the disk and barrel modules must be made separately (see the Flex Hybrid Interface Document).

Figure 3. Block diagram of power and signals routed on the flex hybrid.



The following sections discuss these design parameters and their implementation in more detail. A schematicdiagram, layout drawings and fabrication drawings are provided in the file drawings.pdf. A complete layout drawingwill be presented during the FDR.

5. Electrical RequirementsSignalsThe readout electronics operate from a master clock frequency of 40 MHz. Since the length of the flex hybrid is lessthan ¼ that of the third harmonic of the clock wavelength, transmission line effects are not a concern. This has beenverified by simulations and direct observation. The LVDS signals are generated by current source drivers in themodule electronics. This requires at least one “termination” resistor to provide a sink for the current to develop avoltage across. Therefor, an “H” bus topology is used with a single terminating resistor for the MCC to FE LVDStraces, with a single resistor located near the MCC (see Figure 4). The LVDS signals are routed adjacent and parallelto each other to minimize cross talk to and from other signal traces. The typical value of for these resistors is 100 Ω . Itis expected that the final MCC design will include these resistors within the die itself to reduce radiation cross sectionand simplify the flex hybrid layout.

For LVDS signals associated with the optical links, a single termination resistor is provided on the flex hybrid tominimize reflections in the cables between the flex hybrid and the optical links. Since the optical links have onlyrecently become available and the cables are still under development, the best value for these resistors is unknown.

The CMOS level signals operate at the CCK frequency of 5 MHz. These signals are given a lower priority than powerand LVDS signals with respect to position when routing the flex hybrid.

PowerThere are three power supplies, which must be routed on the flex hybrid to each FE. Although the actual currentrequirements are not known at this time, the estimated worst case maximum after full dose irradiation is listed alongwith each supply in Table 1. The power busses on the flex hybrid will be optimized to minimize the voltage drops oncethese values are known for the final FE and MCC designs.

Voltage CurrentPower Supply Max Nominal Max NominalVDDA 6.000 3. 5 1.2 1. 08VCCA 4.000 1. 75 1. 5 1.44VDD 6.000 4 2 1. 52 0.Vdet 700 - 0.004 -

Table 1 . Power supplies on the flex hybrid.

These values are based on fabrication of the FE and MCC in DMILL. These IC’s will also be fabricated in IBM’s deepsub-micron process, which operates at 2 V. The power requirements for this process are not yet estimated, but areexpected to be no worse than these numbers.

MCC

Figure 4. Simple H bus. Small black rectangle represents a resistor.Red and blue lines represent the meal traces of opposite polarities ofthe same LVDS signal on the flex hybrid.



The original specification for routing of the power signals on the flex hybrid was that the voltage drop to any FE wasto be no more than 50 mV round trip. Experience with the first three prototypes has shown that the voltage drop canbe as high as 100 mV round trip, given the estimated maximum currents in Table 1. The limiting factors are:

1. Available area2. Thickness of the metallization3. Length of the traces

Item three is particularly important for the disk module pigtail, which is connected onto an extension to the base flexhybrid, lengthening the traces (see Flex Hybrid Specifications and Measurements Document). This additional lengthmust also to be routed through a restricted area with respect to width, further raising the resistance in the path to thedisk services pigtail. The effect is of the order of being a 1/3 higher resistance than that of the barrel module powerrouting.

Like the signals, the power traces are routed in an H bus configuration. This helps to minimize voltage drops. A 4.7µF 1206 size capacitor in parallel with a 0.1 µF 0402 capacitor is placed at the junction of each of the busses to supplya modest amount of filtering to the power inputs. More filtering here is desirable, but this is the best compromiseavailable given the envelope constraints and the need to keep the mass on the module low. In addition, each supply isbypassed again at each FE with a 0.1 µF 0402 size capacitor to provide local decoupling. All capacitors are X5Rcharacteristic ceramic. The distances between the capacitor terminals and the corresponding wire bond pads must beminimized to maximize the effective capacitance. This is particularly important in the case of the digital supply,where as little as 0.1 Ω can prevent effective decoupling. A working voltage of at least 10 V is specified to insurereliability over the 10 year operational lifetime of the detector.

The high voltage sensor bias supply is isolated by two 10 kΩ resistors in series with both the supply and return lines ofthe supply. A 1 nF capacitor across the sensor serves to filter noise in the supply.

Simulations are also under development using 3-D models of the flex hybrid and approximations of the FE currentrequirements. Although not exact, these simulations should provide enough information to suggest the bestcompromises with respect to value and placement of the decoupling capacitors for each supply to each FE. This isparticularly important for the final production layout, as it may not be possible to gain much experience withprototypes built with the production FE design before it is required to be complete.

Currently, a grounding pad is provided under the MCC to ensure against lockup. It is not clear at this time whetherthis is required or desirable (because of the constraints imposed upon the layout) in the final design. Tests to date withirradiated DMILL MCC’s working in a test beam indicate that grounding of the MCC substrate is not required.

6. Physical RequirementsThere are many physical requirements on the flex hybrids, due to several factors. The pixel detector’s close proximityto the interaction region requires that its mass be kept to a minimum and that it be able to tolerate up to 1015 n/cm2

irradiation equivalent. The mechanical constraints impose a very tight envelope on the module in r. Since the FE’s,which are bump bonded to the sensor, are glued to the local support, consideration must be given to the CTE(Coefficient of Thermal Expansion) of the various materials in the module: This is covered in the Flex HybridInterface Document.

Polyimide has been chosen as the substrate material, both for low mass and radiation tolerance. In addition, it is themost commonly used material for FCB’s (Flexible Circuit Board). Its CTE is well matched to Si and its dielectricstrength is on the order of 7500 V/mil. For the cover layers, or solder mask, material, both LPI polyimide and du PontPyralux, an acrylic, have been used in prototypes. Pyralux is widely used in industry as a cover layer material, but it’shigh CTE, a factor of 10 greater than polyimide, requires additional consideration for assembly and mounting ofmodules. Although the LPI polyimide material is better both in CTE and voltage hold off, it is currently only availablefrom one vendor.



On one edge of the flex hybrid where the FE bonds reside, the envelope in r (height) is restricted to the range of 0.6mm to 0.9 mm. The specified height for the 0402 capacitors is 500 ± 100 µm. Assembly vendors estimate thatcomponents can be as much as 1 mm in height after reflow soldering. Trial assembly needs to be done in productionmode (automated assembly) so that the process can be tuned to meet the requirements of the pixel modules. In flexhybrid version 2.x, the 0402 capacitors are mounted within the 0.6 mm height restricted area because the present FEwire bonding scheme does not allow enough room between FE bond pad groups to move the capacitors between them(see page 3 of drawing.pdf). In the production layout, the active FE wire bond pads will be moved to the center 2/3 ofthe die edge, so these capacitors will be placed further to the edge of the flex hybrid where there is more verticalclearance.

The sensor high voltage (HV) is routed so as to keep at least 1 mm between all HV traces/components and othertraces/components. In addition, there is a guard ring at sensor bias/flex hybrid ground between all HV traces andadjacent traces. The voltage hold off of air is dependent on humidity and other factors, but all module tests beforeassembly will be made at 200 V or less. Although the detector will be operated in a dry atmosphere, the current plancalls for potting of the entire HV area for safety.

7. Irradiation TestsIrradiation tests have been performed on the flex circuits and components. The FE and MCC have just recently beenfabricated in rad hard processes and are not covered here. Several runs have been completed, the most recent of whichwas in October at the CERN PS. These have not yet been returned for evaluation. Previous runs include 60Co to 30Mrad and 60 Mrad without voltage applied to components. At CERN PS this summer, components were irradiatedwith 3.5 V applied to 1.9x1015 p/cm2 (24GeV).

The flex circuits have been irradiated both with and without Pyralux cover layers. The only effect observed is adarkening of the polyimide. The Pyralux is unaffected. Likewise, evaluation of a wide variety of values, types and sizesof resistors and capacitors shows no deterioration in their performance to date.

8. Prototype FabricationDuring development of the ATLAS Pixel detector system, three prototype flex hybrids have been constructed. Sincethe ROD's will not be available until 2001, and there is a need during development to work with individual modules, a"support card" and VME based DAQ (Data AcQuisition) system is used to enable testing and read out of flex hybridmodules. The support card provides a module heat sink, cable drivers, connectors, hit bus readout and other supportfunctions. The Pixel DAQ system consists of a VME PLL (Pixel Low Level) card, a stand alone PCC (Pixel ControlCard) and software to control the system. The DAQ system provides fully functional control for test, debug andoperation of modules in the lab and in the test beam. The support card and DAQ system were developed jointly byLawrence Berkeley National Laboratory, Seigen University (Germany) and INFN (Genova, Italy).

Since the optical link was not yet available, the first prototypes (Versions 1.0 and 1.x) provide signal I/O throughconnection pads at one end of the Flex Hybrid. Likewise, power connections are made through similar pads at theopposite end. The flex hybrid is mounted on the back side of the sensor (the side opposite the FE's). The MCC ismounted directly on the flex hybrid, and all the bare die connections are made with 0.7 mil or 1 mil wire bonds(including the FE's). On the flex hybrid, power and ground lines, differential signals and CMOS signals are bussedparallel to each other on the bottom metal layer. Traces on the top layer route the signals to the pads and buses.

flex hybrid version 1.0 includes 54 decoupling capacitors: Three tantalum + 3 ceramic capacitors that decouple theAVDD (Analog VDD), AVCC (Analog VCC) and DVDD (Digital VDD) lines coming into the flex hybrid and 48ceramic capacitors that decouple these lines at the traces leading to each pair of FE bond pads (there are twoconnections for each supply on each FE). There are also 14 terminating resistors for the LVDS signals. The ceramiccapacitors and resistors are 0402 size SMD's (Surface Mount Device). The tantalum capacitors are EIA-A size SMD's.All passive components are mounted directly on the Flex Hybrid. Version 1.0 uses a U-bus topology for the bussedsignals, i.e., the signal buses are routed continuously around the flex hybrid in a "U" shape, with a terminating resistorat each end of an LVDS pair. A hole is provided through the flex hybrid to allow access to the bond pad on the back (pside) of the sensor for the bias voltage. Bias is supplied to the n side of the detector through the FE's (analog ground).



CERN was the only vendor for version 1.0 flex circuits. They delivered 25 electrically good circuits in September of1998. These circuits were fabricated on a 50 micron Kapton substrate and did not include cover layers (solder mask).

Flex hybrid version 1.x utilizes an H-bus topology and has only 10 resistors, the terminators for the four MCC signalinputs provided in the previous version having been moved to the support card. The H-bus topology is implemented byplacing the MCC and terminating resistors for the LVDS signals near the center of the flex circuit. Connections aremade from the terminating resistors and MCC to each of the two separate buses near their centers. The H-bus topologyrequires only 1/2 the number of termination resistors for the bussed LVDS signals.

The number of ceramic decoupling capacitors remains the same as in version 1.0. Pads for 0402 resistors were addedfor evaluation of several different grounding schemes and in line with the sensor bias supply and return lines. SMDpads were modified so that either 0402 or 0603 size devices might be used in most locations for ceramic capacitors.The SMD pads for the tantalum capacitors were modified to accept either EIA-A or 0805 sizes. Pads were also addedfor a 1206 size sensor bias decoupling capacitor and a Pt1000 temperature sensor. Additional traces and connectionpads were added on the flex hybrid to route the sensor bias and return, the temperature sensor, Vcal (for internalcharge injection of the FE's) and separate return lines for AVDD, AVCC and DVDD. Flex hybrid v1.x also wasdesigned to allow room for mounting the optical link package, as a proof of concept.

Compunetics delivered the first v1.3 flex circuits in December of 1999. Over the next several months, they delivered atotal of 43 electrically good flex circuits. These were fabricated on a 25 micron Upilex substrate with thick (2.0micron) Au plating over an Ni barrier on the wire bond pads. The solder pads were coated with Intek, a solid organicsolder flux, to protect against corrosion. The patterned cover layers were of Imageflex, a photoimagable solder maskfor flexible circuits. An SEM (Scanning Electron Micrograph) of bond pad cross sections revealed some problems inthe metallization. Although 2.0 microns of Ni over 15 microns of Cu was expected, the SEM revealed that the Cuthickness was 7 microns and the Ni thickness was 8 microns. This is believed to have been caused by mistakes in theAu plating process.

CERN delivered 50 electrically good v1.3 flex in 1999 - 2000. There were some minor layout differences, but withrespect to the component positions and schematically, v1.3 and v1.4 are identical. The CERN circuits were fabricatedon 50 micron Kapton with patterned 25 micron duPont Pyralux PC 1010 acrylic cover layers. Thin (0.1 micron)electroless Au was plated on all traces and pads over a 1.5 micron Ni barrier layer. An SEM of bond pad cross sectionsverified the plated metal thickness’ and a Cu thickness of 16 microns.

Flex Hybrid version 2.x is the first layout to provide for the operation of a module without a support card. This allowsthe construction of functional staves (for the barrel) and sectors (for the disks) of modules. The 51 ceramic decouplingcapacitors have been returned to 0402 size only and the tantalums have been replaced with 1206 ceramic capacitors inthe range of 1.1 µF to 4.7 µF. These component sizes are reduced and their positions adjusted to respect the finalmechanical envelope of the module for the staves, which is limited in some areas to 0.5 mm for component height.The optical link for this version is to be mounted on the flexible pigtail that delivers power to the module. Therefor,the input and output signals are routed with the power inputs. The MCC input LVDS terminating resistors were alsoreturned to the flex hybrid. Provisions have been made in version 2.x to allow connection of a barrel pigtail by wirebonds and a disk pigtail by solder pads. In addition, a 30 pin test connector is provided on a removable section of theflex circuit. This facilitates QA (Quality Assurance) of the flex hybrid throughout assembly and after mounting on themodule. The I/O connections remain at one end of the flex circuit as in previous versions to retain compatibility withv1.x support cards.

Version 2.1 of the flex circuit was received in August, 2000, from Compunetics (Monroeville, PA). These feature athin (0.2 mm) Au plating only to allow for quicker fabrication. This version also includes a patterned 25.4 micronduPont Pyralux PC1010 acrylic cover layer on each side. A total of 41 electrically perfect and 25 electrically defectivecircuits were delivered by Compunetics. The defective circuits are used for assembly practice. Verification of metalthickness’ by resistivity measurements are consistent with the expected values.



CERN delivered 54 electrically good version 2.2 flex circuits in September of 2000. These were identical in layout tothe version 2.1 circuits from Compunetics, but feature 10 micron thick LPI (Liquid Photo-Imagable) Kapton patternedcover layers. This was requested in order to obtain a better CTE (Coefficient of Thermal Expansion) match withsilicon. Although this results in reduced flexibility of the circuit, this is of little importance in the ATLAS Pixelmodule application, since the circuits are planar and static in use. Figure 5 shows a version 2.1 flex hybrid with SMDcomponents mounted.

9. AssemblyAssembly of version 1.0 and most version 1.x flex hybrids was done at collaborating Pixel institutes. A small numberwere assembled by hand at AMA in California. About 35 v2.1 flex hybrids have been assembled at Flex One (Ca) andSurface Mount Depot (Ok), again, by hand. Although most of the assembly work was acceptable from both firms,several problems have come to light as a result of these trials. There has been some leeching of solder under the soldermask and tearing of the test/disk connection tab during rinsing at Flex One. The latter problem can be ameliorated byadding a radius to the inner cuts of the tab. At Surface Mount Depot, the cover layer was damaged in several areas,causing 2 flex to be effectively ruined by solder contaminating wire bond pads. This was traced to damage caused byusing a soldering iron to reflow the solder. A common problem was floating of the components in the solder. Thisresulted in a violation of the module envelope for many components.It is clear that more trials need to be done in assembly with vendors. Automated assembly promises a low cost methodfor assembly and includes the added advantage of testing each component before it is placed on the flex hybrid.

10. Production OrganizationThe current baseline plan calls for 50% of the flex circuits to be produced at CERN and 50% at Compunetics. All flexhybrid components, except the MCC, are to be mounted in industry. Attachment and wire bonding of the MCC is to beperformed at Pixel institutes. Testing of the flex hybrids is to take place at UOK and Albany before mounting of theMCC. These tests will include:• CERN flex tests at Microcontact (Switzerland) – CERN has no in house testing• Test for pinholes in bottom cover layer• Test of complete Flex Hybrid – Ohmic, with probe card• C - F test of power busses to verify connection/value of all decoupling capacitors

Testing of the flex hybrid after mounting of the MCC is split 50 – 50 between US and European labs. This will beperformed with a Genova supplied MCC/hybrid test stand (under development).

A flow diagram for flex hybrid production is shown in Figure 6.

Figure 5. A version 2.1 flex hybrid with components mounted.



The baseline schedule for flex hybrids is shown in Figure 7. This schedule is based on production of rad hardelectronics in IBM deep sub-micron. However, if DMILL technology proves to be successful, the flex hybrid schedulewill be adjusted forward accordingly, as the principle dependency is on the electronics design and fabricationschedules.

Figure 6. Flex hybrid production flow diagram. CERN does not provide testing nor circuitsingulation. Assembly vendors require flex circuits in panel for automated assembly.



Figure 7. Flex hybrid schedule based on fabrication of electronics inIBM deep sub-micron process.

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