company matched funded Innovative electronicsManufacturing Research Centre (IeMRC) Flagshipproject between 3 UK universities and 10 companiesentitled "Integrated Optical and Electronic InterconnectPCB Manufacturing". The project aims to develop ofoptical waveguide design rules, layout software,fabrication methods compatible with commercialproduction, characterisation techniques and opticalconnector design to provide a supply chain for PolymerMultimode Optical Waveguide Printed Circuit Boards(OPCB) for 10 Gb/s board-to-board interconnections.
IntroductionElectronic rack based systems often have a central or
rear printed circuit board known as the backplane ormotherboard into which is plugged multiple smallerprinted circuit boards known as line cards, daughterboards, drive cards or mezzanine cards. For the highestreliability and the lowest maintenance costs, thebackplane should have only passive interconnects asactive components may fail. Conversely, the line cardsare highly populated with active components but it iseasy to unplug them and to replace them if the activecomponents fail, as they are accessible from the outsideof the rack. For highest efficiency, the connectors fromthe line cards to the backplane must have a highaggregate bandwidth and so data on the line cards ismultiplexed to a high bit rate and sent through a multichannel high bit rate connector onto the backplane. Asbit rates increase, the copper tracks or traces on thebackplane are now limiting performance.
At high bit rates copper traces have high frequencydependent loss due to the skin depth effect [1] andelectromagnetic waves are strongly radiated andreceived by other traces leading to severe crosstalk. Ifthe system unit is not shielded, then electromagneticinterference (EMI) radiates from the box and likewisetraces receive interference from outside the box.Frequency dependent loss and intersymbol interferencecan be compensated by using transmitter pulse preemphasis filters and receiver amplifiers with high gainhaving either fixed blind or adaptive equalisation filters.However, these techniques consume power and arecostly. Electromagnetic crosstalk can be reduced byshielding the traces by burying them in a multilayerboard between earth planes; however, there are still highfrequency currents in the earth planes which radiate.
Differential lines can suppress much radiation but atcomers, the asymmetric differential mode can convert tothe common mode, which radiates strongly [2]. At highbit rates parasitic impedances also become important socopper vias between traces on different layers must befabricated to minimise impedance mismatch reflectionsby back drilling buried vias to remove stubs but thisrequires additional processing steps requiring additionalcost. Therefore, engineers are investigating analternative technology to copper traces which does notrequire costly compensations, namely, opticalwaveguide interconnects.
Optical waveguides have very large bandwidths andso are scalable for use at bandwidth well in excess of 10Gb/s. The core and the cladding are both polymer forlow cost and ease of fabrication but the core has aslightly higher refractive index than the cladding. Theyare similar to optical fibres in that they operate by totalinternal reflection but unlike optical fibres, they have asquare core rather than a circular core cross section, asthey are not fabricated by heating and drawing butinstead are formed using techniques compatible withthose already used to fabricate printed circuit boards.The waveguides can be formed on one or more layerswithin or on the surface of a multilayer printed circuitboard as part of the usual lamination process.
Copper traces are very good at low bit rates andbetter than optics for transmitting power so will bepreserved for low data rate connections and powerconnections in the hybrid optical and electronic printedcircuit boards (OPCBs). In order to ease the design ofoptical printed circuit boards by electronic engineersmore familiar with the design of conventional PCBs, inthis project, optical design rules are being establishedand incorporated into existing printed circuit boardlayout programs to layout both copper traces and opticalwaveguide interconnects together. The design rulecheckers and autorouting software will check and layoutthe optical waveguide interconnects on the opticallayers.
If electronic engineers are to use optical waveguidesin preference to copper traces, the cost of a fullinstallation must be kept very low. For example, theoptical pluggable connector is an important part of thecost of a system using optical waveguides so its costmust be minimised by reducing the part count in theconnectors and reducing the fabrication tolerancesrequired in the connector. The connector must bedesigned to give a low loss connection even if it is
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slightly misaligned. Therefore, the waveguides are madeto have large cores of 35 to 75 microns in diameter,which makes them multimode rather than using thesmall '"-'5-9 micron cores of single mode waveguides. Inmultimode waveguides, modal dispersion is the maincause of dispersion but as the lengths of the waveguideare at most 0.5 to 1 metre, this form of dispersion is nota problem. OPCBs will be widely adopted if the cost offabrication is minimised by slightly adapting PCBmanufacturer's existing equipment and processeswithout adding complex fabrication steps.
In an earlier project, "Storlite" also funded byEPSRC together with DTI, three partners of the currentconsortium UCL, Xyratex and Exxelis began researchon optical waveguides. Storlite resulted in [3-17] theestablishment of some design rules and several patentsincluding the design of a novel, low cost, high precisionalignment technique which is being used in the firstcommercial optical backplane waveguide connectorbeing made by US connector manufacturer Samtec.
Project Aims1) To establish waveguide design rules for each of
several waveguide manufacturing techniques photolithography, laser-writing, laser ablation,extrusion and printing - and to incorporate theminto commercial design rule checker and constraintmanager layout software for printed circuit boardsso that PCB designers can easily incorporate opticalconnection layers without detailed knowledge ofthe optics involved. To investigate and understandthe effect of waveguide wall roughness and crosssectional shape on the behaviour of light and theeffect on waveguide loss.
2) To develop low cost manufacturing techniques,including polymer formulation, for integratedOptical and Electronic interconnected PrintedCircuit Boards, OPCBs. To develop and to comparethe commercial and technological benefits ofoptical printed circuit board manufacturingtechnologies - photolithography, laser-writing,laser ablation, extrusion and printing - for high datarate, small and large 19", printed circuit boards sothat it will be clear which technology is best foreach type of PCB. To characterise the behaviour ofoptical waveguide backplane systems in real worldconditions, including cycling temperature, highhumidity and vibration.
The results of the research are being disseminatedthrough a range of public conference presentations andproceedings [18-29], private talks to other polymerwaveguide researchers at IMEC, Ghent University andIBM Zurich, circuit engineering publications for PCBmanufacturers, [30-31] and through peer reviewedacademic journals [32-35].
The Consortium PartnersDepartment of Electronic and
Electrical Engineering, UniversityCollege London, UCL, (Lead),School of Engineering and Physical
Sciences, Heriot-Watt University, UK, Wolfson Schoolof Mechanical and Manufacturing Engineering,Loughborough University, UK, Xyratex TechnologyLtd., BAE Systems Advanced Technology Centre(Photonics Group), Renishaw, Exxelis Ltd, DowComing, Stevenage Circuits, Cadence Design Systemsand National Physical Laboratory (NPL) plus twoassociated corporators: RSoft and Xaar.
University College London (VCL) ResearchUCL is establishing design rules for optical
multimode acrylate polymer waveguides by opticalmeasurement and computer modelling. UCL haveinitially concentrated on waveguides fabricatedphotolithographically from Truemode® acrylateformulation at the partner company Exxelis.Loss Design Rules
The loss of individual waveguide components, suchas straight sections, 90° bends, crossings, tapers andtapered bends must be known across a range of designparameters, such as bend radii or waveguide widths, sothat the combined loss of a cascade of such elementscan be found to determine whether the interconnection'soptical power budget is sufficient to achieve a good biterror rate. UCL characterized the cross-talk betweenadjacent and neighbouring waveguides by laterallymoving an input VCSEL to scan an array of waveguides[35]. When the light source is misaligned to awaveguide core, it emits into the cladding and itstransmitted power drops almost linearly at a rate of0.011 dBmlJlm.
z~~"""'--~··~"""'--;'xU: 1::-:;:-;:-Fig. 1. Cross talk in straight waveguides
UCL used a combination of crossings at straightsections and a curved waveguide to measure loss percrossing and achieving a consistent result with that ofother workers [37,38] for the 90° crossing case. 0.023dB per crossing was achieved at a 90° crossing whichmeans output power dropped down 0.5% at each 90°crossing. The mean loss of each point was found byaveraging 50 measurements at each designed crossingangle. The loss of multimode polymer waveguide bendswas measured for a range of radii of curvature and forseveral waveguide widths to establish design curves toaid optical waveguide interconnect backplane designersto minimise transmission and radiation loss [36]. Theexperimental results were obtained for waveguideshaving a refractive index difference of I1n =1.9% ofcore index and having unpolished end faces which are
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..... LMER
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Fig. 5 Schematic diagram of the laser ablationprocess route for the fabrication ofwaveguides.
The process involves depositing a layer of claddingpolymer onto the substrate, followed by a layer of corematerial, typically using spin coating. Laser ablation isthen used to remove tracks of material to leave astanding structure of core and cladding. While it is notnecessary to ablate into the cladding layer, in general, alittle of it is intentionally machined away to ensurecomplete removal of the core that, if left behind, couldotherwise lead to cross-talk. After machining, a fmallayer of cladding is applied to complete the structure. Inthis work, ablation using two different laser systems isunderway: KrF excimer (248nm) and Nd:YAG(355nm). The Nd:YAG system used here is acommercial device for via drilling in PCB manufactureand, therefore, offers the opportunity to utilise existinginfrastructure to enable the rapid integration of thistechnique into conventional PCB manufacture. Trialswith this system have already been conducted toestablish machining rates and some waveguides havebeen fabricated for subsequent characterisation.
Fig. 4. Comparison of (a) modeling result and (b) experimentalresult of rotation tolerance of different guide width.
The results are compared in fig. 4. ; (a) showsmodelling result of rotation tolerance when axialdistance is 200 Jlm between source and waveguide. (b)Represents experimental measurement corresponding tothe modelling.
Loughborough University ResearchLaser ablation to create waveguide structures
Laser ablation is another technique that is beinginvestigated for the fabrication of waveguides as shownschematically in fig 5.
20
0.18_ 0.18 -1...........•~ .
m:!!. 0.14~ 0.12 -1 \ .
IiB 0.1(; 0.08 -! '-- .
~ ~: -4 =.""-<.:: :_:::_:_:1 0.023 I-J 0.02 -! .
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Mean Loss Per Crossing
IJ
40 60 80 100
Crossing Angle (Degree)
Fig. 2. Transmitted mean power per crossing as a function of crossingangles
Tapered bends can increase misalignment toleranceat the input facet. However, there is a trade-off betweeninsertion loss and misalignment tolerance [34]. VCLfound that the product of these two factors is a constantwhich increases linearly with taper ratio (TR = Win/Wout).
Product = 0.650TR - 0.09independent of bend radii. Based on these measurement,VCL suggest that taper ratios TR ;::: 0.4 may be best fora backplane system.Misalignment Design Rules
Translation and rotation misalignment isinvestigated by modelling and experiment. A VCSEL ischosen as the light source and offsets between VCSELwaveguide and waveguide-receiver are studied to findthe translation and rotation tolerance. Beamproprapagation method (BPM) is used for modellingand the waveguides are multimode with a channelstructure made of Truemode® acrylate polymer. VCLhave measured the translation and rotationmisalignment for several waveguides with deferentwidths. Values along the x axis represent lateralmisalignment between optical source and waveguidecore centre. Values along the z axis represent axialmisalignment between the optical source and thewaveguide. The modelled result fig (3.a) andexperimental result fig (3.b) is shown for the waveguidewidth used in this figure is 50x50 Jlm.
commonly used in OPCB backplane applications. Theoptimum bend radius for polymer waveguidebackplanes is 13.5 mm for 50 Jlm x 50 Jlm, is 15.3 mmfor 75 Jlm x 50 Jlm and is 17.7 mm for 100 Jlm x 50 Jlmwaveguide cores as these provide a balance of transitionand radiation loss versus propagation loss.
Fig. 3. Output power with related to lateral and axialmisalignment between VCSEL and waveguide. (a) Modelling result.
(b) Experimental result.
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those observed on the cladding (Fig. 7 c). However, theheight of these features compared to the width still onlygave an approximately 1:5 aspect ratio indicating thatthis may still not be the optimum approach. In addition,this technique also led to the generation of someunstable features (Fig. 7 d). Further work is underway toestablish if a combination of techniques utilising barrierstructures and areas of different wettability on thesurface will enable the constraint of the core materialsuch that the required shape and form can be achieved.
Fig. 8 Cross section of an optical waveguide usinglaser direct writing
+- Upper cladding
Fig. 7 Inkjet printing of polymer optical waveguidematerial diluted with solvent. (a) tracks formed on a
cladding substrate, and (b, c, d) deposits on ahydrophobic, modified glass surface. Figs a, b, cdeposited with the same droplet frequency and
separation. (a) and (b) 8 pI per drop, (c) 32 pI per drop.(d) 32 pI per drop deposited at 4 times higher frequency
than (c).
Heriot-Watt University ResearchHeriot-Watt University has previously developed a
direct UV-Iaser-writing technique and custom photopolymer so as to form multimode polymer waveguidesand embedded 45° out-of-plane mirrors. In the OPCBproject, the key aim is to explore how these techniquescan be extended to suit optical backplane applications both in the context of scale and manufacturability. Thefigure shows an optical microscope image showing anend-on view of a back-illuminated clad laser-writtenpolymer multimode waveguide core on an FR4substrate. The core was written at 100 mmls i.e. aneffective writing speed of 50 mm/s.
Fig.6 Cross-section through an optical waveguide onFR4 prepared using Nd:YAG laser ablation.
Fig 6 shows a cross-section of a trial waveguidestructure on FR4: in this case the ablation rate was toohigh and even some of the FR4 substrate was removed.Waveguide fabrication is also underway using theexcimer laser which uses a mask projection technique toshape the beam spot such that complex profiles can beachieved. A key aim of this work, is to use the highermachining precision available, to form two and threedimensional structures such as curved mirrors for inplane and out-of-plane interconnection.Inkjet printing ofpolymer waveguide materials
Inkjet deposition offers the potential to selectivelydeposit waveguide materials where they are required,reducing the amount of material used and enablingsubstrates with a variety of geometries to be processed.However, there are numerous challenges to beovercome before core waveguide structures can beprinted directly onto cladding layers. The formulation ofthe "ink" such that it has the required viscosity forsuccessful printing needs consideration and in this workthe optical polymer has been partially diluted with twocandidate solvents in order to meet this requirement.However, it is the control of the spread of the dropletsonce they have impinged on the surface that ispotentially the most significant issue if structures withaspect ratios approaching the typical 1: 1 height : widthof optical waveguides are to be achieved. Initial trialshave shown that directly printing core liquid onto thecladding surface leads to broad tracks with low height,unsuitable for waveguide applications (Fig. 7 a).Control of the contact angle between core and claddingby variation of the cladding surface energy is oneapproach that may enable narrower features to beprinted. In order to investigate the efficacy of thisapproach, the core-solvent mixture was jetted onto aglass substrate that had been modified to make ithydrophobic (low surface energy). The results indicatedthat much less spreading of the core-solvent mixturetook place on the surface. Low volumes of fluid createdclearly separated drops on the surface (Fig. 7 b) whichwith increased volume and I or reduced separationmerged into tracks that were considerably narrower than
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ConclusionsThe paper reviews the IeMRC Flagship Project
known as OPCB, its aims and consortium structure anddetails some ofthe research progress.
AcknowledgmentsThe authors are grateful to EPSRC and the partner
companies for funding via the IeMRC Flagship ProjectOPCB.
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