Integrated Electrical –

Optical Substrate Manufacture

-

The OPCB ProjectDr David A. Hutt

Wolfson

School of Mechanical and Manufacturing Engineering, Loughborough University,

Loughborough, UK

2

Acknowledgements – Authors / Contributors to the Presentation

University College London (UCL):David R. Selviah, Kai Wang, Ioannis Papakonstantinou, Michael Yau, Guoyu Yu, F. Anibal Fernández

Heriot-Watt University (HWU):Andy Walker, Aongus McCarthy, Himanshu Suyal, Mohammad Taghizadeh

Loughborough University (LU):David Hutt, Paul Conway, Shefiu Zakariyah, John Chappell, Tze Yang Hin

Xyratex:Dave Milward, Richard Pitwon

3

Outline

IntroductionOPCB ProjectOptical Waveguide ConstructionMaterial RequirementsFabrication techniquesCharacterisationConclusion

4

Copper reaching the limits for high speed signals:Copper reaching the limits for high speed signals:

Crosstalk

Reflections

Signal dissipation

“Skin effect”

‘Electro Magnetic Compatibility’ IssuesSignal Frequency

Equalisation & Pre Emphasis Circuits

Low Skew ConnectorTechnology

Via Stub Control Processes

Number of layers per board

New Dielectric MaterialsC

osts

COST

IMPLICATIONS

OF

HIGH

SPEED

COPPER

COMMUNICATION

4

FP7 Proposal

| Richard Pitwon

Xyratex

5

THE

LIGHT

ALTERNATIVE

Core

Cladding

Optical Waveguide

0.25 mm

Optical interconnects as an alternative

Fit more optical channels on the board

No interfering radiation leaking outside the box

Send multiple signals simultaneously (WDM)

0.25 mm

Many optical 

channels

Cladding

1 electronic 

channel

5

FS Overview

| Richard Pitwon

Xyratex

6

Potential Environmental Benefits

Reduced materials usageHigher bandwidth of optical layers requires fewer layers than with copper – simplified routingFewer manufacturing stagesReduced board areaLess waste at end of life

Power reductionLower power optical drivers for high speed signalsReduced system cooling

7

The OPCB Project

Integrated Optical and electrical interconnected PCB (OPCB) for 19 inch backplanes and daughter cardsHigh bit rate (10 Gb/s), error-free, reliable, dense connectionsCAD design tools, Fabrication Techniques, Optical-Electrical connectors

Optical and ElectronicInterconnects

Backplane

Mezzanine Board (DaughterBoard, Line Card)

Optical Connector

IeMRC, 3 year, Flagship Project3 universities, 8companies Integration of optical waveguides with electrical printed circuit boards

8

Integration of Optics and Electronics

BackplanesButt connection of “plug-in”daughter cardsIn-plane interconnection

Focus of OPCB project

Out of plane connection

45o mirrorsChip to chip connection possible

Multilayer organic substrate

Core

CladdingVC

SEL

Dau

ghte

r car

d

Det

ecto

r

Dau

ghte

r car

d

DetectorVCSEL

Core

Cladding

Multilayer organic substrate

9

Multimode Waveguide Requirements

For total internal reflection, cladding refractive index lower than core Δn ~ 1%

FR4

Lower cladding

Upper cladding

Core 50-75μm20-75μm

20-75μm

Low optical losses at 850 nm, 1310 nm and 1550 nm wavelengths

Absorption Wall roughness

Good adhesion to substrateAble to withstand manufacturing processes e.g. solder reflow, laminationLong term reliability Easily processed by PCB manufacturers

10

OPCB Project Approach

Project focussed on polymer waveguide materialsRange of manufacturing techniques investigated

Photolithography Laser direct writing Laser ablationInk jet printingEmbossing

Characterisation of techniques for loss, manufacturability etcBest materials and processes identifiedTransfer of technology to industry

Design rules developed Transfer into commercial design software

OPCBs manufactured for end users

11

PHOTOLITHOGRAPHIC

WAVEGUIDE

PATTERNING

Substrate

Core layer

Upper cladding

Lower cladding

1.1.

Deposit lower refractive index polymer (cladding) onto substrateDeposit lower refractive index polymer (cladding) onto substrate

surfacesurface

2.2.

Cure polymer layer with exposure to ultraCure polymer layer with exposure to ultra--violet light to hardenviolet light to harden

3.3.

Deposit higher refractive index (core) polymer onto lower claddiDeposit higher refractive index (core) polymer onto lower cladding layerng layer

4.4.

Align photolithographic mask into positionAlign photolithographic mask into position

5.5.

UV exposure through mask to create hard patterns in the core layUV exposure through mask to create hard patterns in the core layerer

6.6.

Remove uncured portions of the core layerRemove uncured portions of the core layer

7.7.

Deposit lower refractive index polymer onto patterned core layerDeposit lower refractive index polymer onto patterned core layer

8.8.

Cure upper cladding layer with UV lightCure upper cladding layer with UV light

9.9.

Laminate over upper cladding layerLaminate over upper cladding layer

Waveguides

11

FS Overview

| Richard Pitwon

Xyratex

12

•

Direct Laser-writing of waveguides•

Increase writing speeds and manufacturability

•

Writing over large areas (400-500mm long)•

Photo-polymer Formulation•

Optimise for faster writing; alternative polymer systems; possible dry formulation

•

Options:•

Custom multifunctional acrylate

polymer

•

Exxelis

formulations•

Polymer Properties•

Tunable

refractive index & viscosity

•

High glass transition temperature

HWU Contribution to OPCB ProjectAndy Walker, Aongus

McCarthy, Himanshu

Suyal

13

Direct Laser-writing Set-up

•

UV-illuminated square aperture (50 μm)

imaged, 1-to-1, onto polymer- coated substrate, carried on computer-controlled x-y stage.

•

Three beams available –

to write:

(a) vertically-walled features, or (b) plus/minus 45-deg structures.

14

Laser written polymer structures

SEM images of polymer structures written using imaged 50 µm square aperture (chrome on glass)

15

Out-of-plane coupling, using 45-deg mirror (silver)

Microscope image looking down on mirror coupling

light towards camera

OPTICAL INPUT

Waveguide terminated with 45o

mirror

16

Laser Ablation for Waveguide Fabrication

Ablation to leave waveguidesExcimer laser – LoughboroughNd:YAG – Stevenage Circuits

FR4 PCBCladding

Core

FR4 PCBDeposit cladding and

core layers on substrateLaser ablate polymer

FR4 PCBDeposit cladding layer

UV LASER

SIDE VIEW

17

Nd:YAG

Ablation

FRFRFR---4 layer4 layer4 layer

Lower cladLower cladLower clad

corecorecore

upper cladupper cladupper clad

0 0.05 0.1 0.15 0.2 0.25 0.30

100

200

300

400

500

600

Power (Watt)

Dep

th o

f abl

atio

n (μ

m)

Constant variables : Velocity (10 mm/s);Frequency (10 kHz); Repetition ( 6 times)

Nd:YAG laser based at Stevenage CircuitsGrooves machined in polymerAblation depth characterised for machining parameters

18

Excimer

Laser Ablation

Straight waveguide structures machined in polymerFuture work to investigate preparation of curved mirrors for out of plane interconnection

corecorecoreLower cladLower cladLower clad

FRFRFR---4 layer4 layer4 layer260μm70μm

35μm

Plan View

Cross-section

19

Ink Jet Deposition of Polymer WaveguidesLocalised deposition of cladding and / or core materials

More materials efficientActive response to local features

Printing UV cure materialDeposit liquid, then cure

INK

Print head

Ink deposits

Substrate positioning- CAD data

High Speed Camera Images

20

Ink Jet Printing Challenges

Ink formulationViscosity, surface tension

Waveform developmentDrying effects

Coffee stain

PMMA on glass. Deposited by pipette

from solution.

21

Line Stability

Ink / substrate interactions affect droplet spreadWaveform for jetting still to be optimised. Initial observations:

Increasing volume of fluid leads to greater line stabilitySolvent selection aids line stability

Increasing volume of fluid deposited

1mm

Same dropletsize, different solvent

22

Control of Surface WettingNeed to control contact angle of polymer droplet on surface

Wetting angle is an important factor in determining droplet cross-section / printing resolutionControl of surface chemistry (balance of wetting and adhesion)

Increased contact angle leads to unstable features

Droplets on wettable

and non-wettable

surfaces

1mm

Modified glass substrate enables 75μm wide features, 15μm high to be printed

23Copyright © 2007 UCLCopyright ©

2007 UCL

Waveguide Characterisation

at UCL David R. Selviah, Kai Wang, Ioannis

Papakonstantinou, F. Anibal

Fernández

•

Waveguide Key Component Layout Design •

Optical Printed Circuit Board

(OPCB) Design

•

Waveguide Measurement –

Loss, Bit Error Rate, Eye Diagram, Misalignment Tolerance, Wall Roughness

•

Reliability Assessment–

Humidity, temperature cycling, vibration, aging characteristics

•

Modelling and Experimental comparison→ Design rules to be fed into software tools

24

Waveguide Structures•

Straight waveguides 480 mm x 70 µm x 70 µm

•

Bends with a range of radii•

Crossings

•

Splitters•

Spiral waveguides

•

Tapered waveguides•

Bent tapered waveguides

•

In plane mirrors

25

Waveguide Loss Measurements

Output image of 50 μm ×

50 μm VCSEL illuminated waveguide

•

Photolithographicaly fabricated by Exxelis

•

Cut with a dicing saw,

unpolished

850 nm VCSEL

Integrating sphere photodetector

150 μm pinhole

nW Power Meter

50/125 μm step index fibre

mode scrambler

-15 dBm

R

Index matching fluid

Input signal applied to end of waveguide and output measured

26

Transition Loss

w

lin

lout

Rs

Rs +ΔR

Rf = Rs + NΔR

A

B

I

Output

Input

O

Schematic diagram of one set of curved waveguides.

Light through a bent waveguide of R

= 5.5 mm –

34.5 mm

• Radius R, varied between 5 mm < R < 35 mm, ΔR

= 1 mm•

Light lost due to scattering, transition loss, bend loss, and reflection

and back-scattering • Illuminated by a MM fiber

with a red-laser.

27

Loss of Waveguide Bends as a Function of Bend Radius

Width (μm) Minimum Radius (mm) Minimum Loss (dB)50 13.5 0.7475 15.3 0.91100 17.7 1.18

28

Crosstalk in Chirped Waveguide Array

•

Light launched from VCSEL imaged via a GRIN lens into 50 µm x 150 µm waveguide

•

Photolithographically

fabricated chirped waveguide array•

Photomosaic

with increased camera gain towards left

100 µm 110 µm 120 µm 130 µm 140 µm 150 µm

29

Conclusion

OPCB project investigating the fabrication of polymer waveguides for electrical-optical PCB backplanesSeveral materials and fabrication techniques under investigationWaveguides characterised for loss, wall roughness and long term reliability

30

Acknowledgements

EPSRC for funding through the Innovative Electronics Manufacturing Research Centre (IeMRC)Industrial collaborators:

XyratexExxelisBAE SystemsStevenage Circuits LtdCadenceDow CorningRenishawNPL