Electrical-Optical Circuit Board Using Polysiloxane Optical Waveguide Layer

A. Neyer, S. Kopetz, E. Rabe, W.J. Kang, S. Tombrink

Universität Dortmund, Fakultät für Elektrotechnik und Informationstechnik,

Arbeitsgebiet Mikrostrukturtechnik (AG MST)

44221 Dortmund, Tel.: ++49 231 755 3728, Fax.: ++49 231 755 3729,

e-mail: andreas.neyer@uni-dortmund.de

Abstract

This paper reports on a new technology for the realization

of an optical waveguide layer in electrical-optical circuit

boards. The technology is based on casting of transparent

polysiloxanes as low cost, low loss (0,05 dB/cm at 850 nm)

and high temperature stable (> 250°C) material system. The

waveguide layer fabrication will be discussed as well as the

preparation of suitable casting moulds. Further issues are the

material and waveguide properties of optical polysiloxanes,

the coupling to OE-modules, and the lamination of optical

layers into printed circuit boards.

Introduction

Next-generation internet switches and high-end computers

are expected to process aggregate data rates in the order of

Tbit/s. In consequence, the interconnections between the

processing units will have to handle data rates in the order of

10-40 Gbit/s. It is, however, well known from basic physical

laws that electrical interconnections will suffer from high

transmission losses and severe signal integrity problems at

such data rates /1/. In order to overcome the evident high-

speed interconnection bottle-neck, optical interconnects are

considered the preferred option. In Gbit/s-rack-to-rack

interconnections with link lengths in the order of several

meters, the widespread solution is the commercial fibre-ribbon

cable in combination with high speed parallel OE-modules. If,

however, interconnection lengths come down to the order of

1m, e.g. in backplanes, integrated optical waveguides are

considered more economical /2/.

The integration of optical waveguides in printed circuit

boards as well as in backplanes imposes severe requirements

on the materials and processes involved. Some of them are:

High transparency of the waveguide materials (< 0,1dB/cm) in

the standardized interconnect wavelength window of 850 nm,

high temperature stability to overcome standard multilayer

printed circuit board lamination process temperatures at

180°C for two hours and especially the soldering process

temperatures of 230°C, large area processing capability (>

0,5m x 0,5m), and cost effective mass production.

Among the waveguide technologies studied worldwide for

the production of electrical-optical PCBs, photolithography is

the most popular to define the multimode waveguide core

structure. Both, direct laser writing /3/ and mask exposure

techniques /4/ are being applied. A considerable variety of

temperature stable polymers have been developed for this

technology: modified acrylates /4,5/, polysiloxanes /6,7,8/,

and epoxies / 9,10,11/.

Waveguide data obtained with theses techniques and materials

are summarized in Table I.

Company Material Thermal

stability

°C

Optical loss

at 850nm

dB/cm

Luvantix /9/ Epoxy >250 0,04

KIST /10/ Epoxy 220 0,36

NTT /11/ Epoxy >200 0,1

Zen

Photonics /5/

Acrylate >250 0,05

IBM /4/ Acrylate >250 0,04

Daimler

Chrysler /3/

Unknown >250 0,04

RPO /6/ Siloxane >250 0,1

Dow

Corning/7/

Siloxane >200 0,06

Shipley /8/ Siloxane >250 <0,1

Table I: Performance data of multimode waveguides

fabricated by photolithographic methods

Although the photopolymer waveguides reported in Table

I show excellent performance their implementation in large

area boards is critical because of the high material costs.

Furthermore, hot embossing has been investigated as a

suitable technology for multimode polymer waveguide

fabrication /12,13/. However, problems may arise from

insufficient high temperature stability of optical thermoplastic

polymers (for T>200°C) as well as from difficulties with the

required high precision at large areas.

In this paper we present a new waveguide technology

based on casting of thermally curing polysiloxanes which

comprises all essential features for a low cost mass production

of large area electrical-optical circuit boards.

Materials

High transparent polysiloxanes are widely used in

electronics industries, e.g. to encapsulate LEDs. In addition to

the low optical loss, the advantages of polysiloxane for

integrated optical waveguide fabrication in printed circuit

boards are the high thermal stability, the extreme moulding

precision, the large area process ability and especially the low

cost.

Fig. 1: High replication precision in PDMS:

Moulded Y-splitter

The transmission loss of optical grade polysiloxane bulk

samples of different suppliers has been measured to 0,02 -

0,04 dB/cm in the 850 nm wavelength window.

0

0,1

0,2

0,3

0,4

0,5

0,6

300 800 1300 1800

Wavelength [nm]

Pro

pag

ati

on

lo

ss [

dB

/cm

]

0,03dB/cm@850nm

Fig. 2: Spectral transmission of bulk PDMS (Wacker)

In this paper, two-component room temperature cross

linking polysiloxanes of Wacker Chemie GmbH, Burghausen,

Germany, have been used. The cladding materials are standard

commercial polymers, whereas the core polymer is a special

development of Wacker in close cooperation with the

University of Dortmund. The transmission spectra of core and

cladding materials are almost identical (Fig. 2)

The thermal stability of the polysiloxane materials has

been studied by comparing the transmission spectra of bulk

samples after curing them at room temperature, after a

subsequent exposure at 180°C for two hours and after a final

exposure at 230°C for five minutes (Fig. 3). This temperature

treatment simulates the real process conditions at multilayer

board lamination and at reflow soldering. Except in the

ultraviolet region (200 nm to 400 nm) there is no significant

change in the optical transmission loss. A slight increase in

transmission loss is observed only at temperatures above

270°C which makes the material very suitable even for lead

free soldering processes.

0

1

2

3

4

5

6

7

8

9

10

200 700 1200 1700

Wavelenght [nm]

Insert

ion loss [d

B/c

m]

20°C

180°C

250°C

Fig.3: Thermal stability of PDMS

Fabrication of waveguide layer and board integration

Reactive ion etching and UV-curing have been reported

for waveguide fabrication in polysiloxane /14,15/. We have

adopted casting in combination with the doctor blade

technique as a new low cost polysiloxane waveguide

fabrication method. One of the advantages of this technology

is that it has the unique feature of simultaneous fabrication of

optical waveguides together with integrated micro mirrors for

efficient OE-module coupling. Furthermore, casting in

combination with the doctor blade technique is well

compatible with large area printed circuit board production

technologies.

First a casting mould for the waveguide core layer is

generated. This is accomplished by SU-8-photolithography

and subsequent electroplating. In the reported experiments

standard 6”-photoresist technology has been applied, but

currently extension to larger formats (300mm x 400mm) is

under work. In case of large area formats, doctor blading is

used instead of spin coating because of the better thickness

uniformity of the resist on rectangular substrates. The resist is

dried and exposed through a photolithographic mask. After

development of the resist the master mould is finished (Fig. 4).

In order to obtain a mechanical stable mould for mass

production an electroplated copy of the resist mould is

realized.

Fig. 4: SU-8 master form with straight waveguides

The complete production process of the optical layer is

shown in Fig. 5.

core polymer

mould

mould

Substrate carrier

substratepolymer

substrate

substrate carrier

waveguide cores

mould

waveguide cores

Substrate

pressing,

curing

Application of core polymer

Doctor blading,

curing

Demoulding

Superstrate

pressing,

curing

superstrate

polymer

substrate

waveguide coressuperstrate carrier

core polymer

mouldmould

mould

Substrate carrier

substratepolymer

substrate

substrate carrier

waveguide cores

mould

waveguide cores

mould

waveguide cores

Substrate

pressing,

curing

Application of core polymer

Doctor blading,

curing

Demoulding

Superstrate

pressing,

curing

superstrate

polymer

substrate

waveguide coressuperstrate carrier

Fig. 5: Scheme of production process of optical layer

First, the waveguide cores are fabricated by filling the

grooves in the mould by the core polymer (n = 1.43). This is

accomplished by the doctor blading technique which is easily

applicable to large formats. Then the core material is

thermally cured.

The next step is the preparation of the substrate carrier.

One function of this carrier is the mechanical stabilisation of

the thin optical layer during the subsequent production steps.

At the same time, the carrier serves as interface to adjacent

PCB layers in case of a multi layer board. It is obvious to use

conventional circuit board laminates like FR4 or Kapton. It is

advantageous to use copper clad material, since the copper can

be structured by standard processes and these structures are

well suited to define the thickness of the substrate polymer.

Another function of the copper structures is the prevention of

pressure from the waveguide layer during multilayer PCB

lamination.

The waveguide substrate layer is fabricated by applying

the liquid cladding polymer (n=1.41) to the mould (which still

contains the cured cores) and, subsequently, the substrate

carrier is pressed against the mould. Now, the copper

structures will define exactly the thickness of the waveguide

substrate layer. After curing the complete layer comprising

cores, substrate layer and carrier is demoulded. Excellent

adhesion between the substrate carrier and the PDMS

substrate layer could be achieved by using special adhesion

promoters.

The production of the superstrate layer is performed by the

same technique using identical cladding polymer (n = 1.41)

and a superstrate carrier with copper structures to define the

thickness of the superstrate layer

Fig. 6 shows the cross section of a realized electrical-

optical PCB using FR4 substrate carriers. The waveguides

have core sizes of 70µm x 70µm and a numerical aperture of

0.26.

Fig. 6 Cross section of PDMS optical layer

embedded between FR4 laminates

An alternative carrier material in particular with regard to

flexible board applications or very high temperature stability

is the polyimide film Kapton® (Dupont). In Fig. 7 the cross

section of such a laminate with Kapton® is shown.

Fig. 7: Cross section of PDMS optical layer

embedded between Kapton laminates

Kapton

Kapton

Superstrate

Substrate

Waveguide core

FR4

Substrate

Superstrate

Waveguide core

Transmission loss and thermal stability

The waveguide loss has been measured by exciting the

waveguides using a 50µm-GI-fibre and collecting the

transmitted light by a 200µm-SI-fibre. Typical waveguide loss

figures at 850 nm are 0,05 dB/cm measured by the cut back

method.(see Fig. 8) The thermal stability has been tested

against the PCB-lamination temperature at 180°C for 2 hours

followed by an exposure at 230°C for 5 minutes to simulate

reflow soldering conditions. Fig. 8 shows the results of these

thermal stability tests.

0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0,08

1 2 3 4 5 6 7 8 9 10 11 12Waveguide number

Pro

pag

ati

on

lo

ss [

dB

/cm

]

Before annealing

180°C 2h

220°C 5 min

Fig. 8: Thermal stability of PDMS waveguide layer

If embedded between two Kapton carriers the optical

waveguide could be annealed up to 260°C without observable

increase of the optical loss. There is even a tendency of

improvement of the optical transmission after the exposure to

higher temperatures.

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

1 2 3 4 5 6 7 8 9 10

Waveguide number

Pro

pag

ati

on

lo

ss [

dB

/cm

] Before annealing

180°C 2h

220°C 5m

260°C 5m

Fig. 9: PMDS waveguides embedded between two Kapton

carriers (see Fig. 7)

Coupling to OE-modules

Besides the integration of an optical waveguide layer in a

PCB, the optical coupling in and out of the board is of crucial

importance for the success of the concept of optical

interconnections in PCBs. In this paper a micro mirror

technology is proposed /16/ which is well compatible to the

described replication process. The basic idea is to realize the

mirror structures already at the masterform level and apply the

waveguide layer fabrication as outlined.

The micro mirrors are integrated into the masterform at

the SU-8 resist level by micro milling the end faces of the

cured SU-8 ridges under an angle of 45° (Fig. 10). Cured SU-

8 is a very suitable material to be treated by micro milling. To

obtain optimum results with respect to surface roughness of

the mirrors, specially designed diamond milling tools have

been developed. The obtained roughness on the mirror surface

has been measured by white-light interferometry to Ra=70-

80nm which is an acceptable value.

Main drawback of the integrated micro mirror technology

is the need to metallise each replicated waveguide substrate

locally at the mirror positions. Furthermore, a transmission

window must be opened in the superstrate carrier and the top

layer of the board (see Fig. 11) .

Fig. 10: SEM-photo of milled micro mirrors

Fig. 11: Waveguides with integrated micromirrors

and MT–pins for passive OE-module coupling

High speed data transmission

In order to test the high-speed transmission capability of

the fabricated boards, a 850nm emitting VCSEL and a pin-

photodiode have been butt-coupled to the waveguide layer of

a 12 cm long multi layer board at the University of Ulm

(Department of Prof. Ebeling) (see Fig. 12). Digital data at a

rate of 10 Gbit/s have been transmitted with bit error rates less

than 10-12

(Fig. 13). The transmission speed was limited by the

photodiode /17/.

Fig. 12: Electrical-optical circuit board with butt-coupled

VCSEL and pin-photodiode

Fig. 13: Eye diagram of 10 Gbit/s data transmission

Extension to large area formats

Typical format size in industrial PCB manufacturing is

18”x24”. This implies that in electrical-optical PCB

fabrication these formats must be feasible, too. With regard to

the PDMS-waveguide technology there will be a straight-

forward extension to such formats by industrial technologies

implying doctor blading. The development of a commercial

production line for electrical-optical PCBs on the basis of

polysiloxane waveguide layers is the aim of the national

German research project called “ProSPeoS” /18/.

Conclusion

Polysiloxane waveguide technology has been

demonstrated to have low optical transmission loss (0,05

dB/cm at 850 nm), high thermal stability (> 250°C) and large

area process ability using casting and doctor blade techniques.

Being highly compatible with printed circuit board

technologies the casting of polysiloxane fulfils all

requirements for a successful development of large size

electrical optical circuit board production.

Acknowledgement

The close and fruitful cooperation with Wacker Chemie,

Burghausen, in the field of materials development is gratefully

acknowledged. Part of this work has been supported by the

BMBF under the framework of “Research for the production

of tomorrow”, project OPTICON (No 02 PP 2033).

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