..glassPack.. - Photonic Packaging using thin glass foils for Electrical-Optical Circuit Boards(EOCB) and sensor modules

Henning Schroder, Norbert Amdt-Staufenbiel, Lars BrusbergFraunhofer Institute for Reliability and Microintegration IZM

Gustav-Meyer-Allee 25, 13355 Berlin, Gennanyhenning.schroeder@izm.fraunhofer.de

Figure 1: Schematic diagram of the demonstrator systemwith 2 modules per daughter card (only one is shown inthe drawing), double layer optical waveguides andcoupling scheme

Another promISIng field of application ismultifunctional integration for sensors. The thin glasssubstrates available in different fonnats have excellentoptical, electric, thennal, mechanical, and chemical

costs the trend is being set by integrated polymerwaveguide layers, manufactured using planar structuringprocesses, which are embedded into the circuit boardassembly, so called EOCB [6]. The waveguides arealways multimodal step index waveguides with corediameters in the range of 30...70 Jlm. The length of thewaveguides and the obtainable optical attenuation areprimarily detennined by the properties of the variousstructuring technologies themselves [7]. Due to theirmaterial properties, the optical attenuation of polymerwaveguides is higher than for glass, thus, thepower/perfonnance balance for, above all, the opticalconnections in the backplane area is critical. Thetransition to glass based waveguides foils can be regardedas a promising step in tenns of reliability and low opticalattenuation. Furthennore the "glassPack" technology canbe used to realize waveguide based coupling elementslike shown in Figure 11 for vertical out of plane couplingto the e/o modules and between daughtercard and opticalbackplane, respectively. This is exemplary shown in thechallenging architecture presented in Figure 1 providingtwo pluggable modules per daughter card, each modulesupplies 2 x 12 waveguide channels at 10 Gb/s in twovertical stacked waveguide layers [8]. For the 90° lightdeflection special glassPack optical interfaces areproposed with two optical waveguide layers by the sameprocess as for the horizontal waveguide layers itself.These coupling elements are under development currentlyand first results are shown in Figures 12 and 13.

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AbstractThe "glassPack"-concept will be introduced as a new

packaging technologies platfonn for a wide area of opto­electronic applications like optical backplane, electrical­optical circuit boards (EOCB) and sensors. The usage ofthin glass foils of some tens of microns thickness assubstrate and interconnection material is the crucial pointof the concept. First realizations will be presented.

IntroductionPhotonic packaging in such hybrid opto-electronic

systems involves single packages, modules, andsubsystems comprising of at least one optoelectronicdevice, micro-optical element or optical interconnection.The commonly used substrate materials are ceramicplatfonns, silicon, HTCC, LTCC or polymeric substrates.In contrast the main advantage of thin glass is theirtransparency. Thin glass is a commercial available andreliable material for display protection with high thennalresistance and excellent optical properties. Furthennoreglass is a well known material and many technologies likepolishing, plating, etching and refractive index tuning arealready available and can be adopted.

The main ideas of the "glassPack" concept are:selection of suitable glass foils as substrate material,realization of micro system compatible structuringtechnologies like cutting, drilling and milling, integrationof optical waveguides by ion exchange for single andmulti-mode applications, optical interconnects betweenfibres and integrated waveguides by laser fusion,integration of electrical wires and vias, assembly ofelectronical and opto-electronical components, andbonding of the thin glass foils to 3D-stacks andlamination to PCB base materials. Furthennore, theintegration of micro fluidic channels into a "glassPack"will be supported. The technical focus of that paper lieson the waveguide process itself and newly developedinterconnection technologies for the opticallyfunctionalized glass foils. They can be laminated inbetween of polymeric base materials like FR4 for opticalbackplane or EOCB, and complete glass based packageson wafer level can be realized, respectively.

Application fieldsOptical backplanes and PCB with opto-electronic

components using glass fibers are the standard in today'shigh-end area. That led to approaches using theintegration of glass fibers in the circuit board with passiveelements and (MT) connectors. Because of high assembly

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400 500 600 700 800 900 1000 1100 1200 1300

wavelength in nm

Figure 3: Extrapolated refractive index as a function ofwavelength [2]

Ion exchange for optical waveguidesThe required technique for realizing optical

waveguides and lenses if required have to be integrated inthe process cycle of photonic modules and printed circuitsboards. That means it must be produced economically andwith low environmental impact. Ion exchange techniquein molten salts was chosen to reach this aim. At the glassinterface monovalent alkali ions, mostly sodium, becomesvery mobil under influence of hot salt melt. Alkali ions ofthe glass are exchanged by diffusion with alkali ions ofthe salt melt (B+ in Figure 4). If these ions have otherionic radii and polarizability a variation of refractiveindex occurs. Therefore dopants could be Li, K, Cs, Rb,Ag, TI and Cu [3]. But the toxic TI is to handle with care.

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The choice of appropriate glass depends on theapplication. Optical waveguides and fluidic channels willbe implemented inside the thin glass substrates. Electricalfeed troughs' and wires on the surface form electricalcircuits. Integrated waveguides, fluidic channels, orelectrical wires implemented in various thin glasssubstrates can be stacked at wafer level resulting in 3D­packages (Figure 2). As an example a refractometricsensor has been realized (Figure 15) [4].

Thin glass propertiesUsing the display glass D 263 T supplied by Schott

Desag AG the demands can be fulfilled. The glassD 263 T is commercially available in a thickness rangingfrom 0,03 mm up to 1,10 mm with the maximumdimensions of 440 mm x 360 mm [1]. The display glassused as testsamples has a thickness of 100 to 300 Jlm anda size of 4 inch. It is a borosilicate glass which isproduced by melting very pure raw materials (Table 1).As such, it is very resistant to chemical attack. Furtherfeatures are: easy cutting, high luminous transmittancebetween 380....2400 nm and excellent flatness.

Figure 2: glassPack technologies processed on waferlevel

properties, which make it suitable for harsh andhazardous environments or applications in the field ofoptical sensing. One of the most important glassPackfeatures is the suitability for wafer level packaging.

Table 1: chemical composition ofD 263 T(compounds in wI. %)

Si02 B20 3 Ab0 3 Na20 K 20 ZnO Ti02 Sb20 3

64,1 8,4 4,2 6,4 6,9 5,9 4,0 0,1

The refractive index as a function of wavelength isshown in Figure 3 according data given by Schott DesagAG [1]. The fit function was calculated to describe thedispersion relation for wavelength A> 800 nm. Thesewavelength range is important for intra system opticaldata and telecom applications.

Figure 4: Principle of ion exchange in molten salts [3]

Due to the availability of the cat ions sodium andpotassium in D 263 T (Table 1) it is possible to producechannel waveguides using this technique. The exchangeefficiency is affected by temperature, concentration ofdopants in the salt and the glass and the exchange time.Another increase in productivity is possible if the thermalstatistic movement of ions can be enhanced by a directedforce like an electrical field E (Figure 4) or a thermalgradient. If required the optical waveguides can be madealso in a double side ion exchange process as depicted inFigure 5. For this process a electric field must not beapplied.

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The flow can be described as follows:

___- ....~-_.._ ......- Etch mask-t Aluminium mask

ii Thin glass foil

Alignment pins

Figure 6: Diffussion mask for waveguides and alignmentmarks by the ion exchange process

Aluminium mask openingThe same part of the thin glass foil after removal of

the diffusion mask is shown in Figure 7. The parallelwaveguide lines and alignment marks can be clearly seen.

Ion exchange from saltmelt

Aluminium mask removal

Figure 5: Thermal ion exchange for waveguidestructuring from both sides

1) Determination of diffusion process parameters inthe display glass for modifying the refractive index andwaveguide structuring

2) Design of waveguides for optimum coupling ofoptical sources and detectors without signal distortion(size, numerical aperture)

3) Processing of designed waveguides by ionexchange in hot salt melt. Adjusting of optical andmechanical properties after ion exchange is possible withmulti component salt melts. The maximum steady thermaldiffusion speed at 350°C is about 0.5 Jlm per hour. Batchprocessing is possible.

4) Optical and mechanical characterization: measuringdiffusion parameters with inverse WKB-method by modeline spectroscopy and RNF method. The maximumnumerical aperture without glass damage after silver ionexchange is 0.39. For the realized waveguides weprocessed a salt melt for NA = 0.22.

To establish this new "glassPack" technology it isnecessary that materials and processes are compatiblewith each other. This is demonstrated by using sputteringand photolithography to apply a metal diffusion mask forthe optical layer including the waveguide structures andthe alignment structures in the same process. Thewaveguide structures are made by diffused dopands forthe core material by thermal silver ion exchange. InFigure 6 a part of a patterned metal mask on thin glassfoil is shown. The metal mask has to withstand the veryaggressive hot salt melt. On the left side the alignmentmarks are visible. These marks are used to realise the lowloss optical interconnection to the optical couplingelements between horizontal waveguides and e/o- modulsin EOCB applications.

Figure 7: Ion exchanged wavguides after diffussion maskremoval with positioning marks (lx8 cm)

Layers with up to 12 parallel multimode opticalwaveguides of 250 Jlm pitch and single mode waveguidesfor Mach-Zehnder-strctures within thin glass foils havebeen realized. The waveguides are of high thermalreliability and show an attenuation of about 0,1 dB/cm.

Refractive index scan x-direction

8) 80 100 120 1«1

Scan area x-direction (pm]b)

Figure 8: Cross section of double sided planar multimodewaveguides, a) micrograph of optically structured displayglass with 2 x 12 waveguides, b) refractive indexdistribution of one waveguide measured by RNF-method

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In Figure Sa an optical micrograph of a cross sectionis shown. The waveguides are illuminated from thebackside. The bright corona is caused by the graded indexprofile due to the isotropic under diffusion of the mask.

Electrical wiring and component assemblyGlass, being a highly electric isolating material, is

very suitable as a substrate for high-frequencyapplications. Electrical wiring consists of wires, contactpads, and feed throughs. Suitable thin film technologiescan evolve from IC and PCB industry [9]. The remainingchallenge using glass as substrate material is drillingholes and depositing metal in the holes for electric andthermal feed throughs. Using a solid state laser [10],holes are drilled with minimum diameters of 200 Jlmthrough a D263T™ thin glass foil of 500 Jlm thickness asshown in Figure 9. Next, titanium-tungsten and gold isdeposited as plating base. Instead of gold the depositionof copper is possible too, depending on the followedplating process. The deposition of the plating base occursholohedral at the walls inside the holes. As a result,uniform metal deposition inside the hole is possibleduring the plating process. Glass as thermal insulator incombination with ICs or semiconductor optical devicesrequires thick metal wiring and thermal feed throughs forspreadin~ the heat of the devices. Figure 10 shows apolished micrograph section of a laser-drilled hole whichwas filled by gold metallization of 100 Jlm thickness.

Figure 9: Part of a laser-drilled 4"-wafer

Figure 10: Polished micrograph section of a feed throughin D263T™ thin glass foils having a thickness of 500 Jlm.The laser-drilled hole has a diameter of 200 Jlm

Mounting the devices near the feed through helps the heatspread through the glass foils to heat-sinks positionedunderneath. Instead of gold, electro-plating of copper invarious thicknesses from 10 to 100 Jlm in combinationwith a nickel barrier and finally deposited 100 nm flash

gold provides a suitable metallization for soldering andwire bonding.

Optical interconnection

Thin glass substrate with ionexchanged optical waveguide

1--------------- .------ -- ----- --_~iltedand_ :~---.-----.--------------.-------~ polished face

Fiber '.'.'.'., ,.'.'.'.. ' '.'.,.' ',.'.'.'.'.'.-.'.-.'.,.'.'. -

Direct Fusion on glass Ion exchanged lens

Figure 11: Schematic drawing of optical couplingelement using the glassPack technology with beamdeflection.

For the out of plane optical coupling special designedcoupling elements are realized. They consist of planarwaveguide arrays made by ion exchange and reflectivemirror surfaces for the light deflection. The waveguidescan be narrow for single or wide for multimodepropagation. In Figure 11 schematic cross section isshown. The coupling element itself can be realized assingle layer element or as a stacked sandwich to realizemore complex optical functionality and mechanicalproperties. So ion exchanged lenses in the bottom layercan be integrated to focus the out coming light to a smallPhotodiode or vertical grating structure, e.g. to couple tosilicon photonics waveguide chips.

Figure 12: 45 degree polished thin glass substrate withdouble layer ion exchanged optical waveguides. Dashedlines indicate both of the waveguides. A and B indicatethe position of the out of plane coupled beams.

Figure 13: (upper) Light coming outof the position A (Figure 4) and(lower) light coming out of theposition B

But even in single layer glass foilsmore optical functionality can beintegrated by means of a double sideion exchange process as depicted inFigure 12. Waveguides are wellaligned vertically and the 45 degreemirror is polished very precisely inorder to achieve a 90 degree

deflection element for double layer waveguide arrays. InFigure 13 the deflection and the out coupling is

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demonstrated. The light coupling between optical circuitsand a light source, detector or fiber is realized by polishedend faces of the glass foil.

One benefit of using glass foils is the possibility ofdirect fusion bonding to silica fibers. The fiber end face ispositioned in front of the polished end face of theintegrated waveguide. A CO2-Laser beam focused on thebonding zone melts both bond partners about theannealing point and fuses those together [5]. As shown inFigure 14, a reliable bond without intermediate layerresults.

Figure 14: Polished micrograph section of a fused bondbetween a silica fiber and a thin glass foil

Fluidic channelguiding

the analyte

Figure 15: Design of the refractometric sensor withintegrated MZI and fluidic channels, and optoelectroniccomponents. The sensor has a length of 80 mm and awidth of 10 mm

In the sensor demonstrator realized recently [4] thephoto diodes and the laser chips are directly butt coupledto the waveguide chip. This approach is quite commonand the coupling efficiency depends on the beamproperties of the laser as well as the waveguide profile

which can be adopted by controlling the diffusionparameters.

In Figure 15 the positions of the butt coupledcomponents are shown. Most critical is the activealignment of the Mach-Zehnder-waveguide plate to thevery little already assembled laser dies (upper inset).

Conclusion and OutlookWe have demonstrated our generic "glassPack"

technology using thin glass foils with respect to twoapplication excemples: multimode waveguide layers forEOCB and hybrid integrated sensor modules. The benefitof glass results in excellent optical, electric, chemical andmechanical properties. Suitable technologies like ion­exchange, laser drilling, electro-plating, and fusionbonding and direct optical butt coupling are used forEOCB waveguides and the refractometric sensor andshow high potential to realize integrated wafer levelpackages in glass. Current developments focus onwaveguide array coupling elements of very flexibledesigns. So such kind of coupling elements can beapplied for multimode and multilayer optical coupling ofelectrical-optical circuit boards (EOCB) as demonstratedwithin this paper and to interconnect sensors and siliconphotonic wires through high-index contrast verticalgratings to the micro optical periphery.

References[1] Schott Desag AG Display Glas, "Material

information" (Griinenplan, Germany, 1999)[2] Schroder, H., Arndt-Staufenbiel, N., Cygon, M.,

Scheel, W., "Planar Glass Wave Guides for HighPerformance Electri-cal-Optical-Circuit-Boards(EOCB) - The Glass-Layer-Concept -", Proc. 52thECTC 2003, May, 27th.-30th. 2003, New Orleans,USA

[3] Najafi, S. I., "Introduction to glass integratedoptics," Artech House (Boston, London, 1992)

[4] Brusberg, L., Schroder, H., Amdt-Staufenbiel, N.,Wiemer, M., Proc. Photonics Europe 2008,Strasbourg, France, 7-11 April 2008

[5] Amdt-Staufenbiel, N.; Lang, G.; Krissler, J.; et.al.: "Specific glass fiber technologies: lensing andlaser fusion," Proceedings of SPIE, Vol. 5445(2004).

[6] Schroder, H., Bauer, J., Ebling, F., Franke, M.,Beier, A., Demmer, P., Siillau, W., Kostelnik, J.,Modinger, R., Pfeiffer, K., Ostrzinski, U., Griese,E. Invited talk "Waveguide and packagingtechnology for optical backplanes and hybridelectrical-optical circuit boards", Proc. PhotonicsWest 2006, 21.-26.1.2006, San Jose, USA, SPIE,pp.6115-6136.

[7] S. Uhlig, L. Frohlich, M. Chen, N. Amdt­Staufenbiel, G. Lang, H. Schroder, R. Houbertz,M. Popall, M. Robertsson, "Polymer OpticalInterconnects - A Scalable Large-Area Panel

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Processing Approach", IEEE Transactions onAdvanced Packaging, Vol. 29, No.1, pp. 158- 170,Feb. 2006.

[8] Schroder, H., Arndt-Staufenbiel, N., Beier, A.,Ebling, F., Franke, M., Griese, E., Intemann, St.,Kostelnik, J., Kiihler, T., Modinger, R, Roda, I.,Ingolf Schlosser, I.; "Thin Glass Based Electrical­Optical Circuit Boards (EOCB) using Ion­Exchange Technology for Graded-IndexMultimode Waveguides"Proc. 58th ECTC 2008, Lake Buena Vista, Florida,USA, May 27-30,2008

[9] Elshabini-Riad, A.; Barlow, F. D.: "Thin filmtechnology: handbook". McGraw-Hill, New York,USA (1997).

[10] Du, K. and Shi, P., "Surface precision machiningof glass substrates by innovative lasers," GlassScience Technology, Vol. 76, No.2 (2003).

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