To satisfy the urgent need for low-cost multimode planar waveguides, we developed photodefined, multimode-fiber compatible waveguides with low-cost commercially available epoxies showing low losses from 550 to 1100 nm and around 1300nm

Dekker, R.

Diemeer, Mart

Driessen, A.

Hilderink, L.T.H.

English

To satisfy the urgent need for low-cost multimode planar waveghuides, we developed photodefined, multimode-fiber compatible waveguides with low-cost commercially available epoxies showing low losses from 550 to 1100 nm and around 1300 nm

University of Twente Research Information

1624 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 15, AUGUST 1, 2006Low-Cost and Low-Loss Multimode Waveguides ofPhotodefinable EpoxyMart Diemeer, Lucie Hilderink, Ronald Dekker, and Alfred Driessen, Member, IEEEAbstract—To satisfy the urgent need for low-cost multimodeplanar waveguides, we developed photodefined, multimode-fibercompatible waveguides with low-cost commercially availableepoxies showing low losses from 550 to 1100 nm and around1300 nm.Index Terms—Integrated optics, optical interconnections,optical polymers, waveguides.I. INTRODUCTIONI N cost-sensitive areas of short distance optical communica-tions, like datacom, CATV, optical interconnects in printedcircuit boards (PCBs), and in optical sensing, a urgent needhas emerged for low-cost multimode planar waveguide struc-tures to be applied in components like splitters and couplersor integrated in PCBs. These waveguides have to be compat-ible with the standard 50-, 62.5-, or 100- m core size multi-mode silica fibers combined with light-emitting diodes (LEDs)and low-cost vertical-cavity service-emitting lasers (VCSELs)with their emission wavelength at 850 nm. In addition, LEDsand upcoming VCSELs in the 600–1000-nm range and around1300 nm are important application wavelengths. Some conven-tional (C-H containing) low-cost polymeric materials can meetthese requirements because they can be deposited up to 100- mfilm thickness by simple spin or dipcoating techniques; they canbe formulated for direct photopattering; their refractive indexescan be tuned to yield waveguides with fiber-matched numericalapertures (NAs); and their transparency windows can coincidewith the VCSEL wavelengths.The commercially available photodefinable polymers,Truemode (Exxelis) [1] and Ormocer (Micro ResistTechnology) [2], are based on acrylate crosslinking and formwet films after deposition. Photopatterning can only be doneby using masks in the proximity mode, which degrades theresolution and which is not suited for large uneven substrateslike PCBs where conformable (foil) masks have to be used. Thealternative of direct writing is a slow serial process and, there-fore, not cost-effective. Moreover, the acrylate (free radical)crosslinking process requires inerting to avoid cure inhibitionby ambient oxygen, which adds to the production costs.Photocurable epoxies do not possess these drawbacks. Theirdifferent (cationic) cure mechanism allows the processing of dryManuscript received April 10, 2006; revised May 18, 2006.The authors are with the Integrated Optical Microsystems Group, Universityof Twente, 7500 AE Enschede, The Netherlands (e-mail: m.b.j.diemeer@ewi.utwente.nl; l.t.h.hilderink@ewi.utwente.nl; r.dekker@ewi.utwente.nl;a.driessen@ewi.utwente.nl).Digital Object Identifier 10.1109/LPT.2006.879519films with contact masks under normal ambient. In addition,epoxies are well known for their excellent adhesion, durability,dimensional stability, and chemical inertness. The negative pho-toresist SU-8 (MicroChem Corp.) is a well-known epoxy pho-topolymer which was also applied in planar optical waveguideapplications [3]. However, a main drawback is its limited trans-parency at UV/visible wavelengths due to processing inducedabsorption (yellowing) from photoinitiator decomposition prod-ucts and polymer impurities [3], [4]. In this letter, we presentphotodefined, multimode fiber compatible waveguides, basedon commercially available low-cost epoxy materials, showinglow losses from 550 up to 1100 nm and around 1300 nm.II. PHOTODEFINABLE EPOXY FORMULATIONWe have formulated a photodefinable polymer systembased on the most widespread used (in adhesives, coatings,encapsulants) and low-cost epoxy prepolymer, i.e., the digly-cidylether of bisphenol A (DGEBA) [5]. This is available withlow impurity content and different molecular weights .The higher molecular weight versions, starting from aboutg/mol are solid materials. The DGEBA molecularchains contain aromatic rings and have (two) epoxy endgroupswhich can be opened and crosslinked to yield an insolublepolymer network. The stiffness of the rings provide a highglass-transition temperature, while their aromatic characterresults in a high refractive index. Because a low providesa high crosslink density and, consequently, a good photodefini-tion, the solid DGEBA with the lowest (1000 g/mol) (e.g.,Epon 1001F of Resolution or DER 661 of Dow) is used in theformulation.The DGEBA is combined with the fluid epoxy monomer: 3,4-epoxycyclohexylmethyl–3, 4-epoxycyclohexane carboxylate,UVR, (e.g., UVR 6110 of Union Carbide). Its molecules containonly aliphatic rings equipped with two highly reactive epoxygroups to yield a high glass transition temperature network witha low refractive index after crosslinking. It is also commerciallyavailable at low cost and with high purity and is used in UVcurable coatings (PCBs, cans) and castings (LEDs) [5].The combination of epoxies, thus, provides a low-cost for-mulation with high stability and wide refractive index tuningrange. For the waveguide core formulation, the amount of themore reactive fluid UVR in the solid DGEBA is maximized to17%. This results in a just dry, nontacky film suitable for con-tact masking. For the claddings, a larger amount of UVR is takenbased on matching the NA of the waveguides with multimodefibers. These wet layers can be cured by unmasked flood expo-sure.1041-1135/$20.00 © 2006 IEEEDIEMEER et al.: LOW-COST AND LOW-LOSS MULTIMODE WAVEGUIDES OF PHOTODEFINABLE EPOXY 1625Fig. 1. Refractive index at 633 and 850 nm of photocured DGEBA-UVR for-mulations as a function of weight percent UVR.Fig. 2. SEM photograph of a photodefined bare multimode waveguide core ona silicon wafer.The ringopening process of the epoxy groups is catalyzedby a strong acid generated by the decomposition of a photoini-tiator. This photionitiator should be active for the operationalwavelength of the exposure apparatus. Photoinitiators for themost common I-line maskaligners are readily available. Oniumsalts based on triarylsulfonium hexafluoroantimonate arewidely used, including in SU-8, because of their effectiveness[6]. However, in our formulation, we used triarylsulfoniumhexafluorophosphate (UVI 6992 of Union Carbide), becausethis yields less yellowing on postbaking. An amount of 4 wt% isused, which is twice as much as is typically used in SU-8. In thepostexposure bake, the photoacid-catalyzed epoxy crosslinkingtakes place, resulting in insolubility of the latent photoacidimage in the polymer which will not be washed away in thesubsequent development step.III. RESULTSFig. 1 shows the refractive index of the photocuredDGEBA-UVR formulation at 633 and 850 nm as a func-tion of weight percentage UVR as measured in thin photocuredfilms using an ellipsometer (Woolam M44).Any index between 1.58 of the neat aromatic DGEBA and1.51 of the neat cycloaliphatic UVR can be produced. With theformulation for the core containing 17% UVR, waveguides withFig. 3. Microscope photograph of a photodefined multimode waveguide crosssection. (Color version available online at: http://ieeexplore.ieee.org.)NAs up to 0.5 can be produced. This is well beyond the NAs ofall standard multimode fibers NA .We used a 45% UVR formulation as cladding material to ob-tain waveguides with NA . Cyclopentanone was used asa solvent in a 1/1 ratio with the epoxy mixtures to yield a layerthickness of about 25 m for the spin coating speed of 500 r/minthat was used. Layer thicknesses up to 100 m can be producedby using a lower amount of solvent. A silicon wafer with 8- mthermal oxide was coated with a single bottom cladding layer.After photocuring, the core layer was deposited in two coatingsteps. After exposure in a contact mask aligner, using a maskwith 40- m open lines, followed by curing and development,the top cladding was deposited in four spinning cycles followedby photocuring.The processing parameters were identical to SU-8 50 pro-cessing parameters, according to the product data sheet of Mi-crochem Corp. for the SU-8 50- m formulation, with the excep-tion for the exposure times in the 350 W, I-line, mask aligner(Karl Süss MA55). These were derived from the best lithog-raphy results and ten times longer than those for 50- m SU-8layers. A hard bake at 150 C–200 C was not applied.Fig. 2 shows a scanning electron microscope (SEM) photo-graph of the bare multimode waveguide core on a silicon wafer.A smooth sidewall and topsurface can be observed.Fig. 3 shows a microscope photograph of the cross section ofa diced wafer. The waveguide structure and the excellent pla-narization of the polymer cladding can be observed.Waveguide losses were determined from the optical output ofsamples with different lengths (cut-back by dicing) using butt-coupled 50- m multimode fibers, a white light source at theinput and an optical spectrum analyzer (Spectro 320, InstrumentSystems) at the output. The results are shown in Fig. 4, togetherwith the loss spectrum of a comparable SU-8 waveguide [3]. A1626 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 15, AUGUST 1, 2006Fig. 4. Loss spectrum of photodefined DGEBA-UVR (solid) and SU-8(dashed) multimode waveguides.Fig. 5. Loss spectrum of photodefined DGEBA-UVR multimode waveguidesaround 850 nm.zoom-in of Fig. 4 for the communication windows around 850and 1300 nm are shown in Figs. 5 and 6, respectively.The results show a much broader transparency windowfor the DGEBA-UVR waveguides than for the SU-8 waveg-uides. The SU-8 waveguides are brown colored due to strongabsorption by yellowing. The transparency window of theDGEBA-UVR waveguides runs from 550 up to 1100 nm withlosses 0.25 dB/cm and minima of 0.1 dB/cm around 850,625, and 1090 nm. Around 1300 nm there is a low-loss windowof 0.2 dB/cm.IV. CONCLUSIONThe low-loss properties of the novel photodefinable epoxyformulation, combined with its low-cost material and pro-cessing properties and its robustness makes this materialFig. 6. Loss spectrum of photodefined DGEBA-UVR multimode waveguidesaround 1300 nm.attractive for planar waveguides compatible with multimodefibers and low-cost LED and VCSEL sources in particularwith the important 850-nm wavelength used for short distanceoptical communication and sensor applications.ACKNOWLEDGMENTThe authors would like to thank E. Klein, University ofTwente/IOMS, for his mask design.REFERENCES[1] C. Berger, R. Beyeler, G.-L. Bona, R. Dangel, L. Dellmann, P. Dill, F.Horst, M. Kossel, C. Menolfi, T. Morf, B. J. Offrein, M. L. Schmatz,T. Toifl, and J. Weiss, “Optical links for printed circuit boards,” in 16thAnn. Meeting IEEE Lasers & Electro-Optics Soc., Tucson, AZ, Oct.26–30, 2003.[2] M. Popall, A. Zabek, M. E. Robertsson, S. Valizadeh, O. J. Hagel,R. Buestrich, R. Nagel, L. Cergel, D. Lambert, and M. Schaub, “OR-MOCER, Inorganic-organic hybrid materials for e/o-interconnection-technology,” Molecular Crystals and Liquid Crystals, vol. 354, pp.711–730, 2000.[3] A. Borreman, S. Musa, A. A. M. Kok, M. B. J. Diemeer, and A.Driessen, “Fabrication of polymeric multimode waveguides anddevices in SU-8 photoresist using selective polymerization,” in Proc.Symp. IEEE/LEOS Benelux Chapter, Amsterdam, The Netherlands,2002, pp. 83–86.[4] J.-S. Kim, J.-W. Kang, and J.-J. Kim, “Simple and low cost fabricationof thermally stable polymeric multimode waveguides,” Jpn. J. Appl.Phys., vol. 42, pp. 1277–1279, 2003.[5] R. S. Bauer, , C. P. Wong, Ed., “Application of Epoxy Resins in Elec-tronics,” in Polymers for Electronic and Photonic Applications. SanDiego, CA: Academic, 1993, pp. 287–331.[6] J. V. Crivello, “The discovery and development of onium salt cationicphotoinitiators,” J. Polym. Sci., Part A: Polym. Chem., vol. 37, pp.241–4254, 1999.

Low-cost and low-loss multimode waveguides of photodefinable epoxy

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Low-Cost and Low-Loss Multimode Waveguides of Photodefinable Epoxy

Application of Epoxy Resins

Fabrication of polymeric multimode waveguides and devices in SU-8 photoresist using selective polymerization,” in

Inorganic-organic hybrid materials for e/o-interconnectiontechnology,” Molecular Crystals and Liquid Crystals,

Optical links for printed circuit boards,”

Simple and low cost fabrication of thermally stable polymeric multimode waveguides,”

The discovery and development of onium salt cationic photoinitiators,”

Low-cost and low-loss multimode waveguides of photodefinable epoxy

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