ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Optical Waveguide Polymer Curing StudyUsing FTIR Spectroscopy

Chong Siew Kuang', Wong Yuen Yee2 and Sahbudin Shaari, Member, IEEE'Photonics Technology Laboratory

2MEMS Design LaboratoryInstitute of Micro Engineering and Nanoelectronics (IMEN)

Universiti Kebangsaan Malaysia43600 UKM Bangi, Selangor, MALAYSIA

Email: skchong@eng.ukm.my

Abstract The UV polymerization ofperfluorinated acrylic polymer for opticalwaveguide was studied by FTIR spectroscopy.15 pim of ZPU12-450 and 6 ftm of ZPU12-456required 8 and 3 minutes repectively to be fullyUY cured with low intensity of 14 mW/cm_2 atroom temperature in the presence of nitrogenflowing. These results showed that delay ofcuring reaction with increasing depth in thecoating. Besides, UV induced polymerization isfound faster than thermal curing reaction.

I. INTRODUCTION

RECENTLY, polymeric optical waveguide haveattracted much attention for use in opticalinterconnects and in integrated devices for opticalcommunications in the excess network and homenetwork areas [1, 2]. Polymer materials offer anumber of interesting features for fabrication ofoptical waveguides compared to silica glass. Silicaglass has a low optical loss of below 0.01 dB/cmas well as high thermal stability. Organicpolymers, by contrast, have attracted a lot ofattention due to their ease of fabrication andstructure flexibility. These are important factorswith regard to their use as packaging and opticalinterconnects. ln addition, the thermooptic (TO)coefficient of polymers is an order of magnitudelarger than silica glass and some of non-aromaticpolymers have very low birefringence. These aredesirable characteristic in terms of theirapplication to optical integrated devices such asTO switch. Classes of polymers that are used inintegrated optics include acrylates, polycarbonates,polyimides and olefins.

The most simple and widely used method ofpolymer formation is radical-chain additionpolymerization. Polymerization involves initiation,propagation, transfer and termination steps.Initiation is the step where initiating radicals areformed by external stimulation such as light, heat,gamma-radiation and redox process. UV radiationcuring offers a number of advantages over othermethods, such as solvent-free formulation, lowenergy consumption, ambient temperatureoperations and tailor-made properties of thephotopolymer. Indeed most of the UV curing resinsystems are made of acrylic monomers andoligomers due to their high reactivity and low cost.Nowadays, highly crosslinked polymers can beproduced within seconds by photoinitiatedpolymerization of multifunctional acrylicmonomers and oligomers [3]. Photocalorimetry [4]and real time infrared spectroscopy [5] arepowerful tools for studying the complex kineticsgoverning crosslinking photopolymerization.

In this paper, we first report on the curing timeof perfluorinated acrylic resins (ZPU12-450 andZPU12-456) for different thickness of coating inorder to get the optimum curing time for thedesired thickness by using FTIR spectroscopy. It isbecause insufficient curing can cause pooradhesion of the coating to the substrate and affectsthe properties of the layer, such as refractiveindex. These resins is chosen as the core andcladding materials for TO switch because of theirlow propagation loss, high environmental stability,precise control of refractive index, good and easyability to process, excellent adhesion properties,and easy control of film thickness. So far, TOswitch such as directional coupler switch [6] and

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

digital optical switch [7] has been fabricated byusing this material.TO polymer waveguide switches are

advantageous compared to silica-based TO switchfor low-speed applications as bypass or protectionswitches. First, large optical devices can be mademore easily because polymer waveguides areeasier to be fabricated than silica-basedwaveguides. Second, the TO effect of polymers isten times larger than that of silica. TO coefficientis -10-4/°C for polymers and - 10-5/C for silica[8, 9]. This means that the refractive indices ofpolymers are highly dependent on the temperature.Therefore, using polymers instead of silica cansignificantly reduce the electrical power neededfor switching. Conventional silica TO switchesrequire a large amount of electrical power, 400 to500 mW [10]. In contrast, switch power as low as5 mW for a polymeric Mach-Zehnder switch hasbeen demonstrated [1 1].

In UV curing, a liquid acrylic resin istransformed almost instantly into solid polymer byexposure to UV light. Scheme 1 shows themechanism of crosslinking reaction of ZPU12-RI,where Rf represents various perfluorinatedcompound and Ro is free radical species. TheHC=CH double bond was attacked by free radicalspecies induced by photoinitiator and turned tobecome CH-CH single bond. However, it isunstable and reactive because it has radical on CH-CH single bond. Therefore, the intermediateradical species will react with other CH=CHdouble bond, which makes the long chainpolymeric network (chain propagation reaction)

occured. The final termination reaction ariseswhen the two reactive species bonded together.

II. EXPERIMENTAL CONDITIONS

Photopolymerization experiments have beencarried out with perfluorinated acrylate (ZPU 12-450 and ZPU12-456), which were obtained fromZenphotonics Company. These resins were used asreceived. The resins were dispensed onto a cleanedsilicon substrate, spun to achieve desired filmthickness, exposed to UV light with intensity of 14mW/cm-2 in an irradiation chamber with nitrogenflowing, and finally baked at 160°C for an hour byusing hotplate. Infrared spectra were recordedwith a UMA 400 IR microscope attathed to theDigilab Excalibur FTS 3000 FTIR system underdry air purge. High sensitivity MCT detector withspectral resolution of 8 cm-' has been set. Whenthe arcylate coating was exposed to UV, theabsorption band will decrease slowly due todissappearance of C=C double bond of acrylicpolymer. For example, Fig. I shows the decreaseof IR band during UV curing of acrylate coating.The degree of conversion (x) is directly related tothe decrease of IR absorbance, and was calculatedfrom equation

AO-Al 0Conversion, x ( - -' x 100

where A, and A, represent the area of the IR bandcentered at 1636 cm-, before and after UVexposure at time t.

0 011 II

H 2C '-CH _C-OCH2-Rf -CH2-0 CH - 2

UTV ir"avdi,ation Gelneration of free radical species

0 O Ro

H2C. C, cHCH O-CH2--Rf-CH2-0 CH '

° Q2 oCH QC-CH2-Rf-CH2-0 c-CH-CH2

(-11haL'iL DrlD,-kPrntionl iveac:tionl R

0 0

R

Scheme I Mechanism of crosslinking reaction of ZPU 12-RI.

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

Absorbance

0,6

0,4

0,2

1626 1636 1646

Wavenumber (cm-')Fig. I IR band of acryliate before (-) and after (...)

UV curing.

Ill. RESULTS

Fig. 2 and Fig. 3 show the FTIR spectra of 15 ,tmofZPU 12-450 and 6 jim ofZPU12-456 before andafter curing. It can be seen that the functionalgroup of acrylate, C=O (carbonyl group, about1700/cm) was not change the structure before andafter curing process. The HC=CH double bondwith stretching vibration mode at 1636/cm wasattacked by free radical species induced byphotoinitiator and then turned to the CH-CH singlebond. So, the C-C double bond peak wasdisappeared after curing as shown in IR spectra.Besides, another peak shown in 807/cm, whichwas out of plane vibration mode of C=C-H(monosubstituted C-H bond) was disappeared aftercuring.

I_

Before0o- UV curing

I-BeforeUV curing

1j After ~ lf

aftrUV curinlg261) 24(0 MO 211 1D 1603 14(X) U122) 0(0 6(0Wavenumber (cm-')

Fig. 3 FTIR spectra of 6 ltm of ZPUI1 2-456 before andafter UV curing.

Homogeneous through cure of acrylic polymer isessential for attaining optimum properties. In orderto achieve good through cure of the coating UVlight has deeply to penetrate into the layer. Fig. 4shows the conversion of acrylic polymer for 6 utmand 15 ptm thickness of coatings. It was obviousthat the 6 ptm and 15 gim coatings were fully UVcured at 3 minutes and 8 minutes respectively.These results reflect the delay of the curingreaction with increasing depth in the coating.

0

0._

tV)

-0.5-

After-1- UV curing

-I' 1 i1 T111 I T T26( 2400 220 2O) 180) 1600 14(0 12) 100 a (0)

Wavenumber (cm-')

Fig. 2 FTIR spectra of 15 p.m of ZPU12-450 before andafter curing.

1009080706050403020100

0 1 2 3 4 5 6 7 8 9 10

Time (minute)

4 15 umthicknessofZPU12-4506 um thickness ofZPU 12-456

Fig. 4 Depth profile of conversion of the acrylicpolymer for 6 pum and 15 p.m coating.

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

A number of factors are expected to affect theinhibitory process of acrylic resins, such as theoxygen content of atmosphere, the rate at whichair diffuses into the sample, which depends on theresin viscosity and on the temperature, the filmthickness, and the duration of polymerization [12].Therefore, we should define the key parameterswhich will affect the polymerization in order to getthe best film quality.Oxygen inhibiton causes numerous deleterious

effects on free-radical formed by photolysis of theinitiator are rapidly scavenged by oxygenmolecules to yield peroxylradicals. These speciesusually abstract hydrogen atoms from the polymerbackbone to generate hydroperoxide. Moreover,this premature chain termination modifies themechanical properties of the film. Consequently,higher UV energy is needed to consume theoxygen dissolved in the resin [12].The most effective way to overcome oxygen

inhibition is to work in an inert atmosphere. Forexample, by flushing the irradiation chamber withnitrogen and carbon dioxide. In our experiment,the nitrogen gas was used to diffuse through thefilm during flowing step with rate of 12ml/minbefore (approximately 3 min) and during curingprocess. The gas will replace the oxygen dissolvedin the sample. As nitrogen molecules are smallerthan carbon dioxide molecules, their penetrationinto the sample will be easier and efficiently [12].If UV curing process is performed in an inertatmosphere, an effective polymerization can beachieved by means of low intensity UV lamps, inparticular at the coating surface, which receivesthe most intense light [12]. Thus, low lightintensity 14 mWcm-2 was used to cure thepolymer. This amount of intensity was acceptableaccording to technical data sheet provided byZenphotonics Company, where the polymershould be cured at the range of intensity 10 to 16mWcm-2.The other factors that may affect the curing

process is temperature. The acrylic resin hasviscosity of 150 to 300 cps at 25 'C. The viscositydecreases at high temperature, thus acceleratingthe oxygen diffusion into the film and amplifyingits inhibitory effect [12]. This phenomenon willcompete with a higher molecular mobility whichaccelerates the polymerization and increases thefinal conversion. In contrast, the viscosityincreases at low temperature, thus the

polymerization proceeds significantly slower thanat room temperature. Therefore, the irradiationconditions have been intentionally selected so asto overcome oxygen inhibition. Thepolymerization was performed at roomtemperature and strongly inhibited in the presenceof air by flushing irradiation chamber withnitrogen.

In most thermally induced polymerization andcuring reactions, an increase of temperature leadsto an increase of both ultimate conversion and thereaction rate [12]. Fig. 5 shows the FTIR spectraof a 15 ,um coating was not fully cured after 7minutes of UV curing and was fully cured after anhour of postbaking. It can be seen that UV curingreaction is performed faster than thermal curingreaction. Besides, the two disappeared peaks afterfully UV cured in Fig. 2 are same as afterpostbaking in Fig. 5. It means that the machanismof crosslinking was terminated. The purpose ofpostbaking process in our work was to harden thecoating.

After°05 j UV curing

-I

0-

05 1

-1-Afterpostbaking

.~~~~~~~~~~~~I r

2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600Wavenumber (cm-')

Fig. 5 FTIR spectra for 15 pim coating after 7 minutesofUV curing and an hour of postbaking.

IV. CONCLUSION

We have study the curing time of perfluorinatedacrylic polymer for different thickness of coatingin order to get the optimum curing time for thedesired thickness of coatings. 15 [tm of ZPU12-450 and 6 .tm of ZPU12-456 required 7 and 2minutes repectively to be fully cured at intensity of

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ICSE2004 Proc. 2004, Kuala Lumpur, Malaysia

14 mW/cm-2 with nitrogen flowing at roomtemperature. UV curing reaction is faster thanthermal curing reaction. Results from FTIRspectroscopy were same as IR spectra given byZenphotnics Company.

ACKNOWLEDGMENT

The authors thank Zenphotonics Company forproviding the information about the material. Thework is supported by the Malaysian Ministry ofScience, Technology and the Environment underNational Photonics Top Down Research ProjectIRPA 020202T001.

REFERENCES

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[2] L. Eladada, R. Blomquist, L. W. Shacklette, and M. J.McFarland, "High-performance polymeric componentryfor telecom and datacom applications," Opt. Eng., vol.39, pp. 596-609, 2000.

[3] C. Decker, "Light-induced crosslinking polymerization,"Polymer International, vol. 5 1, pp. 11441-1150, Nov2002.

[41 J. S. Young, A. R. Kannurpatti and C. N. Bowman,"Effect ofcomonomer concentration and functionality onphotopolymerization rates, mechanical properties andheterogeneity of the polymer," Macromol. Chem. Phys.vol. 199, pp. 1043-1049, 1998.

[5] K. S. Anseth, C. Decker and C. N. Bowman, "Real- timeinfrared characterization of reaction diffusion duringmultifunctional monomer polymerizations,"Macromolecules, vol. 28, pp. 4040-4043, 1995.

[6] Abu Sahmah Mohd Supa'at, Abu Bakar Mohammad andNorazan Mohd Kassim, "Fabrication of Thermoopticswitch using polymers," IEEE National Symposium onMicroelectronics, pp. 8-11, Sept 2003.

[7] Y.-O. Noh, J.-M Kim, M.-S Yang, H.-J. Choi, H.-J. Lee,Y.-H Won and S.-G. Han, "Thermooptic 2x2asymmetric digital optical switches with zero-voltageoperation state," J. Lightwave Technol., vol. 16, no. 2,pp. 446-448, Feb 2004.

[8] Z. Xu, Q. Xie, Z.Tan, Q. Wu, and Y. Chen, "Heat-resistant optical waveguides using new silicone-basedpolymers," First Joint Symp. Opt-. Microelectronic Dev.Circuits, Nanjing, China, Apr. 2000, pp. 138-141.

[9] M. B. J. Diemeer, "Polymeric thermo-optic spaceswitches for optical communications," Optical Meterials9, pp. 192-200, 1998.

[10] J. Kobayashi, T. Matsuura, Y. Hida, S. Sasaki, and T.Maruno, "Fluorinated polyimide waveguides with lowpolarization-dependent loss and their applications tothermooptic switches," J. Lightwave Technol., vol. 16,no. 6, pp. 1024-1029, June 1998.

[11] Y. Hida, H. Onose and S. Imamura, "Polymer waveguidethermooptic switch with low electric power consumptionat 1.3 /L m," IEEE Photon. Technol. Lett., vol. 5, no. 7,pp. 782-784, July 1993.

[12] K. Studer, C. Decker, E. Beck and R. Schwalm,"Overcoming oxygen inhibition in UV-curing of acrylatecoatings by carbon dioxide inerting, Part I," Progress inOrganic Coatings 48, pp. 92-100, 2003.

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