007) 5439–5443www.elsevier.com/locate/tsf

Thin Solid Films 515 (2

Preparation and characterisation of an epoxy-functional inorganic–organichybrid material system with phenyl side group for waveguiding applications

Shane O'Brien a, Mehmet Çopuroğlu a, Gabriel M. Crean a,b,⁎

a Tyndall National Institute, Cork, Irelandb Department of Microelectronic Engineering, University College Cork, Ireland

Received 13 February 2006; received in revised form 8 November 2006; accepted 29 November 2006Available online 6 February 2007

Abstract

An epoxy-functional inorganic–organic hybrid material system for use in photonic waveguiding applications was synthesised by the sol–gelmethod. The influence of preparation process parameters such as composition and UV irradiation, on the properties of the deposited thin films wasstudied. Crack-free films with thickness up to 37 μm were obtained from a single-step deposition process. A tunable refractive index, at 633 nm,which ranged between 1.480 and 1.515 was observed by modifying the concentration of the refractive index modifier. It was also demonstratedthat UV irradiation resulted in an increase in the refractive index of the system by 0.49%. Planar waveguiding structures were demonstrated. Ahigh thermal stability, up to 275 °C, was achieved compatible with post-processing integration processes.© 2006 Elsevier B.V. All rights reserved.

Keywords: Sol–gel method; Refractive index; Waveguiding; Thermal stability

1. Introduction

In the context of material development for optical waveguidingapplications, inorganic–organic hybrid materials offer significantopportunities [1–8]. In terms of their optical properties, polymershave certain features, which make them useful for waveguidefabrication, in particular they can be processed using a wide rangeof techniques such as lithography and embossing. However, inapplications in which polymer waveguides are utilised in harshenvironments, deterioration in optical propagation can be encoun-tered. On the other hand, glasses have excellent optical trans-mission and thermal stability in the order of hundreds of degreesCelsius. However, glass waveguide production techniques areexpensive and complex. Consequently, a hybrid system, whichwould produce a material with advanced optical and physicalproperties, combined with enhanced thermal and mechanical sta-bility in a convenient process, is of interest. The development ofmulti-functional organically modified silanes, such as alkoxysi-lanes, which contain acrylate or methacrylate crosslinkable

⁎ Corresponding author. Department of Microelectronic Engineering, Uni-versity College Cork, Ireland. Tel.: +353 21 4904256.

E-mail address: gcrean@tyndall.ie (G.M. Crean).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.11.154

groups, has enabled increased control over chemical composition,crosslinking and phase stability [9]. This has resulted in the pro-duction of materials that have the optical transparency of poly-mers, but also have inorganic characteristics, such as the thermaland mechanical properties of a glass or ceramic [10,11]. Suchtunability of properties is of particular interest in the area of opticalwaveguide materials.

Nonetheless, these hybrid material systems cannot be easilyproduced by conventional material processing methods, such aschemical vapour deposition or sputtering and therefore alternativeprocessing routes, such as sol–gel, are required. Using this latterprocess, it is possible to carefully control the microstructure andhence material properties.

In this present work, a preparation method for an epoxy-functional inorganic–organic hybrid material thin film system, viathe sol–gel method, is described. Epoxy functionality potentiallyimparts the required thermal stability to the system to survivesubsequent high temperature post-processing e.g. solder reflow(typically 230 °C for 10 s). The optical properties of the films werestudied by means of refractive index measurements at 633 nm.Fourier transform-IR (FT-IR) spectrometrywas performed in orderto exhibit the chemical structure of the resultant films and also tofollow the influence of UVexposure time on it. Thermogravimetric

Fig. 2. Cross-sectional SEM image of a representative prepared film.

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analysis (TGA) was performed to evaluate the thermal stability ofthe system. Planar waveguide structures were produced by photo-lithography and characterised, at 633 nm, by near-field imaging oftransmitted light and optical propagation loss measurements.

2. Experimental details

A mixture of (3-glycidyloxypropyl)trimethoxysilane(GPTMS) (Fluka) and dimethyldiethoxysilane (DMDS)(Fluka) was prepared with a mass ratio of 66:1, respectively,which formed the main structure. Then, various amounts ofdiphenyldimethoxysilane (DPDMS) (Fluka), which is both amatrix material, refractive index modifier and a potentialthermal stabiliser, were added to the mixture. This mixture wasthen stirred for 15 min and acidified with 0.1 M aqueous HClsolution, which was added dropwise during subsequent stirringover a 20 min period to produce a mixture with a mass ratio of1:0.14, GPTMS:0.1 M aqueous HCl solution. Iso-propylalcohol (IPA), with a mass ratio of 1:10, IPA:mixture, wasthen added over a period of 5 min and the resultant mixture wasrefluxed for 3 h and allowed to cool. A further 0.1 M aqueousHCl solution, with a mass ratio of 1:10, 0.1 M aqueous HCl

Fig. 1. Flow diagram of the synthesis and film preparation.

solution:mixture, was added and the mixture was stirred for 1 h.50% triarylsulphonium hexafluoroantimonate salt in propylenecarbonate (Aldrich) was then added as a photo-initiator, with amass ratio of 1:50, photo-initiator:mixture, to facilitate UV-induced crosslinking during subsequent processing steps.

The sol–gel mixtures were aged in ambient environment fortime durations ranging between 0 and 4 days. Prior to deposition,the mixtures were passed through a 0.45 μm polypropyleneWhatman™ filter. They were then spin-coated onto polishedsilicon wafer substrates using a Laurell Technologies Corp. WS-400A-6NPP/LITE spin-coater, at spin rates ranging from 300 to100 rpm for 30 s. The coated substrates were then pre-baked on ahot-plate at 80 °C for 20 min. Some of the pre-baked films werethen irradiated through a chrome-on-quartz contact mask (Align-Rite Ltd.) with UV light in order to fabricate the waveguides. ADEK 1600UVExposure Systemwas used as theUV light source.The remaining ones were exposed to UV light without using amask for FT-IR spectrometry and TGA. Various UV exposuretimes ranging between 0 and 5 min were employed. All thesamples were then post-baked on a hot-plate at 80 °C for 20 min.The overall flow diagram of the sol–gel synthesis and filmpreparation is shown in Fig. 1.

An Olympus BH-2 microscope, JVC digital 1/2 in. CCD TK-C1381 colour video camera and Hitachi-S4000 scanning electron

Fig. 3. Thickness of the films as a function of sol–gel ageing time for differentspin rates.

Fig. 4. FT-IR spectrum of a representative prepared film.

Table 1Variation of A760/A1260 as a function of UV exposure time

Ratio UV exposure time (s)

0 15 30 45 60 90

A760/A1260 0.491 0.311 0.272 0.279 0.316 0.296

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microscope (SEM) were used to image the waveguide structures.The operating voltage of the SEM was 20 keV. A Metricon 2010Prism Coupler was used to measure the refractive index of thesamples. 10 measurements were performed per sample and theaverage was taken. The standard deviation within each data setwas ±0.001. A waveguide test rig was used to measure thewaveguide performance. The optical propagation loss values at633 nm were calculated using the cleave-back method. Thethickness measurements were performed on a Dektak profil-ometer. The FT-IR analysis was done using a Thermo Nicolet(Nexus 470) FT-IR spectrometer. A Polymer Laboratories(Rheometric Scientific) TGA1000M model thermogravimetricanalyserwas employed in order tomeasure the thermal stability ofthe samples. The measurements were performed in static air, inorder to simulate the real conditions, in which the material systemwill potentially be used. A heating rate of 10 °C/min was appliedin all experiments.

3. Results and discussion

Crack-free films were obtained up to a thickness of 37 μm.The cross-sectional SEM image of a representative preparedfilm (7.1 mol% DPDMS, UV-exposed (30 s)) is given in Fig. 2.

Fig. 5. FT-IR spectra of a representative prepared film with different UVexposure times. (Numbers on the curves correspond to the UV exposure time(min).)

The relationship between sol–gel ageing time, spin-coating rateand resulting film thickness is shown in Fig. 3. The thickness ofthe deposited films, at all spin rates, increased with sol–gelageing time, due to ongoing condensation reactions duringageing [12].

The FT-IR spectrum of a representative prepared film sample(11 mol% DPDMS, UV-exposed (45 s)) is given in Fig. 4. Thegeneric peak assignments were made as follows: the broad peakbetween 3200 and 3600 cm−1 was assigned to O–H bonds; thefeatures at 2870 and 2935 cm−1 were assigned to C–H bonds; thestrong peak centred at 1110 cm−1 was assigned to Si–O bonds.The presence of a broad peak around 2125 cm−1 resulted mostprobably from the overtone absorptions of various species in thestructure. In addition, the peak centred around 760 cm−1 wasassigned to epoxy rings [13], and that at around 1260 cm−1 wasassigned to Si–CH3 species [14]. In order to follow the UV-induced epoxy crosslinking, the relative amount of epoxy rings inthe structure was determined using the ratio of IR absorbance at760 cm−1 (A760) to that at 1260 cm

−1 (A1260). It was assumed thatthe number of Si–CH3 bonds remained constant during the syn-thesis and processing. Therefore any change in absorbance atanother wavenumber could be monitored by normalising theabsorbance at that wavenumber to that of Si–CH3. The corres-ponding regions of the FT-IR spectra of a sample, with 11 mol%DPDMS, are given in Fig. 5 and the numerical data, which wereobtained by using appropriate baseline corrections, are given inTable 1. The significant decrease in A760/A1260, observed after just15 s UV irradiation, demonstrated that the UV-induced epoxycrosslinking occurred via epoxy ring opening reactions. For sub-sequent waveguide fabrication, a 30 s UVexposure was employedas it enabled effective crosslinking (i.e. sufficient to produce ameasurable change in refractive index suitable for waveguidingapplications) within the UV-exposed regions.

Fig. 6. Variation of refractive index as a function of DPDMS content.

Fig. 7. Dynamic thermograms of the unexposed (a) and the UV-exposed (5 min)(b) samples (numbers correspond to the DPDMS content (mol%)).

Fig. 8. Optical microscope image of a representative prepared planar waveguidestructure.

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Further characterization was performed to determine thedegree of tunability of the refractive index of the materialsystem through adjustment of precursor ratios and also toinvestigate the effect of UV irradiation on the refractive index ofthe films. A series of precursor solutions were made, in whichthe ratio of the refractive index modifier (DPDMS) to theprimary matrix material (GPTMS) was varied. As can be seen inFig. 6, the refractive index was tunable over the range 1.480 to1.515. UV irradiation for 30 s caused an average increase of0.49% in the refractive index of the system. This observation iscompatible with the epoxy crosslinking reactions induced byUV irradiation, since it is known that epoxy crosslinks lead to anincrease in the refractive index of similar material systems [1].

Fig. 7 (a) and (b) shows the dynamic thermograms of theunexposed and the UV-exposed (5 min) samples with differentDPDMS content, respectively. The observed mass loss, up to

Table 2Variation of the temperature at 10% mass loss as a function of DPDMS content(Un: unexposed, Ex: UV-exposed (5 min))

Sample DPDMS Content (mol%)

0.00 1.7 3.4 6.6 11

Un 254±4 262±3 261±3 269±3 275±3Ex 270±2 263±2 274±2 269±2 263±2

700 °C, corresponded to the decomposition of organic segmentsof the structure. The difference in profile of this decompositionbetween the unexposed and the UV-exposed samples showedthe effect of UV-induced epoxy crosslinking on the thermalstability of the material system. The variation of the 10% massloss temperature with DPDMS content is given in Table 2. Aslight increase was observed for the unexposed samples, as afunction of DPDMS content. This was due to the presence of Si-phenyl groups in the structure, which increased the thermalstability of the system [15]. This effect was negligible in thecase of UV-exposed samples. This was most probably due to thefact that epoxy crosslinks hindered the segmental movements inthe structure, which therefore prevented the formation of morethermally stable segments. Nonetheless, the temperature valuesat 10% mass loss, which ranged between 250 and 275 °C,demonstrated that the material system has a sufficiently highthermal stability, in the 0–11 mol% DPDMS range to bepotential candidates for waveguiding applications.

The optical microscope image of a representative preparedplanarwaveguide structure (7.1mol%DPDMS) is shown in Fig. 8.The waveguides were examined by projection of the near-field

Fig. 9. Near-field image of light transmitted through a representative preparedwaveguide.

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image of light transmitted through the waveguide onto the camera.The image collected from a representative prepared waveguidecore (7.1 mol% DPDMS) is shown in Fig. 9. It was observed thatlight was effectively confined to the core region of the waveguide.The optical propagation loss measurements were performed for allwaveguide samples. The average propagation loss valuewas foundto be 0.20±0.01 dB cm−1, compatible with short-range wave-guiding applications.

4. Conclusions

An epoxy-functional inorganic–organic hybrid material sys-tem was prepared by the sol–gel method for use in opticalinterconnect and sensing applications. Thin films were depos-ited by spin-coating up to 37 μm in thickness. The resultantfilms were irradiated with UV light in order to obtain waveguidestructures. It was shown that epoxy crosslinking occurred viaepoxy ring opening reactions upon UV irradiation. A tunablerefractive index, at 633 nm, which ranged between 1.480 and1.515 was observed by modifying the concentration ofDPDMS. It was also demonstrated that UV irradiation resultedin an increase in the refractive index of the system by 0.49%.The near-field image of transmitted light, at 633 nm, fromplanar waveguide structures showed waveguiding and efficientlight confinement within the core regions. The opticalpropagation loss, at 633 nm, of the waveguides was found tobe 0.20±0.01 dB cm−1, compatible with short-range waveguid-ing applications. The TGA results demonstrated that the thermalstability of the material system could be improved up to 275 °C,in terms of 10% mass loss, in air. This material system has thepotential for use in waveguiding applications, in particular, inenvironments where high thermal stability is required.

Acknowledgements

This work was supported by the Irish Government,Department of Education and Science, through the HigherEducation Authority Programme for Research in Third LevelInstitutions, Project Eco-Electronics.

The authors would also like to acknowledge Dr. EndaMcGlynn and Ms. Clare Higgins of the School of PhysicalSciences, Dublin City University for their assistance in the FT-IRanalysis.

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