Direct fabrication of PDMS waveguides via low- cost ... - OSA Publishing

Direct fabrication of PDMS waveguides via lowcost DUV irradiation for optical sensing Sebastian Valouch,1 Heinrich Sieber,2 Siegfried Kettlitz,1 Carsten Eschenbaum,1 Uwe Hollenbach,2 and Uli Lemmer1,* 2

1 Light Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, 76131 Karlsruhe, Germany Institute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany * [email protected]

Abstract: We demonstrate the fabrication of single mode optical waveguides by irradiating polydimethylsiloxane (PDMS) with a low cost Hg lamp through a conventional quartz mask. By increasing the refractive index of the irradiated areas, waveguiding is achieved with an attenuation of 0.47 dB/cm at a wavelength of 635 nm. The refractive index change is stable in ambient air and water for time periods of more than 3 months. The excitation of water-dispersed fluorescent nanoparticles in the evanescent field of the waveguide is demonstrated. ©2012 Optical Society of America OCIS codes: (230.7390) Waveguides, planar; (230.3120) Integrated optics devices.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

H. Ma, A. K.-Y. Jen, and L. R. Dalton, “Polymer-based optical waveguides: Materials, processing, and devices,” Adv. Mater. (Deerfield Beach Fla.) 14(19), 1339–1365 (2002). D. G. Rabus, M. Bruendel, Y. Ichihashi, A. Welle, R. A. Seger, and M. Isaacson, “A Bio-Fluidic-Photonic Platform Based on Deep UV Modification of Polymers,” IEEE J. Sel. Top. Quantum Electron. 13(2), 214–222 (2007). Y. Xia and G. M. Whitesides, “Soft lithography,” Annu. Rev. Mater. Sci. 28(1), 153–184 (1998). J. C. McDonald and G. M. Whitesides, “Poly(dimethylsiloxane) as a material for fabricating microfluidic devices,” Acc. Chem. Res. 35(7), 491–499 (2002). V. Lien, Y. Berdichevsky, and Y.-H. Lo, “A prealigned process of integrating optical waveguides with microfluidic devices,” IEEE Photonic. Tech. L. 16(6), 1525–1527 (2004). S. Kopetz, D. Cai, E. Rabe, and A. Neyer, “PDMS-based optical waveguide layer for integration in electrical– optical circuit boards,” AEU, Int. J. Electron. Commun. 61(3), 163–167 (2007). D. A. Chang-Yen, R. K. Eich, and B. K. Gale, “A Monolithic PDMS waveguide system fabricated using softlithography techniques,” J. Lightwave Technol. 23(6), 2088–2093 (2005). M. Okoshi, J. Li, and P. R. Herman, “157 nm F2-laser writing of silica optical waveguides in silicone rubber,” Opt. Lett. 30(20), 2730–2732 (2005). C. N. B. Udalagama, S. F. Chan, S. Homhuan, A. A. Bettiol, T. Wohland, and F. Watt, “Fabrication of integrated channel waveguides in polydimethylsiloxane (PDMS) using proton beam writing (PBW): applications for fluorescence detection in microfluidic channels,” in Proc. SPIE (SPIE, 2008), Vol. 6882, 68820D–8. F. Egitto and L. Matienzo, “Transformation of Poly(dimethylsiloxane) into thin surface films of SiOx by UV/Ozone treatment. Part I: Factors affecting modification,” J. Mater. Sci. 41(19), 6362–6373 (2006). M. Ouyang, C. Yuan, R. J. Muisener, A. Boulares, and J. T. Koberstein, “Conversion of some siloxane polymers to silicon oxide by UV / ozone photochemical processes,” Chemical Vapor Deposition 1591–1596 (2000). Y. Berdichevsky, J. Khandurina, A. Guttman, and Y.-H. Lo, “UV/ozone modification of poly(dimethylsiloxane) microfluidic channels,” Sens. Actuators B Chem. 97(2-3), 402–408 (2004). T. Scharnweber, R. Truckenmüller, A. M. Schneider, A. Welle, M. Reinhardt, and S. Giselbrecht, “Rapid prototyping of microstructures in polydimethylsiloxane (PDMS) by direct UV-lithography,” Lab Chip 11(7), 1368–1371 (2011). N. Bowden, W. T. S. Huck, K. E. Paul, and G. M. Whitesides, “The controlled formation of ordered, sinusoidal structures by plasma oxidation of an elastomeric polymer,” Appl. Phys. Lett. 75(17), 2557–2559 (1999). H.-N. Kim, S.-H. Lee, and K.-Y. Suh, “Controlled mechanical fracture for fabricating microchannels with various size gradients,” Lab Chip 11(4), 717–722 (2011). T. S. Phely-Bobin, R. J. Muisener, J. T. Koberstein, and F. Papadimitrakopoulos, “Preferential Self-Assembly of Surface-Modified Si/SiOx Nanoparticles on UV/Ozone Micropatterned Poly(dimethylsiloxane) Films,” Adv. Mater. (Deerfield Beach Fla.) 12(17), 1257–1261 (2000). H. Oláh, H. Hillborg, and G. J. Vancso, “Hydrophobic recovery of UV/ozone treated poly(dimethylsiloxane): adhesion studies by contact mechanics and mechanism of surface modification,” Appl. Surf. Sci. 239(3-4), 410– 423 (2005).

#176916 - $15.00 USD Received 26 Sep 2012; revised 22 Nov 2012; accepted 29 Nov 2012; published 12 Dec 2012

(C) 2012 OSA

17 December 2012 / Vol. 20, No. 27 / OPTICS EXPRESS 28855

18. F. Schneider, J. Draheim, R. Kamberger, and U. Wallrabe, “Process and material properties of polydimethylsiloxane (PDMS) for Optical MEMS,” Sens. Actuators A Phys. 151(2), 95–99 (2009). 19. R. G. Heideman and P. V. Lambeck, “Remote opto-chemical sensing with extreme sensitivity: design, fabrication and performance of a pigtailed integrated optical phase-modulated Mach-Zehnder interferometer system,” Sens. Actuators B Chem. 61(1-3), 100–127 (1999).

1. Introduction The integration of optical waveguides into microfluidic systems promises exciting ways for optical sensing in lab-on-chip applications. For optofluidic systems optical waveguides are crucial elements to guide light to or collect light from the sensing area. Waveguides are also used as the sensing elements itself. Integrating waveguides into microfluidic systems is done via various methods. One method of particular interest for the fabrication is the change of the refractive index via photomodification, as it allows the formation of waveguides without the difficulty of additional steps of photolithography on microstructured substrates. Photomodification has been investigated for a number of polymers [1], the most widely employed material being poly(methyl methacrylate) (PMMA), which is sensitive to irradiation below about 300 nm [2]. However, as PMMA cannot be structured using soft lithography, methods such as hot embossing have to be employed which makes the material less attractive for prototyping. Polydimethylsiloxane (PDMS) on the other hand has proven to be the workhorse of the microfluidic community due to the ease of rapid prototyping by soft lithography [3], excellent mechanical flexibility and optical transparency [4]. Waveguide fabrication in PDMS has been realized by direct photolithography using photosensitive types of PDMS [5] or by soft lithography with different types of PDMS [6] or varying curing conditions [7] to achieve different refractive indices. These methods, however, are difficult to be combined with microchannels, as the second required solution based processing step usually fills cavities later required for fluidic flow as addressed in [5] using a barrier fluid. Therefore methods without the need of solution processing are desirable and have been investigated for a number of materials. Direct waveguide fabrication in PDMS using ionizing radiation has so far only been shown for 157 nm UV-laser irradiation [8] and proton beam irradiation [9]. Both methods require expensive equipment that is not readily available and not easy to operate. Here, we show a novel method for the fabrication of optical waveguides including optical Y-splitters using a simple, low-cost, facile route based on a low-pressure mercury lamp and a conventional quartz chrome mask. The advantage is the simple and parallel mask-based fabrication, which can also be carried out on substrates already patterned with microfluidic structures. 2. Experimental We carry out our irradiation experiments on flat as well as on microstructured PDMS slabs. The PDMS slabs were fabricated by first degassing a 10:1 mixture of PDMS (Sylgard 184, Dow Corning) for 30 min in vacuum, evenly distributing it onto a silicon wafer substrate and curing at a temperature of 100 °C for 15 min. The resulting substrates had a thickness of 1 mm and were cut into pieces of 20 x 35 mm2 size. For pattern definition the substrates were put in contact with a quartz lithography mask, in which the desired single waveguides were defined as openings in the chrome mask with a distance of 50 µm from the center of each waveguide. For the formation of Y-splitters we designed a pattern containing S-shaped curves with a radius of 50 mm which split the input waveguide into two legs spaced 150 µm apart. PDMS adheres easily and reversibly to the mask surface, which ensures a gapless contact without any pressure applied. As illustrated in Fig. 1(a), illumination was performed with a low pressure mercury lamp (U09W-145ozon, Dinies Feinwerktechnik) with dominating emission bands at wavelengths of 185 nm and 254 nm. The mask with the substrate was placed at a distance of 8 cm from the lamp, resulting in a power density of 10.8 W/m2 for 254 nm and 3.78 W/m2 for 185 nm. The fabrication was carried out in a nitrogen glovebox. Irradiation in ambient air gave similar #176916 - $15.00 USD Received 26 Sep 2012; revised 22 Nov 2012; accepted 29 Nov 2012; published 12 Dec 2012

(C) 2012 OSA


results, but as oxygen is strongly absorbing the UV radiation at 185 nm, irradiation times had to be at least 10 times longer than under nitrogen. The waveguides were cleaved using a commercial razor blade. We irradiated both, unstructured as well as prestructured PDMS substrates. The latter contained 25 µm deep microfluidic channels. This shows the possibility to form waveguides on the bottom of the microfluidic structures. Figure 1(b) depicts a corresponding light microscopy image. For the actual optical measurements unstructured PDMS slabs are used.

Fig. 1. (a) Fabrication of the waveguides via DUV irradiation. A quartz chrome mask defines the waveguides. Irradiation from a low pressure Hg-lamp with a wavelength of 185 nm and 254 nm results in a change of refractive index of the PDMS. After irradiation the PDMS is peeled off the mask. (b) Light microscope image of a single waveguide. (c) Atomic force microscopy cross section of a waveguide. The irradiated area increases in height by approximately 50 nm, but shows a sharp reduction in height at the edges. (d) Atomic force microscopy image showing a 25 x 25 µm2 area of a waveguide.

We determined the refractive index of the substrate and the irradiated areas via m-line spectroscopy using an Abbé-refractometer. We investigated the optical near field using a near field measurement system (Hamamatsu, LEPAS-11) together with fiber coupled single mode lasers of different wavelengths (DL100, Toptica). Light was coupled into the waveguides using a single mode optical fiber (S405-HP, SM600, 780HP, Thorlabs) without the use of index matching fluid. Optical attenuation at 635 nm wavelength was measured by coupling a second single mode fiber (SM600, Thorlabs) to the waveguides and measuring the optical power using an optical powermeter (HP 8153A, Agilent). The dimensions of the waveguides were measured using a conventional light microscope in DIC mode (Axioplan 2 Imaging, Zeiss) and an atomic force microscope (BioAFM, JPK Instruments). To demonstrate fluorescence excitation we used light from a HeNe-Laser (Rofin Sinar, 632.8 nm) coupled via a single mode fiber (SM600, Thorlabs). The particles used for the fluorescence excitation experiment were dark red fluorescent nanoparticles (FluoSpheres (660/680) 0.04 µm F-8789, Invitrogen) dispersed in DI water. 3. Results and discussion 3.1. Refractive index change Waveguides with different widths were characterized. Qualitatively, the refractive index increases with the dosage. An effective refractive index difference of approximately 0.001 was determined for a substrate irradiated with 1.5 J/m2, with the non-irradiated PDMS layer showing a refractive index of 1.4176. The correct determination of the refractive index contrast of our samples is difficult, which is mainly due to the elastic properties of PDMS.


(C) 2012 OSA


The waveguiding behaviour of the PDMS waveguides however is very reproducible. The waveguiding is mainly controlled by the refractive index contrast between irradiated and nonirradiated areas. As a change in refractive index induced by strain or pressure influences both the irradiated and non-irradiated area the waveguiding is not affected as severely. The exact mechanism of the UV-induced changes in PDMS, is not completely understood and is dependent on a multitude of parameters. There are indications from experiments using unpatterned UV-irradiation that a silica-like layer is formed due to decomposition of carbon and oxygen [10]. Yet, the depth of photoinduced changes in the PDMS layer has been a topic of ongoing debates. For thin layers of PDMS a conversion depth on the order of 100 nm has been suggested [11]. On the other hand the conversion to a depth of several microns has also been achieved using the same methods based on UV-irradiation at wavelengths of 185 nm and 254 nm [12]. Furthermore a change in solubility in an aqueous NaOH/ethanol-mixture to a depth of several microns is also reported and utilized for the fabrication of microfluidic channels [13]. 3.2. Mechanical properties, process optimization and stability Atomic force microscopy on the waveguides shows a change in height of the irradiated areas, as shown in Fig. 1(c) and (d). The crosssection in Fig. 1(c) reveals the formation of a plateau, which is flanked by trenches at the interface from the irradiated to the non-irradiated area. We assume that this is caused by the interfacial stress caused by the formation of the oxidized layer [14]. For irradiation with 254 nm only limited changes of the irradiated PDMS are expected [10], accordingly we observe no formation of waveguides when using a different low pressure mercury lamp with 254 nm emission only. Prolonged irradiation of the PDMS layer can result in a brittle silica-like layer, which forms cracks during substrate handling [15]. These cracks on the surface act as sources of optical scattering and have to be avoided for efficient waveguide operation. Furthermore, we observe a tendency of the PDMS to stick to the quartz mask, if the irradiation dose is too high. To avoid both cracking and adhesion to the mask, we kept the dose below 1.5 J/cm2 of 185 nm irradiation. This yields samples with no tendency to form cracks even for repeated bending perpendicular to the waveguide at a bending radius of less than 25 mm proving that the waveguides are robust and easy to handle. To determine the long term stability of the waveguides we stored substrates in ambient air for up to 3 months with no negative effect on the waveguiding properties. This shows that the refractive index change is permanent and stable. The waveguiding mechanism therefore does not depend on the change from a hydrophobic to a hydrophilic surface upon UV irradiation [16], which is usually only temporary for PDMS [17]. Also washing the substrates with DI water had no effect, making this method well suited to the application in PDMS-based lab-onchip systems. 3.3. Waveguide attenuation We tested wavelengths of 405 nm, 635 nm and 808 nm, as these are relevant for their use in lab on chip systems. For 405 nm no proper waveguiding could be observed which we attribute to additional transmission losses introduced by the irradiation damage in the PDMS and due to the higher dispersion in PDMS for shorter wavelengths [18]. For 635 nm and 808 nm waveguiding could be observed. Figure 2 shows the modal profiles of waveguides with different irradiated waveguide widths. It is evident, that the optimum width for singlemode waveguiding is achieved for a width of 5 µm. Below this width the light is only weakly guided. For widths of 10 µm and 15 µm more than one mode can be excited in the waveguide.


(C) 2012 OSA


Fig. 2. Modal profiles at a wavelength of 635 nm of irradiated PDMS waveguides with different widths of UV-irradiation of the PDMS substrate. (a) The narrow waveguide with a width of 2 µm shows only weak guidance. (b) The 5 µm wide waveguide shows singlemode characteristics. (c) + (d) For the 10 µm wide waveguide two modes can be excited. (e) For the 15 µm wide waveguide only one of the numerous possible modes is shown here.

The attenuation of a 34 mm long waveguide at a wavelength of 635 nm was measured using the cutback method. As shown in Fig. 3, we determined an attenuation of 0.47 dB/cm with incoupling losses of 0.75 dB per facet. The inset in Fig. 3 shows the nearfield emission of a 5.4 µm wide waveguide. Single-mode operation with a slightly asymmetric mode profile is observed. The asymmetry of the mode is a direct result of the waveguide being an asymmetric waveguide with a cladding layer of air (n ~1.0) and a substrate layer of PDMS (n ~1.47). The result is a modal distribution with more of the light in the high refractive index PDMS substrate than the low refractive index air.

Fig. 3. Waveguide attenuation determined via the cut-back method. The linear fit results in a slope of 0.47 dB/cm and incoupling losses of 0.75 dB per waveguide facet. Inset: Modal profile of a 5.4 µm wide waveguide at a wavelength of 635 nm coupled to a single mode optical fiber.


(C) 2012 OSA


3.4. Y-Splitters We also investigated optical Y-splitters as used in marker-free sensing schemes [19] fabricated via DUV-irradiation. We coupled in laser light via a singlemode fiber and determined the insertion loss using fiber-to-fiber-coupling. As shown in Fig. 4(a) we detected the emission from the substrate edge using a camera, Fig. 4(b) shows the output from a 20 mm long Y-splitter fabricated with our DUV irradiation method. Single mode operation along with high quality splitting ratio and low stray light levels is observed. We measured the Ysplitter attenuation as 12 dB for each leg and a difference between the two legs of less than 0.5 dB by coupling in and out of the waveguide. The light microscopy image in Fig. 4(c) shows the splitting region of the splitter and reveals excellent resolution of the irradiated areas. The results demonstrate the feasibility of our concept for the fabrication of photonic circuits for sensing applications. The approach allows easy integration and use in integrated optical elements such as Mach-Zehnder-interferometers.

Fig. 4. (a) Measurement setup for the characterisation of the Y-Splitter. (b) Output from the Ysplitter. The distance between the two waveguides is 150 µm. An even power distribution between the two waveguides is observed with a difference of less than 0.5 dB. The overall attenuation is 12 dB for a 20 mm long coupler structure. (c) Light microscopy image of the splitting area of the Y-splitter.

3.5. Fluorescence excitation To demonstrate the functionality of our approach we excited fluorescence in the evanescent field of the waveguides. The detection was accomplished using a camera mounted in a normal direction to the waveguide. The fluorescent nanoparticles with a diameter of 40 nm and a peak emission wavelength of 680 nm were dispersed in DI-water onto the waveguide array. The light from a HeNe-Laser was coupled into one of the waveguides running underneath the fluosphere drop. Stray light from the incoupling fiber causes additional emission visible as haze in Fig. 5(b). To estimate the evanescent field penetration depth into the covering water layer we approximate the total reflection angle at the interface between irradiated and nonirradiated PDMS to 0°, which results in θ1 = 90°. The evanescent field penetration depth into the covering water layer can be approximated by assuming reflection at the PDMS/water interface using equation Eq. (1) with the refractive index n1 of water at 1.33 and the refractive index of PDMS at 1.4176. As evident in Fig. 5(b) the FluoSpheres are primarily irradiating from the area directly above the waveguide. Stray light from the incoupling fiber causes additional emission visible as haze in Fig. (5b). dp =

λ 2π n1 sin 2 θ1 − (

n2 2 ) n1

≈ 220nm

(1)


(C) 2012 OSA


Fig. 5. (a) Schematic of the measurement setup used for fluorescence excitation. (b) Fluorescence from the water-dispersed FluoSpheres measured through a 665 nm cut-off filter to eliminate excitation light. (c) Photograph of an optical waveguide with a 635 nm laser coupled in via a single mode fiber. A drop of water is placed on the substrate. This shows that the light is waveguided with water as cladding layer.

Operation of the waveguides was also shown with a cover of water to show the robustness of the waveguiding properties, as depicted in Fig. 5(c). Here scattering on dust particles present on the substrate makes the position of the waveguide visible. 4. Conclusions We have developed a new method to fabricate optical waveguides in PDMS by means of a low-cost low pressure Hg-lamp and a conventional quartz lithography mask. Single mode operation is demonstrated for 635 nm laser light with a waveguide attenuation of 0.47 dB/cm and incoupling losses of 0.75 dB per waveguide facet. We show that operation with a water layer on top of the waveguide is feasible, making this an attractive method for the integration into microfluidic systems. We also demonstrate excitation of 40 nm diameter fluorescent nanoparticles in the evanescent optical field of the waveguide. The easy and cost-efficient fabrication, mechanical robustness and the long term stability demonstrate the usability of the waveguides in optofluidic lab-on-chip-systems based on PDMS. Acknowledgments This work has been performed within the DFG Research Center for Functional Nanostructures (CFN). We acknowledge support by a grant from the Ministry of Science, Research and the Arts of Baden-Württemberg (Az.: 7713.14-300). We thank C. Kaiser and C. M. Ögün for their help with the UV lamp power measurements. H. Sieber acknowledges the support by the Karlsruhe School of Optics & Photonics (KSOP).


(C) 2012 OSA
