Fabrication of Optical Interconnects with Two Photon Polymerization

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1 Fabrication of Optical Interconnects with Two Photon Polymerization Klaus STADLMANN* 1,Klaus CICHA* 1, Josef KUMPFMÜLLER* 2, Volker SCHMIDT* 3, Valentin SATZINGER* 3, Jürgen STAMPFL* 1, Robert LISKA* 2 * 1 Institute of Material Science and Technology, Vienna University of Technology, Favoritenstraße Vienna, Austria kstad@mail.tuwien.ac.at * 2 Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Wien, Austria * 3 Institute of Nanostructured Materials and Photonics, Joanneum Research GmbH, Franz-Pichler-Straße 30, A-8160 Weiz, Austria Two Photon Polymerization (2PP) is a direct 3D structuring technique based on femtosecond laser technology. One of the key advantages of 2PP is the fact that writing inside a given volume is possible. This enables the fabrication of optical interconnects by using suitable matrix materials, leading to a promising future industrial application. This paper reports how waveguides can be structured directly into a matrix made out of Poly(dimethyl siloxane) (PDMS) using 2PP.To fulfill the economical aspects of a fabrication of optical interconnects, a cost effective, mechanically flexible and temperature-resistant matrix material with a low attenuation in optical applications (smaller than 0.1 db/cm at 850 nm wavelength) such as PDMS is needed. For structuring, a femtosecond laser beam is focused by a 20x microscope objective with a numerical aperture of 0.8 or 0.4, respectively. To increase the structuring speed we used a special 2PP photo initiator developed at Vienna University of Technology. This highly efficient initator is able to increase the structuring window for 2PP applications and offers a high structuring repeatability. To increase the structuring speed from the physical point of view, a new femtosecond oscillator with a maximum average power of over 600mW was utilized. Keywords: Nanostructuring, High-Resolution Parts, Waveguides, Optical interconnects, Femtosecond laser, Two-Photon-Lithography, Optoelectronics, PDMS 1. Introduction Two Photon Polymerization (2PP) is an innovative additive manufacturing technology (AMT) [5]. 2PP has recently gained a lot of interest as fabrication method for complex three-dimensional submicron structures. Parts produced with this technology can be used in various applications e.g. mechanical, electronic and optical micro-devices. In this study a method for evaluating photopolymers in terms of their usability for 2PP is reported. Most of the monomers used in literature for structuring with 2PP contain a one photon initiator. To start the 2PP process the concentration of commercial initiators, for example Irgacure 369, needs to be high. In order to reduce the initiator concentration, several dedicated Two-Photon- Initiators (TPI) were synthesized [7]. These TPI s are designed for a multi photon application; therefore the amount of initiator to start the polymerization process was significantly reduced. By introducing these new TPI s the structuring window was increased towards higher structuring speed. We applied one of the developed TPI s to structure waveguides into a matrix material swollen by a monomer formulation containing the TPI. The major aim of this work was to structure waveguides. To achieve this, a thin film of the PDMS matrix material is coated onto a glass slide and then covered with a liquid monomer. After a swelling time of 4-6 hours, the matrix is infiltrated with the monomer and ready for the structuring process. To reduce the time consuming step of swelling the matrix material with the photo curable monomer, another promising concept was developed. This system makes it possible to cure the PDMS matrix in the presence of a monomer. Optical waveguides were written using 2PP to induce a classical radical polymerization to monomers with a high refractive monomer [4]. To fulfill the need of a high feed-rate for an upcoming industrial application, the following major issues have to be considered: The reactivity of the formulation (matrix material swollen by the monomer) has to be sufficiently high. 1

The utilized initiator has to exhibit a high twophoton cross-section. The optical setup for shaping the waveguides has to be tuned to achieve the targeted diameter of the waveguides.

2 The utilized initiator has to exhibit a high twophoton cross-section. The optical setup for shaping the waveguides has to be tuned to achieve the targeted diameter of the waveguides. Average laser power 2. Experimental setup In the Micro-3-Dimensional Structuring System M3DL we use a femtotrain Ti:Sapphire oscillator which was especially designed by High Q Lasers for the waveguide application. This compact all diode pumped solid state oscillator emits light with a wavelength of 793nm and a typical pulse width of 100fs. The pulse repetition rate is 73MHz and the average output power is 690mW. Fig.1: M3DL scanner setup (left); M3DL detail of the bottom-up structuring layout (right) As shown in the schematic drawing (Fig.2), the laser beam first passes through a collimator positioned after the laser head. Then the first order of the collimated beam passes through a rotating λ/2 wave-plate. By placing a polarized beam splitter after the wave-plate the laser power coupled into the objective can be controlled. The key component of the M3DL is the X - Y scanner. It consists of two linear air-bearing stages. These two stages carry the complete optical setup including a CCD-Camera for live imaging and the microscope objective. These air bearing stages have very high position accuracy at high structuring speed. Of course the stage speed is limited due to the hatch resolution, the resulting acceleration and the deceleration, nevertheless structuring speeds up to 30 mm/s are possible. The M3DL setup is designed to enable the fabrication of large structures (build volume of 150 x 100 x 100mm) with high writing speeds. The use of highprecision air-bearing stages allows the fabrication of structures with an accuracy of 50nm over the whole building envelope. The X-Y scanner and the building envelope of the M3DL is shown in figure 1. The second structuring system which was used is located at the Institute for Nanostructured Materials and Photonics. It consists of a Ti:Sapphire oscillator (Maitai) and a Ti:Sapphire amplifier (Spitfire). The repetition rate provided by the oscillator is 80 MHz and the pulse duration is approximately 80 fs. The average power of the amplifier is 1W. The pulse duration is 150 fs and the repetition rate changes to 1 khz [8],[9]. Fig.2: Schematic drawing of the physical M3DL setup 2.1 Microscope objectives For structuring waveguides into the matrix material we use two different 20x microscope objectives with numerical apertures of 0.8 or 0.4, respectively. This rather low magnification for a 2PP processes, is necessary to achieve suitable waveguide cross section dimensions in the region of 20µm for a single waveguide. The diameter of an embedded optical interconnect is preset by the focal volume of the objective. This objective offers a suitable working distance around 720µm. The typical PDMS film thickness in our experiments varies from typically 300µm up to 500µm. Another benefit of a rather long working distance is that the residual monomer left on the surface cannot coat the objective lens during the structuring process. 2

3. 2PP Initiators Proceedings of LPM2010 The 11th International Symposium on Laser Precision Microfabrication B3K [7] is a novel highly efficient TPI.

Good initiation reactivity towards monomer is brought along by the ketongroup. The chemical structure of B3K is shown in figure 3. Fig.3: B3K 3.1.

Most building parameters (size 50µm; hatchperiod 10µm; layer-distance 0,7µm; 20 layers) of the structure were held constant. The laser power and writing speed were adapted to the needs of the TPI.

3 3. 2PP Initiators Proceedings of LPM2010 The 11th International Symposium on Laser Precision Microfabrication B3K [7] is a novel highly efficient TPI. The large twophoton cross-sections of B3K can be explained by the length and the coplanarity of the pi-conjugated systems. Good initiation reactivity towards monomer is brought along by the ketongroup. The chemical structure of B3K is shown in figure 3. Fig.3: B3K 3.1. Initiator Screening For the TPI screening, arrays of standard structures were fabricated. Most building parameters (size 50µm; hatchperiod 10µm; layer-distance 0,7µm; 20 layers) of the structure were held constant. The laser power and writing speed were adapted to the needs of the TPI. The fabricated structures were classified in terms of quality and thickness of hatch-lines, building- imperfections, geometry-quality and overall impression. Fig.5: Initiator screening evaluation of B3K B3K Fig. 6: Waveguide monomer screening, Parameters: Power 23mW, P=20mW, Speed 50 µm/s V=20 µm/s Fig.4: REM TPI screening picture, B3K in acrylate based resin. 4. Monomers for waveguide application Various ways for structuring waveguides by 2PP have been reported, e.g. by selective curing of a component in a resin mixture [2],[3] or by direct writing into various glass materials [6]. For structuring waveguides into a swollen PDMS matrix only monomers with higher refractive index than the matrix itself were investigated and tested. Beside the refractive index there are more factors which are important for our application: Proper compatibility with the host material 3

Uncured monomer must be removable under vacuum or/and increased temperature Only small evaporation during the structuring process After testing several of monomer mixtures Acrylic acid isobornyl

4 Uncured monomer must be removable under vacuum or/and increased temperature Only small evaporation during the structuring process After testing several of monomer mixtures Acrylic acid isobornyl ester (AIB) and 1,4-Butadien diacrylate (BDA) were chosen for system Sample preparation and structuring weight increase upon swelling [%] RT601 30% AIB 70% BDA(1) 5 60% AIB 40% BDA(2) 70% AIB 30% BDA(3) 50% AIB 50% BDA(4) sample swelling time [h] Fig.9: Weight increase of a 280µm thick silicone matrix after swelling with monomers of different AIB concentration. Fig.7: Basic structuring principle for waveguides. A commercial silicone rubber (RT 601, Wacker Chemie) is used as matrix material. In the first step the silicone rubber compound is coated onto a glass substrate with a thickness between µm. After the curing time of the silicone according to the data sheet the matrix is swollen by a monomer. For the swelling process we formed a monomer reservoir consisting of two glass slides one covered with the matrix material - with a spacer of approximately 500µm in between. The swelling time is defined by the ratio of AIB/BDA in the monomer mixture. The higher the concentration of AIB, the lower the swelling time can be since AIB-containing resins swell the silicone matrix very fast. The reactive monomer formulation contains wt% of the TPI. After 4-6 hours an equilibrium state of the dissolved monomer in the matrix is reached. Figure 9 shows the standard structuring process for AIB/BDA compound (system 1) including the steps swelling, structuring and evaporating the uncured monomer left in the matrix. To avoid the time consuming swelling procedure we alternatively applied an acetoxy condensation curing polysiloxane system with phenyl groups as mentioned in another work [4]. For the chemical compound 2, a mixture of the monomers phenyl thioethyl acrylate (PTEA) as well as BDA were applied. After direct laser writing, the monomers were evaporated from the non-illuminated regions at elevated temperature and reduced pressure. 6. Light microscopic investigation After the evaporation of the uncured monomer the sample was reviewed using a phase contrast light microscope. By cutting the sample into several parts and coupling in light from the microscope condenser, the cross section of the waveguide or the waveguide bundle can be obtained. Fig.8: Sample preparation and writing process 4

Proceedings of LPM2010 The 11th International Symposium on

10: Light microscope image of a waveguide array system 1

13: Light microscopy image of a waveguide bundle cross

14: Light microscopy image of waveguides structured into PTEA

12: Cross section of a waveguide bundle system 1, 60/40wt%

5 Proceedings of LPM2010 The 11th International Symposium on Laser Precision Microfabrication Fig.10: Light microscope image of a waveguide array system 1 60/40wt% (AIB/BDA), average laser power µW Fig.13: Light microscopy image of a waveguide bundle cross section inscribed into system 1 40/60wt% (AIB/BDA), average laser power 180µW Fig.11: waveguide array (AIB/BDA), µm 60/40wt% Fig.14: Light microscopy image of waveguides structured into PTEA (chemical compound 2) Fig.12: Cross section of a waveguide bundle system 1, 60/40wt% (AIB/BDA), average laser power 180µW Fig.15: Phase contrast image of a single waveguide structured in chemical compound 1, 30/70wt% (AIB/BDA), laser power 150mW (laser oscillator) cross section 5

6 7. Summary PDMS was tested as a matrix for optical waveguides structured by 2PP. A flexible PDMS matrix was swollen by a monomer formulation, which was photopolymerized selectively to increase the refractive index in the exposed area. Afterwards the uncured remaining monomer was removed at increased temperature. The degree of swelling was tailored by increasing the amount of BDA, which reduced swelling and acted as a crosslinker during photopolymerization. The achievable change of refractive index using this method was in the range of n = ( n/n = 0.4 % %). The measured optical losses were around 2.3dB/cm. 8. Acknowledgement We acknowledge the financial support by the Österreichische Forschungsförderungsgesellschaft (FFG), project ISOTEC III. 9. References [1] N. Fatkullin, I. Takayuki, H. Jinnai, H.-B. Sun, R. Kimmich, T. Nishi, Y. Nishikawa, and S. Kawata: NMR/3D Analysis. Photopolymerization. 1st ed., pp , Springer, Berlin, glasses with ultrashort pulse laser. Applied Physics Letters 71, no. 23 (1997): [7] N. Pucher, A. Rosspeintner, V. Satzinger, V. Schmidt, G. Gescheidt, J. Stampfl, and R. Liska: Structure Activity Relationship in D-π-A-π-D- Based Photoinitiators for the Two-Photon-Induced Photopolymerization Process. Macromolecules 42, no. 17 (2009): [8] V. Schmidt, L. Kuna, V. Satzinger, R. Houbertz, G. Jakopic, and G. Leising: Application of twophoton 3D lithography for the fabrication of embedded ORMOCER waveguides. In Proceeding of the SPIE, :4760P. Session 7. San Jose, California, USA: Louay A. Eldada, El-Hang Lee, [9] J. Stampfl, R. Inführ, K. Stadlmann, N. Pucher, H. Lichtenegger, V. Schmidt, and R. Liska: Materials for the fabrication of optical waveguides with two photon photopolymerization. In Lasers in Manufacturing 2009, S.:P1-P4. Munich, Germany: A. Ostendorf, [2] J. Ishihara, K. Komatsu, O. Sugihara, and T. Kaino: Fabrication of three-dimensional calixarene polymer waveguides using two-photon assisted polymerization. Applied Physics Letters 90, no. 3 (2007): [3] M. Joshi, H. Pudavar, J. Swiatkiewicz, P. Prasad, and B. Reinhardt: Three-dimensional optical circuitry using two-photon-assisted polymerization. Applied Physics Letters 74, no. 2 (1999): 170. [4] J. Kumpfmüller, N. Pucher, K. Stadlmann, J. Stampfl, and R. Liska: Photo-induced waveguide formation in Polysiloxanes. RadTech Europe 2009, Proceedings presented at the RadTech Europe 2009, Nizza, France, October 14, [5] M. Meadows: NMR, 3D Analysis, Photopolymerization. Advances in Polymer Science, 170, Springer-Verlag: Berlin, Heidelberg x pp. $ ISBN Journal of the American Chemical Society 127, no. 32 (2005): [6] K. Miura, J. Qiu, H. Inouye, T. Mitsuyu, and K. Hirao: Photowritten optical waveguides in various 6

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