214 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 13, NO. 2, MARCH/APRIL 2007 A Bio–Fluidic–Photonic Platform Based on Deep UV Modification of Polymers Dominik G. Rabus, Member, IEEE, Mathias Bruendel, Yasuhisa Ichihashi, Alexander Welle, R. Adam Seger, Student Member, IEEE, and Michael Isaacson, Member, IEEE Abstract—We present a review of our work on deep UV (DUV) modification of methacrylate-based polymers. This technology serves as a platform for realizing planar and ridge waveguide- based devices, fluidic channels, and enables the patterning of living cells including neural cells in vitro. Details on the DUV chemistry and fabrication technologies including hot embossing of multimode interference couplers will be given. Index Terms—Deep UV (DUV), photonic integrated circuits, polymer. I. INTRODUCTION F UTURE micro- and nanosystems require fabrication tech- nologies for integrating several functionalities coming from different disciplines, like photonics, fluidics, and biology. Materials well suited for serving these fields are polymers. Poly- mers are favorable substrates for biophotonic devices due to the biocompatibility, the fabrication flexibility they offer, and their low cost [1], [2]. Polymers also have the advantage of acting as sensitive layers, which is the prerequisite for the optimiza- tion of chemical and biochemical sensors [3]. Consequently, the fabrication of optical waveguides on polymeric substrates has the potential to solve a major integration problem. Using well- known polymers like methacrylate-based polymers opens up the possibility to use readily available micro–nano fabrication tools like lithography or hot embossing. Deep UV (DUV) modification of methylmethacrylate (MMA) polymers is not only useful for creating optical- waveguide-based devices, but also leads to a new surface chem- istry affecting the selective absorption of proteins and the adhe- sion of living cells in vitro. The bi-functionality of the modified polymer chips supporting waveguides and cell anchorage capa- bilities at the same time can serve as a building block for future biophotonic integrated circuits (Bio-PICs). In this paper, we demonstrate that DUV-induced modification of methacrylate polymers can serve as the core technology for Manuscript received September 5, 2006; revised October 6, 2006. The work of D. G. Rabus was supported by Alexander von Humboldt Foundation and Baskin School of Engineering, University of California–Santa Cruz, through the Narinder Kapany Endowment. D. G. Rabus, R. A. Seger, and M. Isaacson are with Baskin School of En- gineering, University of California–Santa Cruz, Santa Cruz, CA 95064 USA (e-mail: rabus@ieee.org; aseger@soe.ucsc.edu; msi@soe.ucsc.edu). M. Bruendel and Y. Ichihashi are with the Institute for Microstructure Technology, Forschungszentrum Karlsruhe GmbH, 76021 Karlsruhe, Germany (e-mail: mathias.bruendel@imt.fzk.de; yasuhisa.ichihashi@imt.fzk.de). A. Welle is with the Institute for Biological Interfaces, Forschungszentrum Karlsruhe GmbH, 76021 Karlsruhe, Germany. He is also with CeloNova Bio- sciences, Inc., 89077 Ulm, Germany (e-mail: alexander.welle@ibg.fzk.de). Digital Object Identifier 10.1109/JSTQE.2007.892073 realizing optical-waveguide-based devices, integrated fluidic channels, and patterning of living cells. The advantage of using this technology is that only a single material is required and no etching or deposition is necessary when compared to other processes. The paper is organized as follows. Section II describes the basic properties of DUV-induced modification. Section III pro- vides a summary on fabricated planar waveguide-based devices. Section IV focuses on ridge waveguide-based devices using hot embossing. Section V gives a basic introduction into flu- idic channels fabricated with DUV lithography in polymethyl- methacrylate (PMMA). Section VI describes DUV patterning of living cells including neural cells. The paper ends with a conclusion in Section VII. II. DUV MODIFICATION OF POLYMERS An extremely useful class of low-cost thermoplastic poly- mers, which exhibit a significant increase in refractive index through application of ionizing radiation, such as ion and DUV radiation, are MMA polymers. It was shown that optical waveg- uides can be created in PMMA homopolymers by ion [4], [5] and DUV radiation [6], [7]. The DUV-induced refractive in- dex change in unreacted MMA is found to depend on the flu- ence. DUV exposure of thermally crosslinked MMA can induce further structural changes, and thus, a refractive index change was found to be useful for creating the core layers in optical waveguides. Two types of polymers have been investigated. PMMA (Hesa@Glas, a homopolymer from Notz-Plastic, Switzerland) and alicyclic methacrylate copolymers that were obtained from Hitachi Chemical as OPTOREZ-series (OZ-series). For DUV modification, two commercial UV-exposure sys- tems are used: the first is UVAPRINT CM, H¨ onle, having a 100-W/cm mercury xenon arc lamp (F-Lamp, H¨ onle) combined with a cold mirror with reflectance in the range of 220–420 nm in the exposure system. The resulting output is 0.6 mW/cm 2 at 240 nm. The second and recently acquired system is a mask aligner EVG620 having a DUV lamp combined with a cold mir- ror with reflectance in the wavelength range of 200–240 nm in the exposure system. UV absorbance spectra were measured in transmission by using a 3-μm-thick PMMA 950 K film on a quartz disk af- ter different irradiation doses. Irradiation was performed in ambient air and vacuum (Fig. 1). The increasing absorption peak below 200 nm is due to the generation and reaction of unsaturated bonds. This peak is due to a π–π ∗ transition of C=C bonds [8], [9]. In air, this absorption peak decreases 1077-260X/$25.00 © 2007 IEEE