Thermally stable multi-mode polymer optical waveguide fabricated by single-step photo-patterning of fluorinated polyimide/epoxy hybrids Yuichi Urano* a , Ningjuan Chen a , Kaichiro Nakano b , Katsumi Maeda b , and Shinji Ando a a Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, 152-8552, JAPAN b Nano Electronics Research Laboratories, NEC Corporation, Nakahara, Kawasaki, Kanagawa, 211-8666, JAPAN ABSTRACT Novel polyimide (PI)/epoxy hybrid material for single-step photo-patterning of optical waveguides was prepared by blending of a semi-aromatic fluorinated poly(amic acid silylester) (PASE), a cycloaliphatic epoxy compound, and a photo-acid generator. A large refractive index change (δn, > 0.01), which is sufficient for multi-mode optical waveguides, was obtained between the polymer films prepared with and without UV irradiation. The refractive index change was generated by cationic reaction between the silylated carbonyl ester groups of PASE and epoxy rings, which was initiated by UV irradiation and promoted by successive thermal curing. The difference in molecular structures, which results in the refractive index changes, were characterized by FT-IR measurements, and it was clarified that the films with and without UV irradiation showed PASE and PI structures, respectively. These films exhibited high thermal stability higher than 230°C, which are desirable for waveguide fabrication for optical inter-connects and lightwave circuits. Using this hybrid material, channel-type optical waveguides were successfully fabricated by the single-step photo-patterning procedure without development by aqueous or organic solvents, which is more facile and economical for waveguide mass-fabrication. Keywords: Polymer optical waveguide, Fluorinated polyimide, Photo-induced refractive index change, Photo patterning 1. INTRODUCTION Polymer optical waveguides have attracted much attention in the fields of integrated optics and optical inter-connects, because of their economic efficiency, good processability and flexibility [1]. However, conventional polymer optical waveguides do not have high thermal stability and demonstrate high propagation losses at the optical communication wavelengths (1.3 and 1.55 µm) in the near-infrared region. The higher optical losses are owing to the stronger vibrational absorptions due to plural kinds of carbon-hydrogen (C-H) bonds compared Si-O(H) bonds in amorphous silica (SiO 2 ). In order to reduce the losses, substitution of fluorine or deuterium for hydrogens in polymers have been reported for poly(methyl methacrylate) and epoxy resins [2−4]. Moreover, optical waveguides using fluorinated polyimide [5], benzocyclobutene [6] or poly(arylene ether sulfide) [7] have been investigated to improve not only propagation losses, but also thermal stability. Among these polymers, polyimides (PIs) have been widely used in microelectronics and opto- electronics industries because of their outstanding characteristics such as thermal stability, mechanical strength, resistance to organic solvents, and good processability [8]. Furthermore, several types of fluorinated and perfluorinated PI have been developed as optical waveguide materials due to their low water sorption and high transparency in the visible and near-IR regions. The polymer optical waveguides have been fabricated by the conventional photo- lithographic procedures, consisting of many processes including photo-resist patterning, development using aqueous or organic solvents, and etching [9−15]. In recent years, there have been demands for ‘single-step photo-patterning procedures’ for direct fabrication of channel- type (buried-type) waveguides without using photolithographic procedures (i.e. photo-resist patterning and dry- or wet- etching processes). Single-step photo-patterning is advantageous in process efficiency, low manufacturing cost and environmental friendliness [16]. Hence, many types of polymer materials, in which photo-chemical reactions can induce changes in refractive indices between exposed and unexposed areas, have been reported [16−21]. For example, polymer films doped with photochromic dyes can induce a large change in refractive index of ~0.01 [19−20], but the removal of remained reactants is a serious problem for fabricating highly transparent films. Further, cycloaliphatic-type epoxy