Integration of organic semiconductor lasers and single-mode passive waveguides into a PMMA substrate Christoph Vannahme a,b, * , Sönke Klinkhammer b,a , Alexander Kolew a , Peter-Jürgen Jakobs a , Markus Guttmann a , Simone Dehm c , Uli Lemmer b , Timo Mappes a a Institut für Mikrostrukturtechnik (IMT), Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany b Lichttechnisches Institut (LTI) and Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany c Institut für Nanotechnologie (INT), Karlsruhe Institute of Technology, 76128 Karlsruhe, Germany article info Article history: Received 14 September 2009 Received in revised form 18 December 2009 Accepted 21 December 2009 Available online 28 December 2009 Keywords: Polymer replication Lab-on-chip Electron beam lithography Organic lasers Waveguides PMMA abstract Efficiently combining active and passive elements in integrated optics is a key ingredient for their suc- cessful employment. Here, we present the fabrication of an optimized PMMA substrate structure for improved coupling of laser light generated by organic semiconductor distributed feedback lasers into sin- gle-mode deep ultraviolet induced waveguides. For production, electron beam lithography on an oxidized silicon wafer and subsequent reactive ion etching is used to form the feedback grating of the laser. After- wards, an aligned second electron beam lithography step on top of the grating allows the fabrication of a topographical step of 1.67 lm on the edges of the grating area. Metal is evaporated on this resulting mas- ter structure serving as a plating base for electroforming of a Ni tool. The tool is then used for hot emboss- ing of the structure into PMMA bulk material. On a length of 500 lm the imprinted grating lines, having a period of 200 nm, are 100 nm wide and 60 nm high. Aligned deep ultraviolet exposure to induce a passive single- or multi-mode waveguide and co-evaporation of the active material Alq 3 :DCM finish the coupling region. This structure optimizes the coupling of laser light generated in the laser structure into the pas- sive waveguide. In combination with microfluidic channels, the laser light can be considered for sensing applications on a PMMA lab-on-chip system. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Lab-on-chip systems (LOC) enable screening and bio-medical or chemical point-of-care analysis [1]. Optical sensing promises a high sensitivity and a reduced response time. Particularly, laser light can excite fluorescent markers very efficiently without spec- tral overlap with the marker emission [2]. On the other hand, mar- ker free detection can be achieved by utilizing optical microcavities [3]. All organic integrated photonic systems [4–6] benefit from the low costs of raw material and feasibility of using mass production technologies to fabricate them. Poly(methyl methacrylate) (PMMA) is well suited as substrate material because it can be structured by hot embossing [7] and is transparent for visible light [8,9]. This allows the integration of organic semiconductor lasers [10] by imprinting a distributed feedback (DFB) grating and thermal evaporation of an organic semiconductor material [5]. Waveguides, guiding visible light [9,11], are defined by deep ultra- violet (DUV) exposure of the PMMA. A microfluidic channel can be imprinted in addition resulting in the LOC shown in Fig. 1. Evanes- cent field coupling to DUV waveguides of amplified spontaneous emission [12] and laser light [5,8] has been shown. However, end-fire coupling promises an improved coupling efficiency. Calcu- lations have proven, that this can be realized by introducing a topographical step between the upper edges of the active region and the passive waveguide [13] (Fig. 1b). Here, we present the overall fabrication process and a characterization of this laser-to- waveguide coupling region. 2. Fabrication process 2.1. Master fabrication by electron beam lithography The grating fabrication is similar to the process described by Wang et al. [14]. For the master structure (Fig. 2a) a film of PMMA resist (MicroChem PMMA 950k A2) is spin-coated at 1000 rpm for 45 s onto an oxidized (600 nm) silicon wafer. The resist is baked out at 180 °C for 90 s. On the resulting 150 nm thick PMMA layer electron beam lithography (EBL) is performed with a dose of 700 lC/cm 2 . Subsequent developing in methyl isobutyl ketone and isopropyl alcohol (MIBK:IPA) 1:3 for 30 s and IPA for 10 s 0167-9317/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2009.12.077 * Corresponding author. Address: Institut für Mikrostrukturtechnik (IMT), Kar- lsruhe Institute of Technology, 76128 Karlsruhe, Germany. Tel.: +49 7247 823845. E-mail address: christoph.vannahme@kit.edu (C. Vannahme). Microelectronic Engineering 87 (2010) 693–695 Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: www.elsevier.com/locate/mee