NOVEL IONIC LIQUID – POLYMER COMPOSITE AND AN APPROACH FOR ITS PATTERNING BY CONVENTIONAL PHOTOLITHOGRAPHY Natalia A. Bakhtina 1 , Anja Voigt 2 , Neil MacKinnon 1 , Gisela Ahrens 2 , Gabi Gruetzner 2 , and Jan G. Korvink 1 1 IMTEK – Department of Microsystems Engineering, University of Freiburg, Germany 2 micro resist technology GmbH, Berlin, Germany ABSTRACT A novel crosslinkable, conductive, highly transparent composite material based on a photoresist and an ionic liquid (the names of the composites are not announced here due to the current procedure of patenting) is presented. The composite possesses a good and stable ionic conductivity (up to 10 mS cm -1 at room temperature) over a wide frequency bandwidth (1 kHz – 1 MHz) and is optically transparent (transmission value of 90 % for a 170 µm thick film). In addition, an approach for the patterning of the composite material by conventional photolithography with a good spatial resolution (line width of 20 – 30 µm) is introduced. The unique properties of the material are utilized for time- and cost-saving direct manufacturing of electrically conductive, highly transparent microcomponents. INTRODUCTION Optically transparent, conductive materials have a wide range of applications. They find use in sensors, solar cells, displays, and other electronic components [1]. Commonly used materials for such applications are Transparent Conductive Oxides (TCOs), such as Indium Tin Oxide (ITO), Intrinsically Conductive Polymers (ICPs), such as poly(3,4-ethylenedioxythiophene) (PEDOT), and Electrically Conductive Polymer Composites (ECPCs). Photoresists have a broad range of applications. The first main application of photoresists is their usage as resist masks or sacrificial layers for the generation of electrically conductive patterns in advanced semiconductor and micro-electrical-mechanical system (MEMS) devices via a pattern transfer process, i.e. lithography [2]. The second main application of photoresists, especially of negative-acting materials, is the direct manufacturing of permanent patterns, which can be used as a device material for the fabrication of precisely patterned and mechanically stable micro- and nano- structures in microfluidic systems [3], waveguides [4], and stamps [5]. There are a variety of crosslinkable materials for permanent applications available [6]. For some applications it is advantageous to have additional properties, like conductivity, already in the direct patternable materials to reduce process steps. There are currently several known ECPC compositions in which a photoresist (an electronic insulator in its pure state) is mixed with various conductive filler particles in order to significantly increase the conductivity of the polymerized material. The examples of conductive fillers are terthiophene (3T) with copper (II) perchlorate [7], silver nanoparticles [8], graphite [9] or carbon black particles [10], protonically doped polyaniline (PANI) nanoparticles [11], and others [12,13]. However, these materials have several limitations: (1) the addition of filler particles requires control of material viscosity by addition of various solvents, influencing the photo-polymerization process; (2) film deposition on wafers when using microparticles may have a great effect on the surface morphology and, as a result, may lead to inhomogeneous conductive layers; (3) the resolution is constrained to 10 – 30 μm because of the light diffraction by filler particles [7, 8, 9]; (4) and finally, the cured material has low optical transparency over the visible range. Ionic liquids (ILs) have appeared in recent years as novel compounds in materials research, and are already used in industrial processes due to several attractive characteristics [14,15]. ILs feature high ionic conductivity (up to 30 mS cm -1 ), good solubility, low volatility, low flammability, and high thermal, chemical, and electrochemical stability. ILs have the immense advantage over ECPCs of being transparent and easy to produce, with a wide variety of anion/cation combinations which can be adjusted to tailor their physical properties. ILs seem to be promising materials for applications not only in flexible electronic devices, such as displays and photovoltaics [16], but also in electrochemical biosensors because of their good compatibility with biomolecules and enzymes, and even whole cells [17,18]. While ILs are conductive, the liquid nature of ionic liquids is an obstacle to applications where a predefined physical shape is required. For these applications, a process of solidification is necessary which constrains the liquid. It comprises the formation of a three-dimensional solid structure in the form of a polymer matrix, which entraps the ionic liquid in the porous network. In this material, liquid-like properties (e.g. charge transport) originate from the ionic liquid, whereas solid-like properties originate from the host polymer, which contributes to material flexibility while preventing the system from flowing. Accordingly, this paper reports a new polymeric material based on a negative-acting photoresist (A, B, or C) and the ionic liquid (the names of the composites are not announced here due to the current procedure of patenting). The IL-polymer composites are ionically conductive (up to 10 mS cm -1 at room temperature) over a wide frequency bandwidth (1 kHz – 1 MHz) and are optically transparent (transmission value of 90 % for a 170 µm thick cast film). Conventional photolithography enables a single-step process for structuring of the material with a good spatial resolution (line width of 20 – 30 µm). Combining the advantages of both the material and the fabrication technique, time- and cost-saving direct manufacturing of electrically conductive microcomponents is demonstrated. 978-1-4799-7955-4/15/$31.00 ©2015 IEEE 97 MEMS 2015, Estoril, PORTUGAL, 18 - 22 January, 2015