Influence of size variation on the strain distribution in YSZ membranes on Si substrates
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Influence of size variation on the strain distribution in YSZ membranes on Si substrates Florian Kuhl1, Markus Piechotka1, Daniel Reppin1, Torsten Henning1, Juergen Janek2, and Peter J. Klar1 1 I. Physikalisches Institut, Justus-Liebig-Universität, Gießen, Germany. 2 Physikalisch-Chemisches Institut, Justus-Liebig-Universität, Gießen, Germany. ABSTRACT The oxygen conductor yttria-stabilized-zirconia (YSZ) is widely used in miniaturized solid oxide fuel cells (µSOFC) and may be suitable for solid state ion emitter applications e.g. as miniaturized ion engines for electric propulsion. Since the YSZ films are not completely free of stress during the growth, cracks in fabricated free-standing membranes are often observed. YSZ thin films were deposited on silicon substrates by radio frequency sputtering. Freestanding YSZ membranes were fabricated by partially removing the Si substrate by anisotropic wet-chemical etching using different masking patterns defined by electron beam lithography. We show how different sizes and etching conditions influence the strain in the fabricated membranes. To characterize these membranes we used optical microscopy and scanning electron microscopy. INTRODUCTION For the application in micro solid oxide fuel cells (µSOFC), yttria-stabilized-zirconia (YSZ) is an important material as electrolyte layer [1]. SOFCs basically consist of an anode, a cathode, and an electrolyte. Chemical fuel cells include a higher energy density than common Li‑ion or Ni‑MH batteries. Of all chemical fuel cells, the SOFCs have the highest energy density, e.g. DMFC (direct methanol fuel cell) and PEMFC (proton exchange membrane fuel cell) are not so efficient [2, 3, 4]. Another advantage over batteries is the possibility of a changeable fuel container [5], so that there is no need for charging cycles of the power source. For common SOFCs the working temperature is very high (e.g. for YSZ above 800 °C). Thus the main goal of the miniaturization of SOFCs is the reduction of the electrolyte thickness, because one can lower the operation temperature due to a smaller diffusion path length. Further improvement in size reduction of the fuel cells makes them an alternative power source in portable devices [2, 6, 7]. Therefore membranes out of YSZ and there fabrication processes have been studied intensively. The sizes of membranes that are reported in literature vary from 20 µm x 20 µm [8] to 1000 µm x 1000 µm for flat membranes [9] and 2000 µm x 2000 µm when using corrugated substrates [10]. The fabrication processes are MEMS-based and there exist different membrane geometries [2]. Concerning free-standing membranes strain issues were observed very often [11, 12, 13]. In this work we describe the correlation between membrane size and its stability. We focus on the anisotropic (100) silicon wet etching process. EXPERIMENT Properties of the used YSZ thin films Thin films of YSZ were grown by radiofrequency sputtering. The sputter target is a commercially available target of ZrO2 stabilized with 9.5 mol% of Y2O3, the dimensio
