Advanced Nanocomposite Polymer Films for Bimorphic Gas Sensors
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Advanced Nanocomposite Polymer Films for Bimorphic Gas Sensors G. Gerlach, M. Guenther, G. Suchaneck, Dresden University of Technology, Institute for Solid State Electronics, 01062 Dresden, GERMANY K. Sahre, K.-J. Eichhorn Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, GERMANY A. Deineka, L. Jastrabik Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praque 8, CZECH REPUBLIC ABSTRACT Thin films of aromatic polymers such as polyimide (PI) and polyethersulfone (PES) find an extensive use in aerospace and electronic applications, in particular, as sensitive to moisture and gas uptake layers for bimorphic sensors. In this work, a complex investigation of the film composition, microstructure and physical properties of ion beam modified polymer films was carried out to optimize the moisture uptake. To modify thin films of polyimide and polyethersulfone 50, 130 and 180 keV boron ions with irradiation doses between 1013 and 1016 B+/cm2 were implanted. It could be shown, that partly destruction of chemical bonding under ion bombardment leads to the creation of new amorphous and graphite-like structures, which increase the modified surface film conductivity by several orders of magnitude and enhances the sensitivity of these nanocomposite films to moisture uptake. INTRODUCTION Thin polymer films are broadly used as functional layers for microelectronic sensors. Humidity-induced changes in the polymer properties are applied in bimorphic humidity sensors [1,2] , which operate by monitoring of the humidity-induced volume expansion of a thin polymer
layer. It has been shown in [3,4] , that water molecules are sorbed in existing micropores throughout the polymer layers, but no specific interactions with characteristic polar groups of the several polymers have been observed. The polymer’s sensitivity to moisture and gas uptake can be significantly increased by surface ion-beam modification [5,6] . The changes in structure and physical properties of these nanocomposite films, induced by boron ions bombardment, are described in the present paper. EXPERIMENTAL DETAILS The polyimides (PI2808: PMDA-ODA; PI2566: 6FDA-ODA; PI2611: BPDA-PPD) and polyethersulfone (PES UE2010) chosen for our study are the same widely used by the semiconductor industry. The samples were deposited by spin coating of polyimide precursor solutions (polyamic acids / N-methyl-2-pyrrolidone) and polyethersulfone solution (20 wt.% PES / NMP) onto silicon substrates. The polymer films were soft-baking at 90° C for 20 min to remove solvent and then KK2.8.1
they were cured in nitrogen at 400° C for 1 h. The completion of this chemical reaction was checked by FTIR spectroscopy. The final film thickness was typically between 1.0 and 3.5 µm. The polymer films were irradiated with B+ ion beam (energy: 50, 130 and 180 keV; dose: 13 10 ...1016 B+/cm2). The ion implantation was performed both at the Forschungszentrum Rossendorf, Germany, and AktivSensor GmbH, Stahnsdorf, Germany. The range of boron ions and the do
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