Characterization of chemical and sensoric properties of ion-beam modified polyethersulfone layers

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Characterization of chemical and sensoric properties of ion-beam modified polyethersulfone layers Karin Sahre, Klaus-Jochen Eichhorn, Frank Simon Institute of Polymer Research Dresden, Hohe Strasse 6, 01069 Dresden, GERMANY Margaritha Guenther, Gunnar Suchaneck, Gerald Gerlach Dresden University of Technology, Institute for Solid State Electronics, 01062 Dresden, GERMANY ABSTRACT Thin polyethersulfone (PES) films were irradiated with boron ions at doses between 101310 B+/cm2 and energies of 50, 130 and 180 keV. The induced changes of structural and optical properties were studied in detail. The influence on sensoric properties (moisture uptake, conductivity) was also in the focus of interest. The results of vibrational spectroscopy and X-ray photoelectron spectroscopy (XPS) indicate that in dependence on dose and energy of implanted boron ions the sulfone bonds are destroyed, and carbon-rich, amorphous and graphite-like structures are formed. This structural changes cause an increase of the conductivity of the surface films to a value of 10-8 Ω -1 at a dose of 1016 B+/cm2 from an asprepared value of 10-15 Ω -1 and an increase of the saturation concentration during the moisture uptake. 16

INTRODUCTION Thin polymer films are often used as functional layers for new microelectronic sensors. A modification of the polymer structure in the film can lead to enhanced mechanical, optical and electrical properties. Ion implantation is a preferred method to modify polymer structures as we had shown on aromatic polyimides recently [1]. PES films are increasingly important to application as such functional layers. Former evaluations [2] had proved its potential suitability for the use as sensitive layer in bimorphic humidity and gas sensors. This paper presents detailed studies of aromatic PES layers modified by boron ions. The investigations were carried out by means of several methods of vibrational spectroscopy (FTIR- and Raman spectroscopy), XPS and spectroscopic ellipsometry . The combination of these analytical methods gives a profound picture of the main processes in the layer during boron ion bombardment. Furthermore, we investigated sensoric properties of PES films especially the swelling behavior in dependence on relative humidity using a corresponding bimorphic humidity sensor. The influence of ion-beam induced modification on the moisture uptake was determined by Freundlich´s coefficient. Results concerning the change of the electrical properties dependent on dose and energy of implanted ions are also reported. EXPERIMENTAL DETAILS PES was dissolved in N-methyl-2-pyrrolidone (20 wt.% PES / NMP) and spin-coated on silicon wafers. Prior to coating the thermal oxide layer on the silicon wafers was modified with a coupling agent. After deposition the solvent was removed at 90 °C for 30 min and then the system was stabilized at 400 °C for 60 min. The final thickness of remaining films was typically between 1.0 and 2.5 µm.

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Fig. 1 Chemical structure of PES PES films were irradiated to modify their su

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