Thin Film Stoichiometry Determination by High Temperature Microbalance Technique
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EE11.4.1
Thin Film Stoichiometry Determination by High Temperature Microbalance Technique H. Fritze1), H. Seh2), O. Schneider1), H. L. Tuller2), G. Borchardt1) 1) Technische Universität Clausthal, Department of Physics, Metallurgy and Materials Science, Robert-Koch-Straße 42, D-38678 Clausthal-Zellerfeld, Germany, [email protected]. 2) Massachusetts Institute of Technology, Department of Materials Science & Engineering, 77 Massachusetts Avenue, Cambridge, MA 02139, U.S.A. ABSTRACT The in-situ determination of small mass changes of thin films became feasible with the availability of high temperature stable microbalances. With this technique, changes of the mechanical properties of thin films deposited on piezoelectric resonators are investigated at temperatures above 500 /C by monitoring the resonance behavior of the resonators. The results are valuable for fundamental understanding of the ionic and electronic transport processes in ceramic materials and for applications such as high temperature gas sensors. This work correlates the electrical and the mechanical properties of TiO2-x at different oxygen partial pressures. TiO2-x films are deposited onto high temperature resonators by laser ablation and characterized by the high temperature microbalance technique as well as electrical impedance spectroscopy at 600 /C. The oxygen partial pressure dependent resonance behavior cannot be attributed solely to mass changes of the TiO2-x film. Changes of the film’s mechanical stiffness have to be taken into consideration to explain the resonance behavior. The simultaneous electrical impedance measurements indicate a n-type conduction behavior of the TiO2-x films. INTRODUCTION Piezoelectric materials such as gallium orthophosphate (GaPO4), langasite (La3Ga5SiO14, LGS) and related compounds are promising candidates for a wide range of new high temperature applications. The operation temperature of piezoelectric devices may, in principle, be extended up to the phase transformation at 970 and 1470 /C for GaPO4 [1] and LGS [2], respectively. Our previous work demonstrated the suitability of LGS as bulk acoustic resonator up to 900 /C [3]. The application in high temperature microbalances (HTMBs) is of particular interest. Very small mass changes during (1) film deposition onto resonators or (2) gas composition dependent stoichiometry changes of thin films already deposited onto resonators can be correlated with the resonance behavior of bulk acoustic wave resonators. In addition, the mechanical properties such as density and stiffness can be extracted. This work demonstrates the oxygen partial pressure dependent behavior of a TiO2-x coated HTMB. Partial reduction of the TiO2-x film is expected with decreasing oxygen partial pressure, pO2. Therefore, the resonance behavior of the HTMB is governed by (1) the mass loss and (2) the decreased shear modulus of the partially reduced film. EXPERIMENTAL The resonators of the high temperature microbalance are prepared from polished LGS plates. They are cut from the same crystal
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