Phase Transformations in Sol-Gel PZT Thin Films
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These deposition methods rely upon depositing the material in a solution
containing the proper
precursor elements via spin coating, with subsequent heat treatments on the substrate - film system. Three heat treating steps are often used: a low temperature heat (=100 'C) is sometimes used to remove solvent from the film; a higher temperature pyrolosis step (=350 °C) removes the organic components; and a high temperature (=700 °C) heat treatment to form the perovskite crystal structure. Two materials dominate the literature in terms of substrates for PZT thin films deposited via solution methods: platinum and ruthenium oxide (RuO 2) coated silicon wafers. It has been suggested that platinum [5] assists in the formation of the perovskite crystal structure, either through a lattice match that would encourage epitaxy or the formation of secondary phases which behave in a similar manner. Additionally, the chemical composition of the film has been shown to be very important if forming the perovskite crystal structure [6]. Two analytical techniques have been used in other studies [7,8] to characterize these materials, transmission electron microscopy (TEM) and x-ray diffraction (XRD). While there have been studies of phase transformations during the crystallization process using XRD, an in situ TEM heating of the pyrolyzed material would allow both crystal structure and morphology to be directly monitored in these thin films, in much the same way which in situ TEM has been used to track the crystallization process of barium titanate thin films made via sol-gel processing [9]. Therefore, this study was undertaken to examine the phase transformations which occur during the crystallization of PZT thin films with and without the presence of a Pt electrode. Of particular interest is the ability to perform in situ TEM heating experiments. As the vacuum conditions in 185
Mat. Res. Soc. Symp. Proc. Vol. 623 © 2000 Materials Research Society
the TEM differ significantly from those in ambient of PbO environments (used by some researchers to promote perovskite formation [6]), the differences between in and ex situ heat treatments will be examined. EXPERIMENTAL PROCEDURE Two substrate systems were chosen for PZT deposition, a platinized NaCI single crystal, and a bare NaCI single crystal. The single crystal was cleaved from a large single crystal to form sections approximately 1 cm square and 2 mm thick. Platinum was then deposited using DC magnetron sputtering to thicknesses of approximately 50 nm. A PZT solution containing the proper stoicheometry of Pb(Zr0.52Ti0.48)O 3 and containing an excess of 10 mol % Pb was formed using the 2-MOE route in previously published methods [1,10]. The excess Pb is present to enhance the formation of the tetragonal, rather than cubic, crystal structure [6]. This solution was spun onto the salt crystals at a rate of 4000 rpm for 15 seconds. Both sets of crystals were
then heated to 90 'C to evaporate solvent and 350 °C to pyrolyze the film. This process was repeated twice to build up PZT films a
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