PZT and PLZT thin films on Cu substrates for dielectric and piezoelectric applications: Effect of processing atmosphere
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PZT and PLZT thin films on Cu substrates for dielectric and piezoelectric applications: Effect of processing atmosphere and film strain Taeyun Kim, Sudarsan Srinivasan* and Angus I. Kingon Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A. ABSTRACT PZT (52/48) and PLZT (7/65/35) thin films were prepared by chemical solution deposition (CSD) on Pt/SiO2/Si and Cu foil substrate, respectively. For PZT thin films on Pt/SiO2/Si substrates, pO2 control during thermal treatment affected both perovskite phase formation and orientation, but did not show significant impact on the ferroelectric and dielectric properties. When PLZT thin films (7/65/35) were prepared on Cu foil to tailor ferroelectric/dielectric properties, it was observed that ferroelectric and dielectric properties were significantly affected by the strains, possibly from the coefficient of thermal expansion (CTE) mismatch between PLZT and Cu foil during cooling after high temperature crystallization. It was also experimentally observed that, depending on the direction of applied strain, the PLZT films could be cycled between ferroelectric and paraelectric phases. It was suggested that PZT based thin films on base metal substrate requires very careful processing for optimized electrical performance. INTRODUCTION PZT based high permittivity thin films are actively investigated for a variety of applications, including piezoelectric actuator, pyroelectric sensor, FRAM and embedded capacitor applications [1,2]. Of these, PZT thin films for embedded capacitor have the advantage of lower processing temperature compared with BaTiO3 thin films on base metal electrodes [3], while retaining a similarly attractive set of properties. We have previously reported on the processing of PZT on Cu base metal electrodes [4,5], but the effect of processing parameters on the electrical properties have not been extensively investigated. Specifically, we have previously shown that it is essential to control the pO2 during processing. However, we have not independently separated the effect of pO2 on the electrical properties of PZT from the indirect effect of the same reduced pO2 on the properties, for example via interface reactions. This is undertaken in the present study. Furthermore, the properties of PZT are typically modified by dopants or compositional substitutions, such as La [6,7]. Thus, the research focus has included PLZT films (7/65/35) on Cu foil, to investigate whether the usual approach of La doping to shift the Curie point to achieve paraelectric compositions for dielectric applications can be adopted. Thus PLZT (7/65/35) thin films were prepared directly on Cu foil by CSD. The effect of stress (strain) on the electrical properties of PLZT thin films on Cu foil was also experimentally investigated. EXPERIMENTAL DETAILS To study the impact of oxygen partial pressure, PZT (52/48) thin films were prepared on Pt/SiO2/Si by chemical solution deposition [4]. The Pt/SiO2/Si substrates are considere
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