Intermetallic Formation in PZT for MEMS Structures
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Intermetallic Formation in PZT for MEMS Structures
Kanu priya Sharma, Thomas Oseroff, Leda Lunardi Dep. of Electrical and Computer Engineering, NC State University, Raleigh, NC-27695, U.S.A ABSTRACT Crack free lead zirconate titanate (PZT) films for piezoelectric based MEMS devices have been prepared by a multiple coating sol gel process on platinized silicon (100) substrates. Rapid thermal annealing and Conventional furnace annealing were used for densification and crystallization of the amorphous PZT films. Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and Atomic force microscopy (AFM) were used to observe surface film morphology and grain growth. The phase content of the films was analyzed using X-ray diffraction. The role of intermetallics formed during the heat treatment in the growth of different orientations has also been observed. Film aging critical for device performance has been observed and methods to revert aging effects have been examined and discussed. INTRODUCTION Lead zirconate titanate is a widely used material in piezoelectric micro-electromechanical (MEMS) sensors and actuators. Film quality plays an imperative role in device performance and reliability. PZT films have been deposited by Metal organic chemical vapor deposition (MOCVD) [1], sputtering [2], pulsed laser deposition [3], molecular beam epitaxy [4], and solgel. Sol-gel as a film fabrication technique provides more composition control and allows suitable manipulation of precursor solutions to develop films, powders, fibers or bulk films [5]. The performance of MEMS devices is primarily based on the dielectric and ferroelectric response of the functional material. These properties evaluated in terms of the dielectric constant, dielectric loss, extent of polarization as indicated by remnant polarization and electrical and mechanical coupling depend on surface film morphology, microstructure, phase content and crystal orientation to name a few governing factors. In this study, films have been deposited by a multiple coating sol-gel process as outlined in [6]. The dependence of film microstructure, its dielectric and ferroelectric properties on multiple factors such as precursor solution preparation, gel composition, annealing conditions and formation of intermetallics at the interface between the functional material layer (PZT) and the metal electrode (Pt) have been examined. EXPERIMENTAL DETAILS For the reliable operation of PZT based MEMS devices, the material used for the bottom electrode plays a critical role. Platinum as a material not only is highly conductive and resistant to oxidation, but also helps in developing good texture and high quality PZT films. However, platinum is known to react with silicon at temperatures as low as 400°C. To avert this diffusion, silicon dioxide is used as a barrier layer. With the silicon dioxide now in place, the platinum
requires a thin adhesive layer such as titanium to stick to the oxide. The titanium also prevents the diffusion of lead from PZT into the silicon (to
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