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Multilayered Composite Dielectric Thin Films on Flexible Copper Foil for Advanced Packaging Applications
Keywords: Dielectric thin film, embedded passive, Printed Wiring Board
As an approach to replace the surface mounted technology (SMT) based passive capacitor components for advanced printed wiring board (PWB) systems, high permittivity ceramic thin film dielectrics on flexible copper foil stacks were investigated aimed at decoupling /bypass functional applications. In our study, multilayered composite thin film capacitors were prepared on flexible copper foil by chemical solution deposition and the electrical properties were evaluated. Dielectrics for our multilayered heterostructures were lead lanthanum zirconate titanate (PLZT) with barium strontium titanate (BST) or strontium titanate (SrTiO3) as buffer layer, respectively, due to the high permittivity value and paraelectric response at room temperature. Capacitor-on-foil stacks were processed at high temperature (above 650oC) under controlled reducing atmosphere to evade the oxidation of copper substrates and at the same time to fully crystallize the dielectric ceramic composition for better dielectric properties. Dielectric and ferroelectric properties of capacitors were significantly affected by the underlying buffer layer composition and thickness. Without the incorporation of a buffer layer, PLZT thin films showed hysteretic response on copper foil. For buffer layered composite capacitors, it was found that voltage dependent dielectric properties could be modulated between ferroelectric, hysteretic responses to almost hysteresis-free, paraelectric responses, which is desirable for most embedded capacitors in use. By the selection of optimized stack structure, room temperature permittivity of more than 700 and dielectric loss of less than 0.03 could be attained. Temperature dependent dielectric properties were also investigated for selected multilayered capacitors to evaluate temperature coefficient of capacitance (TCC). Correlations between thermal processing parameters, buffer layer materials/thickness, and thermal properties of capacitor/substrate materials were also discussed.
Taeyun Kim, Post Doctoral Research Associate
North Carolina State University
Raleigh, NC

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