Embedded Capacitor Technology Using Aerosol Deposition

Embedded passives, which achieve miniaturization, cost reduction, and higher performance, are regarded as one of the most promising technologies for a future RF module substrate. Currently, a BaTiO₃/Polymer composite is being used for the embedded capacitors in printed wiring boards. One of the drawbacks of this composite is its relatively low dielectric constant, because the polymer component with a low dielectric constant suppresses the dielectric constant of the whole composite. We propose a resin build-up circuit board with passive functions embedded in a ceramic film without any polymer component for the next-generation low-cost RF modules. We have already manufactured a prototype board with ceramic capacitors embedded in an FR-4 substrate using a unique ceramic deposition technology: aerosol deposition (ASD) in which many kinds of ceramics can be deposited on a substrate at room temperature by making use of accelerated ceramic nanoparticle aerosol bombardment with a nozzle. In this study, first we examine the effects of the characteristics of raw ceramic powder on the crystal structure and the dielectric properties of ASD films. As a result, we confirmed that dense BaTiO₃ dielectric films can be deposited when raw powder without strain is used. From the resulting polarization versus electrical field (P–E curve), we confirmed that paraelectric was observed in the dense films, while the porous BaTiO₃ films deposited using milled powder exhibit a small hysteresis loop. We also clarified that dense BaTiO₃ dielectric films exhibit a nanostructure with a texture consisting of particles under 10 nm in diameter. We also examine the interfacial behavior between BaTiO₃ dielectric films and the Cu electrode, in order to investigate the deposition temperature and the reliability of a BaTiO₃ ASD film under high temperature (250°C), high humidity (100 Rh%), thermal cycle condition (−55°C to 150°C), and bias DC voltage (5 V). We clarified that the BaTiO₃ ASD film satisfies the criteria of reliability in the microelectronic packaging area.