Copper Compatible Barium Titanate Thin Films for Embedded Passives

Abstract

Barium titanate thin films have been prepared by chemical solution deposition on 18 μ m thick, industry standard copper foils in the absence of chemical barrier layers. The final embodiment exhibits randomly oriented BaTiO₃ grains with diameters between 0.1 and 0.3 μ m, and an equiaxed morphology. The average film thickness is 0.6 μ m and the microstructure is free from secondary or interfacial phases. The BaTiO₃ films are sintered in a high temperature reductive atmosphere such that copper oxidation is avoided. Subsequent lower-temperature, higher oxygen pressure anneals are used to minimize oxygen point defects. Permittivities of 2500 are observed at zero bias and room temperature, with permittivities greater than 3000 at the coercive field. Loss tangents under 1.5% are demonstrated at high fields. The BaTiO₃ phase exhibits pronounced ferroelectric switching and coercive field values near 10 kV/cm. Temperature dependent measurements indicate a ferroelectric transition near 100^∘C with very diffuse character. Combining the approaches of the multilayer capacitor industry with traditional solution processed thin films has allowed pure barium titanate to be integrated with copper. The high sintering temperature—as compared to typical film processing—provides for large grained films and properties consistent with well-prepared ceramics. Integrating BaTiO₃ films on copper foil represents an important step towards high capacitance density embedded passive components and elimination of economic constraints imparted by traditional noble metallization.