Low-cost embedded capacitor technology with hydrothermal and sol-gel processes

Fabrication of high-k embedded capacitors on printed wiring board (PWB) is limited due to the inherent low-temperature process required for organic packaging. Current dielectrics for embedded capacitors are mostly organic with insufficient dielectric constant. Integration of high-k thin films on PWB is hindered by high processing temperature of ceramics. The goal of this work, is to develop low-cost/low-temperature, aqueous/non-aqueous based processes for embedded capacitors. Hydrothermal synthesis was used for synthesis of nanograined barium titanate films. Films synthesized at 95°C on titanium foils yielded nanograined films (<80 nm grains) with higher capacitor yield in comparison to conventional hydrothermal films from Ti precursors or sputtered Ti. Films with capacitance of more than 1.0 pF/cm² and thickness of 300 nm (corresponding to a dielectric constant of above 350) were developed. Oxygen plasma treatment of hydrothermal films was found to lower the loss significantly to 0.06 from 0.28. Sol-gel technique was also explored an alternate low cost large area process for synthesis of high K low loss films. Sol-gel derived films are typically crystallized and densified at temperatures that are not compatible with organic build-up processes. This limitation has been addressed using a modified sol-gel process to deposit films on a carrier foil that was subsequently laminated onto the printed wiring board. All high-temperature processing steps required by the oxide dielectrics were performed before the embedding process. High-k barium titanate (BaTiO₃) and strontium titanate (SrTiO₃) thin film capacitors were synthesized on base-metal nickel (Ni) and titanium (Ti) foils as carrier. Rapid thermal processing (RTP) lowers the process time for the development of a well-crystallized titanate film to 3 minutes as opposed to the few hours of processing time required for conventional heat treatment. Capacitance densities ranging from ∼45-700 nF/cm² have been achieved by varying the film thicknesses from ∼250 to 900 nm and the heat treatment conditions. By following the RTP with a 1 hr heat treatment in nitrogen (N₂) atmosphere, the dielectric loss was reduced to 0.005. These sol-gel and hydrothermal films were su- bsequently integrated onto organic boards using conventional lamination and lithography methods, followed by low-cost wet etching.