Synthesis of Ceramic Thin Films for Organic Board Compatible Integral Capacitors - Processing and Characterization

Integration of passive elements like resistors, inductors and capacitors remains a major barrier to miniaturization of convergent microsystems. Thin films (sub-micron) of barium titanate and strontium titanate can easily achieve capacitances of the order of μF/cm ² and hence satisfy the requirements for most integral capacitors with low loss. Conventional sintering and vacuum deposition processes such as RF sputtering are not compatible with low-cost large-area PWB processes. Hence, novel PWB-compatible low-cost processes need to be explored. This paper presents two possible options for processing and integration of barium titanate films on organic Printed Wiring Boards. Films synthesized by hydrothermal techniques (< 100°C) on textured titanium foils (6-12 μm) were found to yield nanograined films (<80 nm grains) with higher yield in comparison with conventional hydrothermal films from titanium precursors or sputtered titanium. However, the films synthesized by this technique have high loss from the entrapped OH groups and defects in the lattice resulting in poor electrical performance. Oxygen plasma treatment of hydrothermal films was found to lower the loss significantly to 0.06. The films were subsequently integrated into organic build-up processes by laminating and patterning the Ti foils before hydrothermal treatment and were metallized using conventional electroless plating process to make it compatible with large-area board technology. Films with capacitance of more than 1.0 μF/cm ² and thickness of 300 nm (corresponding to a dielectric constant of above 350) were developed and integrated into the organic build-up process. As an alternative low-cost large-area compatible thin film deposition process, sol-gel derived ceramic films were also investigated. These films were synthesized on metallic titanium and nickel foils and laminated onto PWBs after heat treatment. The capacitance and dielectric loss of sol-gel barium titanate and strontium titanate films ranged from 250-400 nF/cm ² and 0.03-0.05 respectively. The dielectric loss can be further reduced with process optimization and control of the interface and lattice defects.