Ultra-thin Ceramic Films for Low-temperature Temperature Embedding of Decoupling Capacitors into Organic Printed Wiring Boards

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Balaraman, Devarajan
Tummala, Rao R.
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As microprocessors move towards higher frequencies, lower operating voltages and higher power consumption, supplying noise-free power to the ICs becomes increasingly challenging. Decoupling capacitors with low inductance interconnections are critical to meet the power supply impedance targets. A variety of capacitors are used today to provide decoupling at different frequencies. Surface-mount multi-layer ceramic capacitors currently used at package level provide decoupling only till about 100 MHz because of the component and lead inductances. Embedding thin film capacitors into the package can expand the operating range of package level capacitors to low GHz frequencies. Thin films with capacitance of several microfarads and organic-compatible processes are required for embedding decoupling capacitors at package level. The organic-compatible high-permittivity materials available today do not provide adequate capacitance for the application on hand. While ferroelectric thin films can provide the required capacitance, processing temperatures over 300o C are required to achieve crystalline films with high permittivity. Hence, there is a need to develop novel materials and processes to integrate decoupling capacitors into currently prevalent organic packages. To this end, hydrothermal synthesis and sol-gel synthesis of BaTiO3 films were explored in this study. BaTiO3 films were synthesized by low temperature hydrothermal conversion of metallic titanium. Hydrothermal process parameters such as bath molarity and temperature were optimized to obtain thin films with grain sizes close to 100 nm, at temperatures less than 100o C. Novel post-hydrothermal treatments were developed to improve the dielectric properties of the films. Sol-gel process requires sintering at >700o C to obtain crystalline BaTiO3 films. However, the films can be synthesized on free-standing copper foils and subsequently integrated into organic packages using lamination. Prevention of foil oxidation during sintering is critical. Nickel and titanium barriers explored in this study were ineffective due to instabilities at the interfaces. Hence, films were synthesized on bare copper foils by controlling the oxygen partial pressure during sintering. Using these techniques BaTiO3 thin films with capacitances of 400 1000 nF/cm2 and breakdown voltages of 6 15 V were demonstrated. The films synthesized via either techniques exhibited stable dielectric properties up to 8 GHz owing to fine grain sizes.
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