Approaches for High Permittivity in Barium Titanate
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2008-12-04
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DANIELS, PATRICK R. High Permittivity Barium Titanate Thin Films. (Advisor: Jon-Paul Maria).
As the demand for smaller, faster and more robust electronics increases, the sophistication of system design and the optimization of material properties must improve. In order to accommodate these goals, electrical components, such as capacitors, must be placed closer to the integrated circuit chip and consume less area on a printed wiring board. One approach to this end embeds these passive components into the printed wiring board directly beneath the integrated circuit chip. Previous work at NCSU pioneered a thin film ferroelectric capacitor technology that satisfies the principle demands. This thesis describes a set of advancements to this technology of the thin film dielectrics on flexible copper and platinum foils that target specifically enhanced permittivity through microstructure control.
Barium titanate was deposited on copper and platinum foils by a chemical solution deposition process with the intent of investigating the effects of the A:B site ratio on the microstructure and electrical properties. The primary investigation involved preparing a range of dielectric compositions from 0 to 5% excess barium and annealing them to temperatures ranging from 900 to 1200°C for 20 hours. The annealing atmospheres and maximum temperature limits were chosen with respect to preserving the integrity of the metallic foil substrates.
On Pt substrates, as annealing temperature and the amount of excess barium increased the average grain size increased dramatically. Average grain size grew from 70 nm for a stoichiometric film annealed at 900°C to 800 nm for films with 4% excess barium annealed at 1200°C. The grain size decreased in films with 5% excess barium annealed at 1200°C due to the development of a second phase identified by x-ray diffraction. This data is in sharp contrast to existing descriptions of the BaTiO3 binary phase diagram that suggest ppm levels of solid solubility associated with the BaTiO3 intermediate compound and present interesting new questions regarding stability of Ba excess crystals.
Guided by this Pt substrate reference data, the microstruture – dielectric property relationships were explored for BaTiO3 films on copper foil. Compositions of 1:1 barium titanate and 3% excess barium annealed at 900 and 1060°C were prepared and characterized respectively. The room temperature permittivity increased from 1800 to 4000 with the addition of excess barium and increased annealing temperature. The grain sizes ranged from x1 nm to x2 nm respectively. These results demonstrate a completely new method of controlling grain size in BaTiO3 with Ba excess, and the success of these methods to engineer extrinsic permittivity contributions consistent with well-prepared bulk ceramics.
In addition, thin films of Ba0.7Sr0.3TiO3 were deposited on copper foils via RF magnetron sputtering with the intent of investigating the effects of process flow on the percent yield of working capacitors with respect to electrode size – a second challenge in the development of a viable embedded high value capacitor technology. In the previously established conventional process, electrode metallization was performed after annealing at 900°C. In the newly developed co-firing process, electrode metallization is performed after sputtering deposition but before annealing at 900°C. By changing the process flow, of the fraction of working 5 mm diameter capacitors increased from 0% to 100%. These capacitors were prepared on copper foils with a dielectric thickness of less than 1 µm in the absence of clean room conditions. A model involving curvature-controlled de-wetting is proposed to explain the success of this method to obviate the short circuits that typically accompany geometric asperities associated with polycrystalline thin films and rough substrates.
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dielectric, thin films, Barium Titanate
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Degree
MS
Discipline
Materials Science and Engineering
