Hydrothermal Processing of Barium Strontium Titanate Composites on Printed Circuit Board for Thin Film Varactor Applications
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Thin film Ba0.7Sr0.3TiO3 has been deposited onto printed circuit boards at low temperatures for low and high frequency applications. The fabrication method involves spin coating sol-gel composites (a mixture of BST powder suspended in an amorphous BST sol), and treating the resultant films at 175°C. The treatment is done in an autoclave in an aqueous environment comprised of barium hydroxide, strontium hydroxide, and water, and is carried out at elevated temperature and pressure. This causes the amorphous BST sol to recrystallize, forming bridges that link the BST powders together, creating a dense network, leading to a higher dielectric constant. Three different types of powders have been explored for the composite. They are low-temperature derived, ceramic derived, and sol-gel derived powder. Low-temperature derived powders are the smallest in size, in the nanometer range. Due to such small sizes, the ferroelectric properties are lost, leading to no tunability and very low dielectric constant. The permittivity ranges from 30 to 55. Sol-gel derived powders are very reactive to the hydrothermal process. The resultant film permittivity is the highest, ranging from 200 to 260, but with a limited tunability of 1%. Ceramic derived powders are compatible with the process. The permittivity is less than that of sol-gel derived, with values from 130 to 140, but the tunability ranges from 4 to 8% at low frequencies (500 kHz to 20 MHz). For high frequency characterization (45 MHz to 1.15 GHz), films containing ceramic derived powder are deposited onto grounded coplanar waveguide capacitors. The dielectric constant significantly reduced, reaching as low as 18, but tunability increased to 18%.