Microwave Overlay Capacitor Fabrication on Printed Circuit Boards

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Liu, Bicheng
Barium Strontium Titanate , PCB , Microwave Filter , Ferroelectric , Film Deposition
A technique for making varactors in the form of parallel plate overlay capacitors, using barium strontium titanate (BST): a ferroelectric, polymer composite thick film, has been developed in this work. BST was selected due to the commercial availability of powder. This involves the use of standard air brush techniques to deposit ferroelectric thick films, and planarization of printed circuit boards (PCB) using soldermask to allow for film deposition which was not reported in the literature. A new method to have the PCB prepared appropriately at the manufacturing stage was employed. A bandpass filter on PCB was designed for high frequency measurements and BST composite material electrical characterization. This is due to relative permittivity changing with frequency. The BST composite film recipes were developed using commercial powder and two types of polymer. An existing recipe using cyclic olefin copolymer (COC) was adapted for spray coating, while a new recipe based on poly methyl methacrylate (PMMA) was designed for spray coating. To complete the capacitors, a metal layer was deposited using a Voltera V-One printer with the Elegant Eel conductive ink of thickness 120 πœ‡π‘š. Ferroelectric film thicknesses were measured using a Mitutoyo micrometer to be 15βˆ’120 πœ‡π‘š. Surface profile sweeps were taken with a profilometer as high texture made the profilometer unsuitable for thickness measurements. The capacitor area was measured with a calibrated optical microscope. Low frequency measurements were conducted with a Solartron 1170 Frequency Response Analyzer and were consistent with capacitor theory. High frequency measurements used a TR5048 Vector Network Analyzer (VNA) from Copper Mountain to measure passband characteristics, during which a Sentry 500VA Hipot Tester was used to apply bias to the filters for tunability measurements. The structures were simulated using High Frequency Structure Simulator (HFSS), to provide a relationship between frequency shift and relative permittivity. The relative permittivity of the BST-COC films was πœ€π‘Ÿ=11 at 1.98 𝐺𝐻𝑧, while the BST-PMMA film was πœ€π‘Ÿ=20.5 at 2.09 𝐺𝐻𝑧. The tunability of the films was a 5% relative tuning at 17.5V/ πœ‡π‘š for BST-PMMA films while the BST-COC films had a 3% tunability at the same field intensity.
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