Passivation of miniature microwave coplanar waveguides using a thin film fluoropolymer electret

The insertion losses of miniature gold/silicon-on-insulator (SOI) coplanar waveguides (CPW) are rendered low, stable, and light insensitive when covered with a thin film (95 nm) fluoropolymer deposited by a trifluoromethane (CHF₃) plasma. Microwave characterization (0-50 GHz) of the CPWs indicates that the fluoropolymer stabilizes a hydrogen-passivated silicon surface between the CPW tracks. The hydrophobic nature of the fluoropolymer acts as a humidity barrier, meaning that the underlying intertrack silicon surfaces do not re-oxidize over time-something that is known to increase losses. In addition, the fluoropolymer thin film also renders the CPW insertion losses insensitive to illumination with white light (2400 lx)-something potentially advantageous when using optical microscopy observations during microwave measurements. Capacitance-voltage (CV) measurements of gold/fluoropolymer/silicon metal-insulator-semiconductor (MIS) capacitors indicate that the fluoropolymer is an electret-storing positive charge. The experimental results suggest that the stored positive charge in the fluoropolymer electret and charge trapping influence surface-associated losses in CPW-MIS device modelling supports this. Finally, and on a practical note, the thin fluoropolymer film is easily pierced by commercial microwave probes and does not adhere to them-facilitating the repeatable and reproducible characterization of microwave electronic circuitry passivated by thin fluoropolymer.