Microwave Characterization of Parylene C Dielectric and Barrier Properties

Parylene C thin films are commonly used as a passivation layer, protective coating, or substrate material in implantable medical devices. However, fluid or vapor may permeate through Parylene C films over time through defects, film edges, or bulk diffusion and lead to device failure in vivo. Such failure can be difficult to detect using electrochemical impedance spectroscopy (0.1 Hz – 1 MHz) which does not capture the dielectric contributions of fluid at higher frequencies.We present a method for tracking broadband (1 MHz – 110 GHz) microwave dielectric properties of polymer-fluid interfaces. Using 2D electromagnetic modeling, we simulated measurement sensitivity to potential changes in Parylene C film dielectric properties and estimated sensitivity to changes in film relative permittivity as small as ±0.1 and fluid layers under films as small as 5 nm. Results were used to aid in the interpretation of measurements of soaked Parylene C films. These results are informative development of robust, hermetic polymer films in implantable medical devices and other RF applications in the analysis of complex fluids.