Electrical modeling and analysis of the impact of slits on microstrip lines in thin film polymer layers

In this contribution, the impact of slits on reference planes of microstrip lines in thin-film polymer layers is studied. The return-currents of the microstrip lines excite the slits, causing them to behave like slot transmission lines that are short-circuited at both ends. Consequently, strong electromagnetic coupling occurs, especially when the slits are resonant within the frequency range of interest. We realized that the intensity of this coupling and hence, the amount of power lost from the microstrip lines depends on whether the fields that are coupled away from the lines through the slits penetrate into the silicon on which the thin film polymer layers are developed. For example, a resonant 5 mm long slit causes approximately 55% of power to be lost from a non-resonant 2.5 mm long microstrip line, when the fields penetrate into silicon. When the fields are shielded by a copper layer, the power lost from the line reduces to 15%. To experimentally validate the modeling approach, test structures were designed, fabricated and measured. A very good correlation was obtained between measurement and simulation results. Finally, guidelines to minimize power lost from microstrip lines in the presence of slits were deduced.