Passive Intermodulation on Microstrip Induced by Microstructured Edge

Considering the distributed electrothermal coupling effect, passive intermodulation (PIM) products in very long microstrip lines (MLs) have been deeply studied theoretically and experimentally; however, the complex PIM behaviors of short MLs (SMLs) arising from edge defects are still not well presented, and the nonlinearity mechanism has not been revealed until now. In this article, we propose the microscale single- and double-triangle structures to represent intended random defect patterns on the edge of SMLs. Then, we fabricate multiple 20-mm-long SML samples on sapphire crystal wafers employing the metal lift-off technology (MLOT). By PIM measurements and scanning electron microscope (SEM) image observations, we find statistically that the PIM levels of the SMLs with nanoscale depressions and holes on the top edge are over 10 dB higher than that of the SMLs with dense and smooth edges. Based on the statistical approach, we confirm that the discontinuous nanoscale defects on the top edge are the nonlinearity origin. And the microscale structure will enhance the localized electrical field concentration according to the electrical simulations. Considering the electrical field enhancement effect of microscale structure and the nanoscale defects as nonlinearity sources, an analytical PIM model for SMLs is established.