Effective diameter: A new parameter for THz transmission line fabrication

Largely ignored parameters in transmission line fabrication of low-frequency devices must be considered in terahertz monolithic integrated circuits (TMIC), according to Ma et al. In doing so, they propose a new parameter, called the “effective diameter.”

Commonly used ground coplanar waveguides and other transmission lines in TMIC tend to degrade drastically beyond 100 GHz, largely due to the inherent parasitic capacitance in wide-bandgap materials.

To address this issue, researchers have turned to substrate integrated waveguide (SIW) technology in which the metal sidewalls of a traditional transmission line are replaced with rows of via holes (metallized holes), in the printed circuit board.

Known for its low-loss performance, SIW is easily integrated in existing manufacturing techniques. However, integral aspects of the fabrication process, particularly the via hole shape and size, have rarely been considered.

Ma et al. found that the material hardness of silicon carbide increased the difficulty level of etching, which resulted in conical via hole formation instead of cylindrical ones. They studied the conical holes and discovered that when the inclination changes from 90 degrees to 75 degrees, the cutoff frequency changes by more than 15 GHz.

“It’s predictable that the conical hole will cause a change in cutoff frequency. But what surprised us was the large value of the frequency shift,” Ma said. “For this reason, the via hole inclination must be fully considered in THz SIW circuit design, otherwise great errors will result in both theoretical calculations and experiments.”

From this discovery, the researchers came up with the effective-diameter parameter in which they equalized the conical holes by using the cylindrical diameter to simplify the design process and improve the consistency between simulation and experiment.

Source: “Terahertz monolithic integrated waveguide transmission lines based on wide bandgap semiconductor materials,” by Yang Li, Xiao-Hua Ma, Lin-An Yang, Jin-Ping Ao, and Yue Hao, Journal of Applied Physics (2019). The article can be accessed at https://doi.org/10.1063/1.5083097 .