Overcrowding of signals at RF and microwave frequencies has created growing interest in millimeter-wave and even terahertz (THz) frequency bands for short-range applications. In turn, circuit designers are exploring ways to integrate different components and transmission lines at higher frequencies.
One of the transmission-line approaches for THz frequencies consists of a U-shaped silicon (Si) guiding channel attached to a glass substrate, such as Pyrex glass—a structure known as U-shaped silicon-on-glass (U-SOG). The Si guiding channel is etched from below the guiding channel to reduce interaction of the modal fields with the Pyrex substrate material, resulting in a low attenuation constant for the waveguide structure.
Nazy Ranjkesh and fellow researchers from the University of Waterloo (Ontario, Canada) designed a U-SOG waveguide structure with the aid of High Frequency Structure Simulator (HFSS) modeling software from Keysight Technologies along with the same company’s PNA-X vector network analyzers (VNAs) and a specially designed test fixture. The attenuation constants for the U-SOG waveguide structure compare closely between simulated and measured values, with low attenuation from 800 GHz through 1.1 THz.
Simulated values are usually less than the measurements, due to the difficulty of achieving proper alignments between the U-SOG structure and the standard metallic waveguide test fixtures. But compared with other transmission-line technologies, including metallic waveguide, the U-SOG approach does quite well in preserving signal power even at these high frequencies, making it a good candidate for on-chip transmission lines and integrated circuits at THz frequencies.
See “1.1 THz U-Silicon-On-Glass (U-SOG) Waveguide: A Low-Loss Platform for THz High-Density Integrated Circuits,” IEEE Transactions on Terahertz Science and Technology, Vol. 8, No. 6, November 2018, pp. 702-709.