Silicon Micromachined Multilayer Waveguide Components and Reconfigurable Systems for  Sub-Terahertz Applications

QC 20260504

Abstract

Terahertz (THz) and sub-terahertz (sub-THz) technologies are attracting growing interest for applications such as radio astronomy, sensing and imaging, high-resolution radar, and next-generation communication systems. At these frequencies, silicon micromachining based on deep reactive ion etching (DRIE) and silicon-on-insulator (SOI) technology offers a promising platform for realizing waveguide devices with high precision, low loss, and strong integration capability. This thesis investigates multilayer silicon-micromachined waveguide components and MEMS-based switching circuits for THz and sub-THz applications, with an emphasis on low-loss integration, high-performance switching, and mechanical reliability. 

The first part of the thesis studies multilayer waveguide integration. Vertically and horizontally stacked waveguides are compared to clarify how stacking orientation, current distribution, and DRIE-induced surface roughness influence conductor loss. Based on this analysis, design guidelines are established for minimizing propagation loss in multilayer architectures. In addition, a broadband on-chip 90° waveguide twist is developed to enable orthogonal integration of H-plane and E-plane subsystems within a compact silicon-micromachined platform, demonstrating low insertion loss and good matching over 220–325 GHz. 

The second part of the thesis investigates an interference-based MEMS reconfigurable-surface concept for low-loss waveguide switching. A 220–290 GHz single-pole single-throw (SPST) switch validates the switching principle, and the concept is then extended to a four-port crossover switch for multi-port signal routing. Particular focus is placed on a 163.6–182.55 GHz crossover switch developed for the internal calibration switching subsystem of a spaceborne radiometer operating near the 183.31 GHz water-vapor absorption line. Measurements show good agreement with simulations and confirm excellent RF performance across the required band. To further investigate the influence of fabrication, two distinct DRIE-based process flows are implemented and compared. The results show that both cavity surface quality and geometric underetch must be considered, with their relative impact depending on the specific device implementation. Although geometric deviations can be partly incorporated into the design through simulation after process characterization, surface roughness remains one of the main limitations to achieving low insertion loss. 

The final part of the thesis addresses metallization-induced residual stress in suspended SOI MEMS structures. Since the gold metallization required for lowloss wave propagation can cause severe out-of-plane deformation and switching failure, a post-deposition thermal annealing strategy is developed and experimentally validated. The results show that annealing effectively relaxes or reverses the stress gradient, restores structural flatness, and enables reliable actuation. 

Overall, this thesis advances silicon-micromachined THz and sub-THz waveguide technology through complementary studies on multilayer integration, waveguide switching, fabrication strategies, and stress control. The presented work provides both device-level demonstrations and practical design guidelines for compact, low-loss, and mechanically reliable waveguide components and subsystems. 

Link to DiVA