**6. Discussion & Conclusions**

In this paper, a THz MEMS switch design process is presented. To validate this process, THz MEMS switches are realized on both silicon and fused quartz as examples of both high and low dielectric constant substrates respectively.

In the device design, electrostatic actuation is selected to control the switches in consideration of its advantage over device size, integration challenges, switching speed, power consumption, and RF performance. Manufacturing limits and mechanical reliability together determined the minimum dimensions and thus the switches' geometries. To integrate elevated actuators with CPW, the bias pad geometry is engineered to provide an optimized impedance match. The designed MEMS switches are modeled through transmission line analysis as well as finite element-based electromagnetic simulations. The comparison suggests the transmission line model captures the major electrical features successfully; meanwhile, the finite element model can also evaluate certain minor coupling, resonance, and frequency-dependent conductor loss. Fabrication flow of the THz MEMS switch is provided, in which a diffusion barrier layer is used to prevent forming Au-Al compound [26]. A new RIE/wet-etch combined process is critical to selectively etch certain metal layers. Both silicon and fused quartz switches are calibrated through the two-port TRL method from 140 to 750 GHz. The measurements fit previous modeling and simulation results well and serve to verify this THz MEMS switch design process.

**Author Contributions:** Conceptualization, Y.F.; methodology, Y.F.; software, Y.F.; validation, Y.F. and H.-y.T.; formal analysis, Y.F.; investigation, Y.F. and N.S.B.; data curation, Y.F.; writing-original draft, Y.F.; writing-review and editing, N.S.B.; supervision, N.S.B.; project administration, N.S.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors wish to acknowledge the U.S. National Ground Intelligence Center (NGIC) for their financial support of this work through contract W911W5-11-C-0013.

**Conflicts of Interest:** The authors declare no conflict of interest.
