*Article* **THz MEMS Switch Design**

**Yukang Feng \*, Han-yu Tsao and N. Scott Barker \***

Department of Electrical and Computer Engineering, University of Virginia, Charlottesville, VA 22904, USA; ht5fy@virginia.edu

**\*** Correspondence: yf4rs@virginia.edu (Y.F.); nsb6t@virginia.edu (N.S.B.)

**Abstract:** In this work, an mm-wave/THz MEMS switch design process is presented. The challenges and solutions associated with the switch electrical design, modeling, fabrication, and test are explored and discussed. To investigate the feasibility of this design process, the switches are designed on both silicon and fused quartz substrate and then tested in the 140–750 GHz frequency range. The measurement fits design expectations and simulation well. At 750 GHz the measurement results from switches on both substrates have an ON state insertion loss of less than 3 dB and an OFF state isolation larger than 12 dB.

**Keywords:** millimeter-wave; terahertz; MEMS; switch; transmission line model

#### **1. Introduction**

Compared with conventional diode-based RF switches, MEMS switches have significant advantages in RF performance including higher isolation, lower insertion loss, and fewer intermodulation products. Meanwhile, since the MEMS switch does not require constant DC bias current in the static ON and OFF states, it consumes nearly zero power [1]. Because of these advantages, significant effort has been made to develop MEMS switches in the centimeter band [2–6], and some had also been successfully introduced in commercial applications [7,8]. In comparison, MEMS switch development in the millimeter-wave or THz spectrum faces more challenges.

In such frequencies, the switch's physical dimensions are comparable with its RF signal wavelength. Instead of regarding the switch as a lumped element, one needs to model the circuit from a transmission line perspective. The EM finite analysis is also frequently applied in RF optimization. Meanwhile, at such high frequencies, the switch RF performance improvement requires adjusting the circuit's physical features in dimensions of micrometers. Inevitably, the electrical design has to trade off with the fabrication techniques and limitations; which makes mm-wave/THz MEMS switch design more challenging.

In 2010, a DC contact MEMS switch operating at 50–100 GHz was reported [9]. Its center conductor in the coplanar waveguide (CPW) was actuated through a long cantilever beam from one side of the switch. The cantilever was driven by a comb-electrode actuated folded spring structure, which significantly complicated the bias structure and increased the circuit size. Another CMOS-based MEMS switch was successfully demonstrated at 220 GHz [10]. In this design, the air bridge structure that supported the actuator introduced a large parasitic capacitance and limited the isolation performance. Two switches had to be placed in series to provide 12 dB isolation. The switch reported in 2017 [11] successfully integrated a MEMS switch with a BiCMOS process. In the 220–320 GHz band this capacitive switch achieved 1 dB insertion loss and 12 dB isolation. Another 500–750 GHz waveguide switch [12] reported in 2017 used a MEMS-reconfigurable surface to block & unblock the wave propagation through the waveguide; however, this structure is incompatible with planar RF circuits.

In our previous work [13], we demonstrated an RF MEMS switch on a silicon substrate and provided the preliminary measurement in the 500–750 GHz (WR1.5) band. To

**Citation:** Feng, Y.; Tsao, H.-y.; Barker, N.S. THz MEMS Switch Design. *Micromachines* **2022**, *13*, 745. https://doi.org/10.3390/mi13050745

Academic Editors: Lu Zhang, Xiaodan Pang and Prakash Pitchappa

Received: 1 April 2022 Accepted: 4 May 2022 Published: 8 May 2022

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investigate the feasibility of this design process on different dielectric substrates in a wider millimeter/THz spectrum, the switches were designed on both high resistivity silicon and fused quartz substrate and then tested in the 140–750 GHz frequency range. In this work, we provide a method to design RF MEMS switches in the mm-wave/THz frequency band. The challenges associated with the switch electrical design, modeling, fabrication, and test are discussed and the solutions are provided.
