# Comprehensive Review of RF MEMS Switches in Satellite Communications
## 2 Advanced Technologies and Mechanisms in RF MEMS Switches
### Contact Mechanisms
#### 2.2.2 Ohmic RF MEMS Switches  
Ohmic RF MEMS switches operate based on resistive tuning, achieving signal path disconnection by physically separating the input and output. Ohmic switches are typically used in low-frequency circuits, as they form an actual electrical connection through metal contact points in the closed state, providing a low-resistance path to ensure signal integrity and minimize loss in low-frequency signals. Since a physical connection is formed in the closed state, ohmic switches maintain good signal integrity, which is crucial for analog  
Figure 2: Structures of capacitive RF MEMS switches: (**a**) [PERSON] et al. [51]. Reprinted/adapted with permission from [51]. Copyright 2016, with permission from Elsevier. (**b**) [PERSON] et al. [8]. Configuration diagram of flexible RF MEMS switch in different modes: (i) On state and (ii) Off state of MEMS switch with flat substrate; (iii) On state and (iv) Off state of MEMS switch with curved substrate. Reprinted/adapted with permission from [8].  
signals in low-frequency applications. With special design, series MEMS switches can also achieve good RF performance at frequencies up to 110 GHz [64].  
Manufacturing MEMS with superconducting materials, though still in development, has the potential to greatly reduce circuit losses. [PERSON] et al. [65] designed a niobium-based superconducting DC contact RF MEMS switch, using standard superconducting microelectronics (SME) technology and multi-step post-processing. This study validated the feasibility of operating MEMS switches at extremely low temperatures, such as 4K. However, at superconducting temperatures, the increased rigidity of the beam membrane required higher actuation voltages. They then proposed an innovative eight-mask fabrication process, integrating low-temperature superconducting niobium RF circuits with gold-based RF MEMS switches [66]. The research demonstrated significant effects of the adopted gold-based MEMS process on the superconducting properties of niobium by comparing RF performance at room temperature and low temperatures.  
Ohmic RF MEMS switches are susceptible to stiction and contact degradation. Minimizing static friction can be achieved by modifying the cantilever [67], but altering the structure requires additional steps, thereby increasing the complexity and cost of the process. Instead of modifying the cantilever beam [68], bumps are introduced in the pull-in electrode manufacturing process. These bumps reduce the contact area by 84%, thus minimizing adhesion significantly. The manufactured switch achieves lower driving voltage with fewer manufacturing steps, as shown in Figure 3a. Traditional MEMS switches require a trade-off between RF performance and driving voltage [20]. Enhancing RF performance, such as by increasing the air gap to reduce insertion loss [69], typically results in an increase in driving voltage. Conversely, lowering the driving voltage may compromise RF performance. Utilizing residual stress in thin films, [PERSON] et al. [9] reported an innovative switch design with three-dimensional (3D) wavy microstructures. Figure 3b shows the three states of the switch. This design breaks the limitations of traditional planar cantilever beams, providing additional degrees of design freedom, which can further optimize the switch networks currently used in satellite communications. It utilizes a geometric shape that can be adjusted in three-dimensional space, achieving a lower driving voltage and enhanced RF performance, surpassing conventional flat cantilever switches. However, compared to existing designs [20; 70], the downside of this topology is the longer switching time, likely due to its relatively greater length.  
Figure 3: Structures of ohmic RF MEMS switches: (**a**) [PERSON] et al. [68]. Reprinted/adapted with permission from [68], 2019, IEEE. (**b**) [PERSON] et al. [9]. Reprinted/adapted with permission from [9].