# Comprehensive Review of RF MEMS Switches in Satellite Communications
## 3 Strategies for Optimizing RF MEMS Switch Performance
### Excellent RF Performance
#### 3.2.2 Increasing Capacitance Ratio  
In capacitive MEMS switches, the ON/OFF states are achieved by changing the capacitance between the MEMS bridge and the transmission line. According to the previous Equation (1), a small up-state capacitance helps to reduce insertion loss, while a large down-state capacitance aids in increasing isolation. Therefore, the capacitance ratio (C\({}_{r}\)) directly affects the RF response of the switch. A higher capacitance ratio is desired for better RF response [142; 146], making it one of the key methods to optimize the RF performance of capacitive MEMS switches. Theoretically, the capacitance ratio of the switch can be calculated using Equation (2) [142]. The equation shows that the capacitance ratio can be effectively increased by using materials with a high dielectric constant, reducing the thickness of the dielectric layer, and increasing the air gap between the electrodes.  
\[C_{r}=\frac{C_{d}}{C_{u}}\approx\frac{g_{0}\varepsilon_{r}}{t_{d}} \tag{2}\]  
where \(g_{0}\) is the air gap between two electrodes; \(\varepsilon_{r}\) and \(t_{d}\) are the permittivity and thickness of the dielectric layer, respectively; \(C_{d}\) and \(C_{u}\) are the down-state and up-state capacitance, respectively.  
Researchers have explored using materials with high dielectric constants to increase the capacitance ratio. High dielectric constant materials can enhance the RF performance of the switch, but the reliability of the dielectric must be carefully analyzed. Si3N4 (silicon nitride), with its moderate dielectric constant, has been the most commonly used dielectric material for capacitive RF MEMS switches [147]. Approaches using high dielectric constant materials like HfO\({}_{2}\) (hafnrium dioxide) [148; 149], AlN (aluminum nitride) [147], Ta\({}_{2}\)O\({}_{5}\) (tantalum pentoxide) [150], and STO (strontium titanate oxide) [151] have been proposed to achieve high capacitance ratios in MEMS switches. Table 2 shows the RF performance and capacitance ratios achieved with these materials.  
Figure 9: (**a**) The frequency reconfigurable capacitive shunt RF MEMES switch [142]. Reprinted/adapted with permission from [142]. Copyright 2019, with permission from Elsevier. (**b**) The T-match RF MEMS capacitive switch [143]. Reproduced with permission from Springer Nature. (**c**) The SP12T RF MEMS Switch [145]. Reprinted/adapted with permission from [145], 2018, IEEE.  
The reliability of capacitive RF MEMS switches is limited by leakage current and charging phenomena in the dielectric layer. A common technique to increase the capacitance ratio is to enlarge the air gap between the beam membrane and the dielectric layer, but this method increases the drive voltage, and changing the height of the beam also affects the mechanical characteristics of the RF MEMS switch. Additionally, reducing the dielectric thickness and increasing the dielectric constant are limited due to dielectric breakdown and charge-trapping effects.  
Besides the above methods, the capacitance ratio can also be improved by changing the structure of the MEMS switch, such as the double-buckling beam switch shown in Figure 10a [152]. This design increases the effective capacitance area in the downward state by introducing buckling bimetal beams in the switch. As a result, this switch achieves a capacitance ratio of up to 170 without thin dielectrics or high dielectric constant materials and exhibits good RF performance. The switch shown in Figure 10b, consisting of a composite metal-dielectric buckling membrane [153], uses a buckling plate structure to decrease the upstate capacitance and increase the downstate capacitance, achieving a capacitance ratio of 91:1.