# Comprehensive Review of RF MEMS Switches in Satellite Communications
## 3 Strategies for Optimizing RF MEMS Switch Performance
### High Reliability
#### 3.1.4 Using Appropriate Packaging  
Operational environmental factors like temperature, humidity, and particulate matter in the environment significantly affect the reliability of switchable devices [19; 129; 130]. This is particularly challenging in space applications, where devices must withstand extreme temperature variations, humidity fluctuations, or high levels of radiation [24], while still fulfilling their intended mission lifespans. In this context, the packaging of RF MEMS becomes crucial for ensuring their reliability and a high degree of integration. Consequently, researchers have developed multiple technologies to ensure MEMS switches maintain their performance in harsh environments. MEMS switches require hermetically sealed packaging to prevent any residual moisture or organic contaminants from affecting the internal moving parts. Comprehensive studies are needed on the reliability and electrical performance of devices under different voltage levels and various noise conditions, such as high temperatures and humidity. Understanding the failure mechanisms of RF switches in harsh environments, and their electrical performance under different input frequencies, control voltages, and power levels, remains a key issue to be addressed. WLP (wafer-level packaging) technology is one of the most promising methods for packaging MEMS devices due to its high performance, ease of integration into semiconductor foundry processes, and low cost. [PERSON] et al. [131] proposed a novel packaging method using BCB (benzo-cyclo-butene) as an adhesive layer for encapsulating shunt capacitive RF MEMS switches, as shown in Figure 8a. This packaging method, applied to 50 \(\Omega\) CPW transmission lines, underwent microwave characterization through circuit modeling. Based on the microwave characterization results, a packaging structure for the shunt capacitive contact of RF MEMS switches was designed, achieving a fault-free lifespan of up to 10.2 billion cycles. Al-Ge eutectic bonding is a widely used bonding technique in microelectronics [132; 133], offering significant advantages over other eutectic wafer bonding processes, like Au\(-\)Ge and Au\(-\)Si, in terms of compatibility with standard CMOS wafer manufacturing processes [134]. Al-Ge eutectic bonding provides not only a conductive path between two substrates but also allows for patterning. [PERSON] et al. [135]successfully integrated Au-Ru/AlCu contacts into the MEMS manufacturing process using Al-Ge eutectic bonding for wafer-level packaging, as depicted in Figure 8b. WLP allows for the creation of sealed cavities within MEMS-specific process flows, providing robust protection against external environments. However, the resultant component thickness from this packaging method may pose integration challenges with low-cost RF PCB (printed circuit board) subsystems, especially at millimeter-wave frequencies. The addition of a sealing ring around the chip during the packaging process not only increases the overall size of the component but also adds to the losses. In response to this challenge, an alternative method is TFP (thin-film packaging), integrated directly into the MEMS manufacturing process. This method significantly reduces the final component cost and substantially enhances the reliability of such RF MEMS. Figure 8c shows a zero-level packaged MEMS switch capacitor [136]. This minimized packaging is achieved through standard microelectronic process steps within the production flow. Its hermeticity was verified through simple testing, and the device could withstand subsequent PECVD (Plasma-Enhanced Chemical Vapor Deposition) processes for long-term protection. The packaging shown in Figure 8d is also achieved at the wafer level through thin-film packaging technology [72],  
Figure 8: (**a**) Proposed wafer-level packaging approach for RF MEMS devices using BCB bonding [131]. Reprinted/adapted with permission from [131], 2019, IEEE.