# Comprehensive Review of RF MEMS Switches in Satellite Communications
## 3 Strategies for Optimizing RF MEMS Switch Performance
### High Reliability
#### 3.1.3 Improvement of Contact Materials and Structures  
Contact stiction and degradation are among the most critical failure modes in RF MEMS switches [68]. These reliability issues are closely related to the contact area between metals, especially in ohmic RF MEMS switches. Stiction usually originates from cold welding or hot welding [115]. Cold welding is primarily caused by atomic adhesive forces between contact materials, while hot welding is particularly crucial for switches, as it depends on the current passing through the contact point. This current elevates the temperature, leading to localized welding at the points of micro-roughness. Ideal contact materials should possess low electrical resistivity, low roughness, high hardness, and high resistance to chemical corrosion and contamination. Gold is a common material for making ohmic switch contacts due to its low resistivity and high chemical corrosion resistance. However, being a soft metal, pure gold contacts often fail to achieve the desired reliability. Therefore, strategies to improve contact life often involve alloying gold with other metals (such as ruthenium, rhodium, or nickel) to increase hardness [116]. However, these strategies have significant limitations, as hardness enhancement is limited at low-alloy metal concentrations, while high concentrations increase resistivity and may introduce brittleness and surface contamination issues. Most metals, when exposed to atmospheric environments, are prone to oxidation or reactions with organics [117], forming non-conductive layers on the surface, which must be removed to establish good electrical contact. Surface contamination is associated with the presence of reactive metals on contact surfaces that easily oxidize or react with organic materials, forming friction polymers, which lead to a rapid increase in contact resistance [118]. Gold has a significantly lower affinity for forming friction polymers and does not react with common contaminants. From this perspective, the best approach is to increase the hardness of the gold contact points without introducing other metals on the outer surface. [PERSON] et al. [115; 116] demonstrated that stacking platinum layers between gold not only increased the hardness of the contact points but also improved the contact surface performance. This improved gold-platinum multi-layer contact material significantly outperforms pure gold in terms of stability and shows lower serial resistance. Ideally, understanding the failure mechanisms of MEMS switch surfaces and interfaces requires extensive research under various environmental, temperature, and material conditions to reveal the interactions of various mechanisms and their impact on the overall system response. Recent advancements in CVD (Chemical Vapor Deposition) technology [119; 120] have made it possible to precisely control the morphology and structure of high-quality graphene, leading to its application in various micro- and nanodevices [121]. [PERSON] et al. [122] evaluated graphene as a reliability-enhancing contact material for M/NEM switch devices. Atomic force microscopy experiments and quantum mechanical calculations showed that using a few layers of graphene as the contact material on nickel surfaces can achieve energy-saving features of mechanical contact separation. Compared to bare nickel surfaces, the graphene coating reduced energy dissipation by 96.6%. The DFT (Density Functional Theory) optimized atomic structures and their F-z (force-displacement) curves during the contact separation process on nickel and graphene substrates indicate that graphene almost achieves elastic contact separation, i.e., almost no wear at the atomic scale, including plastic deformation, fracture, and atomic-level wear, as shown in Figure 6. Additionally, under thermal switch conditions, the graphene coating significantly extended the switch's lifespan, reaching 1000 times that of uncoated switches.