# Research and Analysis of MEMS Switches in Different Frequency Bands
## 3 The Research Status of MEMS Switches in Different Frequencies
### Research on Reliable Contacts of MEMS Switches  
Failures in contacts are the main reasons to limit the application of metal-contact MEMS switches, and the performance of this type of switch is primarily determined by the contact area. Contact stiction and contact degradation are the main failures of MEMS switches. Stiction is caused by cold or hot welding and is primarily related to the contact metal structure and the geometry of the contact area. Stiction caused by cold welding depends on the atomic adhesion forces between the contact materials, and stiction caused by hot welding, which is more important for MEMS switches, is related to the electrical current flowing inside the contact, which causes a rise in temperature that causes small asperities to weld to each other [6; 87]. Moreover, if the RF power level is increased, the contact failure is exacerbated by the raised temperature in the contact area [88]. In order to reduce contact failure, satisfy higher power handling capability, and improve lifetime at hot switching conditions, much research has been performed. The selection of the contact material is a key for achieving reliable contacts. The most widely used contact material in MEMS switches is gold (Au) [89; 90; 91], because Au has very low resistivity, high conductivity, and high oxidation resistance. Switches [91] based on a nickel cantilever beam and Au-Au contact have a current handling capability of 150 mA and lifetimes exceeding \(1\times 10^{9}\) cycles. A lateral switching relay structure [90] was designed to provide Au-Au contact with as low as 70 m\(\Omega\) contact resistance and 0.45 current-carrying ability. But contact wear, deformation, and adhesive failure are prone to occurring due to the low hardness and low melting point of Au [92; 93]. Thus, the selection of the electrical contact material and packaging are probably the most critical steps. The contact materials should have high hardness for protection against surface degradation, low resistivity for low contact resistance, and high resistance against chemical corrosion. Many researchers have focused on various contact materials with low roughness, low resistivity, high hardness, and high chemical resistance to contamination and corrosion instead of Au-Au contact. Strategies to improve hardness usually make use of bimetallic contacts and gold alloying with other metals, such as ruthenium (Ru), nickel (Ni), palladium (Pd), or platinum (Pt), or with other noble metals such as silver. Table 8 summarizes some commonly used contact materials. [PERSON] et al. (2018) showed that the failure point of Au-Au was 1.5 \(\times\) 10\({}^{5}\), Au-Pt was 1.6 \(\times\) 10\({}^{6}\), and Pt-Pt was 1.4 \(\times\) 10\({}^{7}\) under hot switching of 100 mA. The measured contact resistance of Au-Au was about 0.1 \(\Omega\), Au-Pt was about 0.25 \(\Omega\), and Pt-Pt was about 0.4 \(\Omega\). [PERSON] et al. (2014) reported that the minimum average contact resistance of microswitches with Au and Au/(6.3%)Pt alloy contacts were measured at 1.17 and 1.87 \(\Omega\), respectively. The \"hot-switched\" life cycles test results were 1.02 \(\times\) 10\({}^{8}\) and 2.70 \(\times\) 10\({}^{8}\), respectively. The lifetime of switches with Ir-Ir and Au-Ir contacts were 1.3 \(\times\) 10\({}^{7}\) and 1.8\(\times\) 10\({}^{6}\), respectively. The contact resistance of Ir-Ir and Au-Ir contacts were 0.79 \(\Omega\) and 0.49 \(\Omega\), respectively, under hot switching of 100 mA ([PERSON] et al., 2018). [PERSON] et al. (2018) presented a switch with an Au-Ru contact, and the switch achieved a contact resistance of 2.4-1.8 \(\Omega\) at 90-100 V in open laboratory environments (nonpackaged). [PERSON] et al. (2017) designed a MEMS switch with RuO\({}_{2}\)-Au contacts. The lifetime of the switch was 1 \(\times\) 10\({}^{11}\) cycles, and the resistance was less than 4 \(\Omega\). Metal contact switches with binary alloy (Au/Pd, Au/(3.7 at%)Pd) and ternary alloy (Au/Pt/Cu, Au/(5.0 at%)Pt/(0.5 at%)Cu) contacts were designed in ([PERSON] et al., 2018).