# Research and Analysis of MEMS Switches in Different Frequency Bands
## 3 The Research Status of MEMS Switches in Different Frequencies
### Research on Dielectric Charging of MEMS Switches  
MEMS capacitive switches exhibit excellent performance in the field of wireless communications; however, reliability issues are still the bottleneck that limits their commercialization [62]. The main failure of RF MEMS switches is \"adhesion\" caused by dielectric charging [63; 64]. The dielectric charging phenomenon has been studied by a number of researchers since it was first observed by [PERSON] et al. [65] in 1998. The dielectric charging effect arises from charge injection and the dipole orientation, displacement, and trapping that occurs under the strong electric field during the down-state of MEMS capacitive switches [66]. The electric field, generated by pull-down voltage across the dielectric layer, reaches up to 1-3 MV/cm when the switch is in down-state. A leakage current is produced in the dielectric material by different conductive mechanisms [67]. The charge in the leakage current is captured by the trap in the dielectric material when the bridge contacts the dielectric layer, which causes the charge to accumulate. The electric field, generated by the accumulated charge, will affect the electric force on the driving electrode, resulting in the drift of the pull-down voltage and the adhesion problem. [PERSON] et al. [68] found that peak voltage, duty factor, and temperature were significant acceleration factors on charging, whereas frequency had little effect on charging. The steady-state leakage current and the steady-state charge density increase at elevated temperatures, whereas the restoring force and spring constant of the membrane decrease at elevated temperatures; therefore, the switch is more prone to stiction due to dielectric charging the when temperature increases [69]. The dielectric charging was strongly affected by temperature and increased exponentially with temperature, especially with high temperature [70]. The increase of charging temperature assists the charge trapping and the increase of discharging temperature leads to an increase in the magnitude of the bulk discharge current [71]. [PERSON] et al. [72] studied the impact of humidity on dielectric charging in RF MEMS capacitive switches and found that bulk charging dominated in dry air, but surface charging increased linearly with increasing humidity. [PERSON] et al. [73] found that thinner dielectric films had larger induced surface potential, which lead to higher charge trapping; thus, thicker dielectric films would be better for less dielectric charging. Charges were also found to decay faster in SiN dielectric films deposited on metal layers compared with dielectric films deposited on silicon substrates, and high-frequency SiN dielectric films were more resistant to dielectric charging compared with low-frequency ones [73]. The accumulated charge increased with the dielectric film thickness of the low-frequency material but could not be affected by the thickness of the high-frequency material [74]. The high electric field injection leads to defect generation in low-frequency material [75]. [PERSON] et al. [76] found that dielectric charging occurred when the actuation voltage had not reached the pull-down voltage and called this phenomenon contactless charging. The contactless charging occurs in gaps between the dielectric layer and the membrane due to the contact surface roughness when the membrane is in the down-state. [PERSON] et al. [77] found that the current density of rough contact was several orders  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Band** & **Insertion Loss** & **Isolation** & **Dielectric Layer** & **Substrate** & **Bridge** & **Transmission** & **Reference** \\ \hline \multirow{4}{*}{\(\mathrm{c}\mathrm{c}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{i  
of magnitude greater than planar contact based on a study of the effect of the surface roughness on dielectric charging, which affects the reliability of the switch.