RF MEMS Shunt Capacitive Switches Using AlN Compared to $\hbox {Si}_{3}\hbox {N}_{4}$ Dielectric

RF microelectromechanical systems (MEMS) capacitive switches for two different dielectrics, aluminum nitride (AlN) and silicon nitride (Si₃N₄), are presented. The switches have been characterized and compared in terms of DC and RF performance (5-40 GHz). Switches based on AlN have higher down-state capacitance for similar dielectric thicknesses and provide better isolation and smaller insertion losses compared to Si₃N₄ switches. Experiments were carried out on RF MEMS switches with stiffening bars to prevent membrane deformation due to residual stress and with different spring and meander-type anchor designs. For a ~300-nm dielectric thickness, an air gap of 2.3 μm and identical spring-type designs, the AlN switches systematically show an improvement in the isolation by more than -12 dB (-35.8 dB versus -23.7 dB) and a better insertion loss (-0.68 dB versus -0.90 dB) at 40 GHz compared to Si₃N₄. DC measurements show small leakage current densities for both dielectrics (<;10^-8 A/cm² at 1 MV/cm). However, the resulting leakage current for AlN devices is ten times higher than for Si₃N₄ when applying a larger electric field. The fabricated switches were also stressed by applying different voltages in air and vacuum, and dielectric charging effects were investigated. AlN switches eliminate the residual or injected charge faster than the Si₃N₄ devices do.