Microfabrication of RF MEMS Switches Through Thin Flim Stress Control

RF MEMS switches are miniature electromechanical devices that use the movement of metallic membranes to achieve open and short circuits. Due to their mechanical nature, RF MEMS switches are far less affected by frequency and are more suitable for high-frequency applications. Their miniature size and near-zero power consumption make them especially attractive for use in a wide range of portable devices and satellites. Unfavorable stress often causes unwanted deformation and distortion of thin film membranes. Realizing free-standing membranes is critical to successfully fabricating RF MEMS switches. This research demonstrates that, by using multi-layer configurations, stress and stress gradient can be purposely tailored to repeatedly and reliably achieve curving or non-curving metallic membranes by using different metal layers that are made by different methods or different processing parameters. The membranes can be a couple of millimeters long and there is no limitation on membrane width. This is beneficial for avoiding impedance discontinuity caused by the presence of different widths along RF paths. The membranes obtained are robust because they are totally metallic. This stress control technique has been used to develop various switches. More than seventy-five switches with different types (bridge or cantilever, ohmic contact or capacitance coupling, and slot or hole) and different dimensions have been exploited and tested. Membrane thickness is 8-12kÅ and the sacrificial layer is 3.5 μm thick for these devices. The actuation voltage is 30~40 V for the capacitive-coupling switch and 50~60 V for the cantilever ohmic contact switches. For a cantilever switch, its actuation voltage can be significantly decreased by using a thinner sacrificial layer without compromising its other RF properties. RF MEMS switches have been integrated in situ with double-band annular slot antenna and a penta-band planar inverted-F antenna to realize multiple frequency bands. The microfabrication approach developed in this research has potential to simplify fabrication and benefit monolithic integration.