Hermeticity testing of capacitive RF MEMS switches

RF MEMS capacitive switches can be used for band switching and adaptive antenna matching in cell phone Front End Modules. They are extremely linear and, if made in standard silicon technology, can be integrated into other applications in the phone. Many efforts have been put in the intrinsic reliability of the RF MEMS capacitive switches. The main failure mechanism is stiction due to dielectric charging. The progress made in RF MEMS processing has shifted the focus towards the package. It is generally accepted that a hermetic package is needed to keep the effect of charging within specification. Since packages are very small (typically 300nl), new hermeticity tests are needed because the leak rate tests are not sensitive enough for these small packages. In this paper we demonstrate the reliability of a capacitive RF MEMS switch package. The product is assembled using a AuSn bonding process in which a Silicon cap wafer is assembled on top of the MEMS die. The package is filled with pure N₂ to the desired pressure. Hermeticity is determined by measuring the pressure dependence of the mechanical resonance of the MEMS top electrode. First we will describe the method we have used to determine the pressure inside the cavity and how to apply that to hermeticity testing. Second, we describe how the requirements for the leak rate of the package can be determined. Finally we show with the use of temperature, humidity and high pressure N₂ testing that our package fulfills these requirements. Although the method may not be new, the combination of extensive reliability testing and understanding the RF MEMS device using FEM simulations has not been reported before.