Hermetic Wafer-Level Packaging for RF MEMs: Effects on Resonator Performance

The work presented here details the wafer-level fabrication and integration of aluminum nitride (AlN) micro resonators into hermetic micro environments. By etching cavities into the lid wafer and then bonding the lid wafer to a wafer of AlN micro resonators, a hermetic micro environment is created. After bonding, the lid wafer is thinned by plasma etching to expose individual die. This sequence presents the opportunity to perform resonator release on a wafer level while providing protection from dicing and other fabrication steps. We present here, fabrication and integration specifics on the wafer-level-packaging (WLP). Further we detail challenges encountered during the integration process including: elimination of micro voids created during eutectic wafer bonding, the use of plasma etching of lid wafers as a replacement to polish based wafer thinning, techniques to confirm hermetic environments, and significant failure mechanisms of the process limiting yield. Finally, we quantify improvements of the AlN micro resonators by correlating quality factors and integrated Pirani gauges.