A Robust Wafer-Level Capping Approach for MEMS Devices

Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment, especially at wafer-level, until they are packaged in final form. The harsh back-end (BE) operations that the MEMS devices have to endure include dicing, pick-and-place, wire bonding and molding. During these processing steps, the MEMS device is exposed to particles and contaminants. Therefore, protection at an early stage is a fundamental requirement. In this work, we describe a silicon nitride thin-film capping, which is processed using a sacrificial layer technique only with front-end technology. This approach is suitable for mass production of MEMS devices, owing to the fact that, it is more cost-effective when compared to other approaches such as wafer-to-wafer bonding and die-to-wafer bonding. A Bulk Acoustic Wave (BAW) resonator, that finds application in the Radio Frequency (RF) front end, e.g., in cell phones, is taken as a MEMS vehicle for our work. It is an example of an extremely sensitive MEMS device, because the resonance frequency shifts significantly when additional mass is accidentally deposited on its surface. The thickness of the silicon nitride capping that is required to withstand all the BE steps, in particular transfer molding, is estimated using simple analytical calculations and finite element model (FEM) simulations. The pressure acting on the thin film capping and the thermal load during molding are included in the FEM model. Using this, the minimum thickness required for the capping is determined. We prove that, a BAW resonator capped with silicon nitride at wafer-level can be wafer-thinned, diced, wire bonded and molded without major degradation in its performance.