A Robust Thin-Film Wafer-Level Packaging Approach for MEMS Devices

摘要

Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment,rnespecially at wafer-level, until they are packaged in final form. The harsh back-end (BE)rnoperations that the MEMS devices have to endure include dicing, pick-and-place, wire bonding andrnmolding. During these processing steps, the MEMS device is exposed to particles and contaminants.rnTherefore, protection at an early stage is a fundamental requirement.rnIn this work, we describe a silicon nitride thin-film capping, which is processed using arnsacrificial layer technique only with front-end technology. This approach is suitable for massrnproduction of MEMS devices, owing to the fact that, it is more cost-effective when compared tornother approaches such as wafer-to-wafer bonding and die-to-wafer bonding.rnA Bulk Acoustic Wave (BAW) resonator, that finds application in the Radio Frequencyrn(RF) front end, e.g., in cell phones, is taken as a MEMS vehicle for our work. It is an example of anrnextremely sensitive MEMS device, because the resonance frequency shifts significantly whenrnadditional mass is accidentally deposited on its surface. The thickness of the silicon nitride cappingrnthat is required to withstand all the BE steps, in particular transfer molding, is estimated using simplernanalytical calculations and finite element model (FEM) simulations. The pressure acting on the thinrnfilm capping and the thermal load during molding are included in the FEM model. Using this, thernminimum thickness required for the capping is determined. We prove that, a BAW resonator cappedrnwith silicon nitride at wafer-level can be wafer-thinned, diced, wire bonded and molded withoutrnmajor degradation in its performance.