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

Abstract

Micro-electromechanical systems (MEMS) devices are extremely sensitive to their environment, especialiy at the 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.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, for example, 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 the wafer level can be wafer-thinned, diced, wire bonded, and molded without major degradation in performance.

A review of MEMS oscillators for frequency reference and timing applications

“…Obviously, this places restrictions on the materials and etch processes being used for processing the thin-film cavity. In [34,35] process flows are described that are compatible with the AlN-based resonator, the process flow in [34] uses the SU-8 epoxy capping layer while in [35] a flow is described for processing an AlN-based resonator using a PECVD nitride capping layer.…”

A Ratiometric Readout Circuit for Thermal-Conductivity-Based Resistive CO2Sensors

“…One chip, with the sensitive transducers, was packaged in a ceramic DIL package of which the cavity is exposed to the ambient air, and the other, with the reference transducers, was packaged in a DIL package of which the cavity is sealed. Note that there are potential solutions for sealing the reference transducers at the wafer-level, such as wafer-to-wafer capping, die-to-wafer bonding and waferlevel thin-film capping [30]. In this way, both types of transducers can in the future be integrated in the same package.…”

Self-Sealing Complex Oxide Resonators

“…Moreover, the pressure at which the sealing layer is deposited is often fixed by the process, such that the reference pressure in the cavity cannot be freely controlled. 13 − 16 …”

“…Recently reported sealing protocols have enabled improvements in the hermeticity of vdW material membranes of up to a factor 10 000, , but scaling them to high volume production is difficult, since depositing and patterning of sealing layers on top of ultrathin vdW material membranes is often detrimental to device performance in particular if high temperatures are needed. Moreover, the pressure at which the sealing layer is deposited is often fixed by the process, such that the reference pressure in the cavity cannot be freely controlled. − …”