An MEMS Sandwich Wafer-Level Package With Precisely Symmetrical Out-of-Plane Electrodes

This article shows the design, fabrication, test verification, and theoretical analysis of a proposed sandwich wafer-level packaging (WLP) structure with double-sided out-of-plane electrode configuration and precise symmetry adjustment method of 5- $\mu $ m out-of-plane gaps for micro-electromechanical system (MEMS) devices requiring control and detection of out-of-plane movement. Common WLP schemes are typically fabricated based on the silicon-on-insulator (SOI) process and direct silicon bonding, which makes it difficult to place double-sided out-of-plane electrodes. In this article, a glass-silicon–glass sandwich WLP scheme is designed to configure symmetrical out-of-plane electrodes on top and bottom layers. Anodic bonding and Au-Sn solid-liquid interdiffusion (SLID) bonding are used for the WLP encapsulation of triple stacks. Gaps between out-of-plane electrodes and suspended mass are controlled by the height of etched silicon anchors and standoffs. Extremely thin metallization layers with a total thickness of less than $3~\mu $ m are precisely deposited by a novel online calibration method to reach the required Au-Sn composition for a stable liquid phase transition during bonding, matching standoff height for out-of-plane gap control. After fabrication, capacitance is measured to evaluate the double-sided symmetry of the out-of-plane electrode gaps. WLP dies with standoffs have a relative capacitance difference of less than 1.77%, while WLP dies without standoffs have a relative capacitance difference of 7.82%. A theoretical symmetry analysis based on process data is carried out to resolve capacitance variance into several sources, achieving a high estimation accuracy of 87.6%. The designed WLP framework is aimed at a fabrication implementation based on universal MEMS processes, without a requirement for an advanced MEMS platform.