Wafer-Level Vacuum Sealing by Transfer Bonding of Silicon Caps for Small Footprint and Ultra-Thin MEMS Packages

Vacuum and hermetic packaging is a critical requirement for optimal performance of many micro-electro-mechanical systems (MEMS), vacuum electronics, and quantum devices. However, existing packaging solutions are either elaborate to implement or rely on bulky caps and footprint-consuming seals. Here, we address this problem by demonstrating a wafer-level vacuum packaging method featuring transfer bonding of 25- $\mu \text{m}$ -thin silicon (Si) caps that are transferred from a 100-mm-diameter silicon-on-insulator (SOI) wafer to a cavity wafer to seal the cavities by gold–aluminum (Au–Al) thermo-compression bonding at a low temperature of 250 °C. The resulting wafer-scale sealing yields after wafer dicing are 98% and 100% with sealing rings as narrow as 6 and 9 $\mu \text{m}$ , respectively. Despite the small sealing footprint, the Si caps with 9- $\mu \text{m}$ -wide sealing rings demonstrate a high mean shear strength of 127 MPa. The vacuum levels in the getter-free sealed cavities are measured by residual gas analysis to be as low as 1.3 mbar, based on which a leak rate smaller than ${2.8}\times {10}^{-14}$ mbarL/s is derived. We also show that the thickness of the Si caps can be reduced to 6 $\mu \text{m}$ by post-transfer etching while still maintaining excellent hermeticity. The demonstrated ultra-thin packages can potentially be placed in between the solder bumps in flip–chip interfaces, thereby avoiding the need of through-cap-vias in conventional MEMS packages. [2018-0257]